ViewCompat：android官方实现兼容的一个帮助类

类代码：

package android.support.v4.view;

import android.content.res.ColorStateList;
import android.graphics.Paint;
import android.graphics.PixelFormat;
import android.graphics.PorterDuff;
import android.graphics.Rect;
import android.graphics.drawable.Drawable;
import android.os.Build;
import android.os.Bundle;
import android.support.annotation.FloatRange;
import android.support.annotation.IdRes;
import android.support.annotation.IntDef;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.view.accessibility.AccessibilityNodeInfoCompat;
import android.support.v4.view.accessibility.AccessibilityNodeProviderCompat;
import android.util.Log;
import android.view.MotionEvent;
import android.view.VelocityTracker;
import android.view.View;
import android.view.ViewConfiguration;
import android.view.ViewGroup;
import android.view.ViewParent;
import android.view.accessibility.AccessibilityEvent;

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.WeakHashMap;

/**API4以后引入的实现兼容的一个帮助类
 * Helper for accessing features in {@link View} introduced after API
 * level 4 in a backwards compatible fashion.
 */
public class ViewCompat {
    private static final String TAG = "ViewCompat";

    //常量 略，本节旨在观察此类的结构

//这个接口列出了这个类支持的方法，这些方法在此类中都有实现，我们也可以实现此接口进行重写
    interface ViewCompatImpl {
        public boolean canScrollHorizontally(View v, int direction);
        public boolean canScrollVertically(View v, int direction);
        public int getOverScrollMode(View v);
        public void setOverScrollMode(View v, int mode);
        public void onInitializeAccessibilityEvent(View v, AccessibilityEvent event);
        public void onPopulateAccessibilityEvent(View v, AccessibilityEvent event);
        public void onInitializeAccessibilityNodeInfo(View v, AccessibilityNodeInfoCompat info);
        public void setAccessibilityDelegate(View v, @Nullable AccessibilityDelegateCompat delegate);
        public boolean hasAccessibilityDelegate(View v);
        public boolean hasTransientState(View view);
        public void setHasTransientState(View view, boolean hasTransientState);
        public void postInvalidateOnAnimation(View view);
        public void postInvalidateOnAnimation(View view, int left, int top, int right, int bottom);
        public void postOnAnimation(View view, Runnable action);
        public void postOnAnimationDelayed(View view, Runnable action, long delayMillis);
        public int getImportantForAccessibility(View view);
        public void setImportantForAccessibility(View view, int mode);
        public boolean isImportantForAccessibility(View view);
        public boolean performAccessibilityAction(View view, int action, Bundle arguments);
        public AccessibilityNodeProviderCompat getAccessibilityNodeProvider(View view);
        public float getAlpha(View view);
        public void setLayerType(View view, int layerType, Paint paint);
        public int getLayerType(View view);
        public int getLabelFor(View view);
        public void setLabelFor(View view, int id);
        public void setLayerPaint(View view, Paint paint);
        public int getLayoutDirection(View view);
        public void setLayoutDirection(View view, int layoutDirection);
        public ViewParent getParentForAccessibility(View view);
        public boolean isOpaque(View view);
        public int resolveSizeAndState(int size, int measureSpec, int childMeasuredState);
        public int getMeasuredWidthAndState(View view);
        public int getMeasuredHeightAndState(View view);
        public int getMeasuredState(View view);
        public int getAccessibilityLiveRegion(View view);
        public void setAccessibilityLiveRegion(View view, int mode);
        public int getPaddingStart(View view);
        public int getPaddingEnd(View view);
        public void setPaddingRelative(View view, int start, int top, int end, int bottom);
        public void dispatchStartTemporaryDetach(View view);
        public void dispatchFinishTemporaryDetach(View view);
        public float getX(View view);
        public float getY(View view);
        public float getRotation(View view);
        public float getRotationX(View view);
        public float getRotationY(View view);
        public float getScaleX(View view);
        public float getScaleY(View view);
        public float getTranslationX(View view);
        public float getTranslationY(View view);
        public int getMinimumWidth(View view);
        public int getMinimumHeight(View view);
        public ViewPropertyAnimatorCompat animate(View view);
        public void setRotation(View view, float value);
        public void setRotationX(View view, float value);
        public void setRotationY(View view, float value);
        public void setScaleX(View view, float value);
        public void setScaleY(View view, float value);
        public void setTranslationX(View view, float value);
        public void setTranslationY(View view, float value);
        public void setX(View view, float value);
        public void setY(View view, float value);
        public void setAlpha(View view, float value);
        public void setPivotX(View view, float value);
        public void setPivotY(View view, float value);
        public float getPivotX(View view);
        public float getPivotY(View view);
        public void setElevation(View view, float elevation);
        public float getElevation(View view);
        public void setTranslationZ(View view, float translationZ);
        public float getTranslationZ(View view);
        public void setClipBounds(View view, Rect clipBounds);
        public Rect getClipBounds(View view);
        public void setTransitionName(View view, String transitionName);
        public String getTransitionName(View view);
        public int getWindowSystemUiVisibility(View view);
        public void requestApplyInsets(View view);
        public void setChildrenDrawingOrderEnabled(ViewGroup viewGroup, boolean enabled);
        public boolean getFitsSystemWindows(View view);
        public boolean hasOverlappingRendering(View view);
        void setFitsSystemWindows(View view, boolean fitSystemWindows);
        void jumpDrawablesToCurrentState(View v);
        void setOnApplyWindowInsetsListener(View view, OnApplyWindowInsetsListener listener);
        WindowInsetsCompat onApplyWindowInsets(View v, WindowInsetsCompat insets);
        WindowInsetsCompat dispatchApplyWindowInsets(View v, WindowInsetsCompat insets);
        void setSaveFromParentEnabled(View view, boolean enabled);
        void setActivated(View view, boolean activated);
        boolean isPaddingRelative(View view);
        ColorStateList getBackgroundTintList(View view);
        void setBackgroundTintList(View view, ColorStateList tintList);
        PorterDuff.Mode getBackgroundTintMode(View view);
        void setBackgroundTintMode(View view, PorterDuff.Mode mode);
        void setNestedScrollingEnabled(View view, boolean enabled);
        boolean isNestedScrollingEnabled(View view);
        boolean startNestedScroll(View view, int axes);
        void stopNestedScroll(View view);
        boolean hasNestedScrollingParent(View view);
        boolean dispatchNestedScroll(View view, int dxConsumed, int dyConsumed, int dxUnconsumed,
                int dyUnconsumed, int[] offsetInWindow);
        boolean dispatchNestedPreScroll(View view, int dx, int dy, int[] consumed,
                int[] offsetInWindow);
        boolean dispatchNestedFling(View view, float velocityX, float velocityY, boolean consumed);
        boolean dispatchNestedPreFling(View view, float velocityX, float velocityY);
        boolean isLaidOut(View view);
        int combineMeasuredStates(int curState, int newState);
        public float getZ(View view);
        public boolean isAttachedToWindow(View view);
        public boolean hasOnClickListeners(View view);
        public void setScrollIndicators(View view, int indicators);
        public void setScrollIndicators(View view, int indicators, int mask);
        public int getScrollIndicators(View view);
    }

    static class BaseViewCompatImpl implements ViewCompatImpl {
        private Method mDispatchStartTemporaryDetach;
        private Method mDispatchFinishTemporaryDetach;
        private boolean mTempDetachBound;
        WeakHashMap mViewPropertyAnimatorCompatMap = null;


        public boolean canScrollHorizontally(View v, int direction) {
            return (v instanceof ScrollingView) &&
                canScrollingViewScrollHorizontally((ScrollingView) v, direction);
        }
        public boolean canScrollVertically(View v, int direction) {
            return (v instanceof ScrollingView) &&
                    canScrollingViewScrollVertically((ScrollingView) v, direction);
        }
        public int getOverScrollMode(View v) {
            return OVER_SCROLL_NEVER;
        }
        public void setOverScrollMode(View v, int mode) {
            // Do nothing; API doesn't exist
        }
        public void setAccessibilityDelegate(View v, AccessibilityDelegateCompat delegate) {
            // Do nothing; API doesn't exist
        }

        @Override
        public boolean hasAccessibilityDelegate(View v) {
            return false;
        }

        public void onPopulateAccessibilityEvent(View v, AccessibilityEvent event) {
            // Do nothing; API doesn't exist
        }
        public void onInitializeAccessibilityEvent(View v, AccessibilityEvent event) {
         // Do nothing; API doesn't exist
        }
        public void onInitializeAccessibilityNodeInfo(View v, AccessibilityNodeInfoCompat info) {
            // Do nothing; API doesn't exist
        }
        public boolean hasTransientState(View view) {
            // A view can't have transient state if transient state wasn't supported.
            return false;
        }
        public void setHasTransientState(View view, boolean hasTransientState) {
            // Do nothing; API doesn't exist
        }
        public void postInvalidateOnAnimation(View view) {
            view.invalidate();
        }
        public void postInvalidateOnAnimation(View view, int left, int top, int right, int bottom) {
            view.invalidate(left, top, right, bottom);
        }
        public void postOnAnimation(View view, Runnable action) {
            view.postDelayed(action, getFrameTime());
        }
        public void postOnAnimationDelayed(View view, Runnable action, long delayMillis) {
            view.postDelayed(action, getFrameTime() + delayMillis);
        }
        long getFrameTime() {
            return FAKE_FRAME_TIME;
        }
        public int getImportantForAccessibility(View view) {
            return 0;
        }
        public void setImportantForAccessibility(View view, int mode) {

        }
        public boolean isImportantForAccessibility(View view) {
            return true;
        }
        public boolean performAccessibilityAction(View view, int action, Bundle arguments) {
            return false;
        }
        public AccessibilityNodeProviderCompat getAccessibilityNodeProvider(View view) {
            return null;
        }
        public float getAlpha(View view) {
            return 1.0f;
        }
        public void setLayerType(View view, int layerType, Paint paint) {
            // No-op until layers became available (HC)
        }
        public int getLayerType(View view) {
            return LAYER_TYPE_NONE;
        }
        public int getLabelFor(View view) {
            return 0;
        }
        public void setLabelFor(View view, int id) {

        }
        public void setLayerPaint(View view, Paint p) {
            // No-op until layers became available (HC)
        }

        @Override
        public int getLayoutDirection(View view) {
            return LAYOUT_DIRECTION_LTR;
        }

        @Override
        public void setLayoutDirection(View view, int layoutDirection) {
            // No-op
        }

        @Override
        public ViewParent getParentForAccessibility(View view) {
            return view.getParent();
        }

        @Override
        public boolean isOpaque(View view) {
            final Drawable bg = view.getBackground();
            if (bg != null) {
                return bg.getOpacity() == PixelFormat.OPAQUE;
            }
            return false;
        }

        public int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {
            return View.resolveSize(size, measureSpec);
        }

        @Override
        public int getMeasuredWidthAndState(View view) {
            return view.getMeasuredWidth();
        }

        @Override
        public int getMeasuredHeightAndState(View view) {
            return view.getMeasuredHeight();
        }

        @Override
        public int getMeasuredState(View view) {
            return 0;
        }

        @Override
        public int getAccessibilityLiveRegion(View view) {
            return ACCESSIBILITY_LIVE_REGION_NONE;
        }

        @Override
        public void setAccessibilityLiveRegion(View view, int mode) {
            // No-op
        }

        @Override
        public int getPaddingStart(View view) {
            return view.getPaddingLeft();
        }

        @Override
        public int getPaddingEnd(View view) {
            return view.getPaddingRight();
        }

        @Override
        public void setPaddingRelative(View view, int start, int top, int end, int bottom) {
            view.setPadding(start, top, end, bottom);
        }

        @Override
        public void dispatchStartTemporaryDetach(View view) {
            if (!mTempDetachBound) {
                bindTempDetach();
            }
            if (mDispatchStartTemporaryDetach != null) {
                try {
                    mDispatchStartTemporaryDetach.invoke(view);
                } catch (Exception e) {
                    Log.d(TAG, "Error calling dispatchStartTemporaryDetach", e);
                }
            } else {
                // Try this instead
                view.onStartTemporaryDetach();
            }
        }

        @Override
        public void dispatchFinishTemporaryDetach(View view) {
            if (!mTempDetachBound) {
                bindTempDetach();
            }
            if (mDispatchFinishTemporaryDetach != null) {
                try {
                    mDispatchFinishTemporaryDetach.invoke(view);
                } catch (Exception e) {
                    Log.d(TAG, "Error calling dispatchFinishTemporaryDetach", e);
                }
            } else {
                // Try this instead
                view.onFinishTemporaryDetach();
            }
        }

        @Override
        public boolean hasOverlappingRendering(View view) {
            return true;
        }

        private void bindTempDetach() {
            try {
                mDispatchStartTemporaryDetach = View.class.getDeclaredMethod(
                        "dispatchStartTemporaryDetach");
                mDispatchFinishTemporaryDetach = View.class.getDeclaredMethod(
                        "dispatchFinishTemporaryDetach");
            } catch (NoSuchMethodException e) {
                Log.e(TAG, "Couldn't find method", e);
            }
            mTempDetachBound = true;
        }

        @Override
        public float getTranslationX(View view) {
            return 0;
        }

        @Override
        public float getTranslationY(View view) {
            return 0;
        }

        @Override
        public float getX(View view) {
            return 0;
        }

        @Override
        public float getY(View view) {
            return 0;
        }

        @Override
        public float getRotation(View view) {
            return 0;
        }

        @Override
        public float getRotationX(View view) {
            return 0;
        }

        @Override
        public float getRotationY(View view) {
            return 0;
        }

        @Override
        public float getScaleX(View view) {
            return 0;
        }

        @Override
        public float getScaleY(View view) {
            return 0;
        }

        @Override
        public int getMinimumWidth(View view) {
            return ViewCompatBase.getMinimumWidth(view);
        }

        @Override
        public int getMinimumHeight(View view) {
            return ViewCompatBase.getMinimumHeight(view);
        }

        @Override
        public ViewPropertyAnimatorCompat animate(View view) {
            return new ViewPropertyAnimatorCompat(view);
        }

        @Override
        public void setRotation(View view, float value) {
            // noop
        }

        @Override
        public void setTranslationX(View view, float value) {
            // noop
        }

        @Override
        public void setTranslationY(View view, float value) {
            // noop
        }

        @Override
        public void setAlpha(View view, float value) {
            // noop
        }

        @Override
        public void setRotationX(View view, float value) {
            // noop
        }

        @Override
        public void setRotationY(View view, float value) {
            // noop
        }

        @Override
        public void setScaleX(View view, float value) {
            // noop
        }

        @Override
        public void setScaleY(View view, float value) {
            // noop
        }

        @Override
        public void setX(View view, float value) {
            // noop
        }

        @Override
        public void setY(View view, float value) {
            // noop
        }

        @Override
        public void setPivotX(View view, float value) {
            // noop
        }

        @Override
        public void setPivotY(View view, float value) {
            // noop
        }

        @Override
        public float getPivotX(View view) {
            return 0;
        }

        @Override
        public float getPivotY(View view) {
            return 0;
        }

        @Override
        public void setTransitionName(View view, String transitionName) {
        }

        @Override
        public String getTransitionName(View view) {
            return null;
        }

        @Override
        public int getWindowSystemUiVisibility(View view) {
            return 0;
        }

        @Override
        public void requestApplyInsets(View view) {
        }

        @Override
        public void setElevation(View view, float elevation) {
        }

        @Override
        public float getElevation(View view) {
            return 0f;
        }

        @Override
        public void setTranslationZ(View view, float translationZ) {
        }

        @Override
        public float getTranslationZ(View view) {
            return 0f;
        }

        @Override
        public void setClipBounds(View view, Rect clipBounds) {
        }

        @Override
        public Rect getClipBounds(View view) {
            return null;
        }

        @Override
        public void setChildrenDrawingOrderEnabled(ViewGroup viewGroup, boolean enabled) {
            // noop
        }

        @Override
        public boolean getFitsSystemWindows(View view) {
            return false;
        }

        @Override
        public void setFitsSystemWindows(View view, boolean fitSystemWindows) {
            // noop
        }

        @Override
        public void jumpDrawablesToCurrentState(View view) {
            // Do nothing; API didn't exist.
        }

        @Override
        public void setOnApplyWindowInsetsListener(View view,
                OnApplyWindowInsetsListener listener) {
            // noop
        }

        @Override
        public WindowInsetsCompat onApplyWindowInsets(View v, WindowInsetsCompat insets) {
            return insets;
        }

        @Override
        public WindowInsetsCompat dispatchApplyWindowInsets(View v, WindowInsetsCompat insets) {
            return insets;
        }

        @Override
        public void setSaveFromParentEnabled(View v, boolean enabled) {
            // noop
        }

        @Override
        public void setActivated(View view, boolean activated) {
            // noop
        }

        @Override
        public boolean isPaddingRelative(View view) {
            return false;
        }

        public void setNestedScrollingEnabled(View view, boolean enabled) {
            if (view instanceof NestedScrollingChild) {
                ((NestedScrollingChild) view).setNestedScrollingEnabled(enabled);
            }
        }

        @Override
        public boolean isNestedScrollingEnabled(View view) {
            if (view instanceof NestedScrollingChild) {
                return ((NestedScrollingChild) view).isNestedScrollingEnabled();
            }
            return false;
        }

        @Override
        public ColorStateList getBackgroundTintList(View view) {
            return ViewCompatBase.getBackgroundTintList(view);
        }

        @Override
        public void setBackgroundTintList(View view, ColorStateList tintList) {
            ViewCompatBase.setBackgroundTintList(view, tintList);
        }

        @Override
        public void setBackgroundTintMode(View view, PorterDuff.Mode mode) {
            ViewCompatBase.setBackgroundTintMode(view, mode);
        }

        @Override
        public PorterDuff.Mode getBackgroundTintMode(View view) {
            return ViewCompatBase.getBackgroundTintMode(view);
        }

        private boolean canScrollingViewScrollHorizontally(ScrollingView view, int direction) {
            final int offset = view.computeHorizontalScrollOffset();
            final int range = view.computeHorizontalScrollRange() -
                    view.computeHorizontalScrollExtent();
            if (range == 0) return false;
            if (direction < 0) {
                return offset > 0;
            } else {
                return offset < range - 1;
            }
        }

        private boolean canScrollingViewScrollVertically(ScrollingView view, int direction) {
            final int offset = view.computeVerticalScrollOffset();
            final int range = view.computeVerticalScrollRange() -
                    view.computeVerticalScrollExtent();
            if (range == 0) return false;
            if (direction < 0) {
                return offset > 0;
            } else {
                return offset < range - 1;
            }
        }

        public boolean startNestedScroll(View view, int axes) {
            if (view instanceof NestedScrollingChild) {
                return ((NestedScrollingChild) view).startNestedScroll(axes);
            }
            return false;
        }

        @Override
        public void stopNestedScroll(View view) {
            if (view instanceof NestedScrollingChild) {
                ((NestedScrollingChild) view).stopNestedScroll();
            }
        }

        @Override
        public boolean hasNestedScrollingParent(View view) {
            if (view instanceof NestedScrollingChild) {
                return ((NestedScrollingChild) view).hasNestedScrollingParent();
            }
            return false;
        }

        @Override
        public boolean dispatchNestedScroll(View view, int dxConsumed, int dyConsumed,
                int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow) {
            if (view instanceof NestedScrollingChild) {
                return ((NestedScrollingChild) view).dispatchNestedScroll(dxConsumed, dyConsumed,
                        dxUnconsumed, dyUnconsumed, offsetInWindow);
            }
            return false;
        }

        @Override
        public boolean dispatchNestedPreScroll(View view, int dx, int dy,
                int[] consumed, int[] offsetInWindow) {
            if (view instanceof NestedScrollingChild) {
                return ((NestedScrollingChild) view).dispatchNestedPreScroll(dx, dy, consumed,
                        offsetInWindow);
            }
            return false;
        }

        @Override
        public boolean dispatchNestedFling(View view, float velocityX, float velocityY,
                boolean consumed) {
            if (view instanceof NestedScrollingChild) {
                return ((NestedScrollingChild) view).dispatchNestedFling(velocityX, velocityY,
                        consumed);
            }
            return false;
        }

        @Override
        public boolean dispatchNestedPreFling(View view, float velocityX, float velocityY) {
            if (view instanceof NestedScrollingChild) {
                return ((NestedScrollingChild) view).dispatchNestedPreFling(velocityX, velocityY);
            }
            return false;
        }

        @Override
        public boolean isLaidOut(View view) {
            return ViewCompatBase.isLaidOut(view);
        }

        @Override
        public int combineMeasuredStates(int curState, int newState) {
            return curState | newState;
        }

        @Override
        public float getZ(View view) {
            return getTranslationZ(view) + getElevation(view);
        }

        @Override
        public boolean isAttachedToWindow(View view) {
            return ViewCompatBase.isAttachedToWindow(view);
        }

        @Override
        public boolean hasOnClickListeners(View view) {
            return false;
        }

        @Override
        public int getScrollIndicators(View view) {
            return 0;
        }

        @Override
        public void setScrollIndicators(View view, int indicators) {
            // no-op
        }

        @Override
        public void setScrollIndicators(View view, int indicators, int mask) {
            // no-op
        }
    }

    static class EclairMr1ViewCompatImpl extends BaseViewCompatImpl {
        @Override
        public boolean isOpaque(View view) {
            return ViewCompatEclairMr1.isOpaque(view);
        }

        @Override
        public void setChildrenDrawingOrderEnabled(ViewGroup viewGroup, boolean enabled) {
            ViewCompatEclairMr1.setChildrenDrawingOrderEnabled(viewGroup, enabled);
        }
    }

    static class GBViewCompatImpl extends EclairMr1ViewCompatImpl {
        @Override
        public int getOverScrollMode(View v) {
            return ViewCompatGingerbread.getOverScrollMode(v);
        }
        @Override
        public void setOverScrollMode(View v, int mode) {
            ViewCompatGingerbread.setOverScrollMode(v, mode);
        }
    }

    static class HCViewCompatImpl extends GBViewCompatImpl {
        @Override
        long getFrameTime() {
            return ViewCompatHC.getFrameTime();
        }
        @Override
        public float getAlpha(View view) {
            return ViewCompatHC.getAlpha(view);
        }
        @Override
        public void setLayerType(View view, int layerType, Paint paint) {
            ViewCompatHC.setLayerType(view, layerType, paint);
        }
        @Override
        public int getLayerType(View view)  {
            return ViewCompatHC.getLayerType(view);
        }
        @Override
        public void setLayerPaint(View view, Paint paint) {
            // Make sure the paint is correct; this will be cheap if it's the same
            // instance as was used to call setLayerType earlier.
            setLayerType(view, getLayerType(view), paint);
            // This is expensive, but the only way to accomplish this before JB-MR1.
            view.invalidate();
        }
        @Override
        public int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {
            return ViewCompatHC.resolveSizeAndState(size, measureSpec, childMeasuredState);
        }
        @Override
        public int getMeasuredWidthAndState(View view) {
            return ViewCompatHC.getMeasuredWidthAndState(view);
        }
        @Override
        public int getMeasuredHeightAndState(View view) {
            return ViewCompatHC.getMeasuredHeightAndState(view);
        }
        @Override
        public int getMeasuredState(View view) {
            return ViewCompatHC.getMeasuredState(view);
        }
        @Override
        public float getTranslationX(View view) {
            return ViewCompatHC.getTranslationX(view);
        }
        @Override
        public float getTranslationY(View view) {
            return ViewCompatHC.getTranslationY(view);
        }
        @Override
        public void setTranslationX(View view, float value) {
            ViewCompatHC.setTranslationX(view, value);
        }
        @Override
        public void setTranslationY(View view, float value) {
            ViewCompatHC.setTranslationY(view, value);
        }
        @Override
        public void setAlpha(View view, float value) {
            ViewCompatHC.setAlpha(view, value);
        }
        @Override
        public void setX(View view, float value) {
            ViewCompatHC.setX(view, value);
        }
        @Override
        public void setY(View view, float value) {
            ViewCompatHC.setY(view, value);
        }
        @Override
        public void setRotation(View view, float value) {
            ViewCompatHC.setRotation(view, value);
        }
        @Override
        public void setRotationX(View view, float value) {
            ViewCompatHC.setRotationX(view, value);
        }
        @Override
        public void setRotationY(View view, float value) {
            ViewCompatHC.setRotationY(view, value);
        }
        @Override
        public void setScaleX(View view, float value) {
            ViewCompatHC.setScaleX(view, value);
        }
        @Override
        public void setScaleY(View view, float value) {
            ViewCompatHC.setScaleY(view, value);
        }
        @Override
        public void setPivotX(View view, float value) {
            ViewCompatHC.setPivotX(view, value);
        }
        @Override
        public void setPivotY(View view, float value) {
            ViewCompatHC.setPivotY(view, value);
        }
        @Override
        public float getX(View view) {
            return ViewCompatHC.getX(view);
        }

        @Override
        public float getY(View view) {
            return ViewCompatHC.getY(view);
        }

        @Override
        public float getRotation(View view) {
            return ViewCompatHC.getRotation(view);
        }

        @Override
        public float getRotationX(View view) {
            return ViewCompatHC.getRotationX(view);
        }

        @Override
        public float getRotationY(View view) {
            return ViewCompatHC.getRotationY(view);
        }

        @Override
        public float getScaleX(View view) {
            return ViewCompatHC.getScaleX(view);
        }

        @Override
        public float getScaleY(View view) {
            return ViewCompatHC.getScaleY(view);
        }

        @Override
        public float getPivotX(View view) {
            return ViewCompatHC.getPivotX(view);
        }
        @Override
        public float getPivotY(View view) {
            return ViewCompatHC.getPivotY(view);
        }
        @Override
        public void jumpDrawablesToCurrentState(View view) {
            ViewCompatHC.jumpDrawablesToCurrentState(view);
        }

        @Override
        public void setSaveFromParentEnabled(View view, boolean enabled) {
            ViewCompatHC.setSaveFromParentEnabled(view, enabled);
        }

        @Override
        public void setActivated(View view, boolean activated) {
            ViewCompatHC.setActivated(view, activated);
        }

        @Override
        public int combineMeasuredStates(int curState, int newState) {
            return ViewCompatHC.combineMeasuredStates(curState, newState);
        }
    }

    static class ICSViewCompatImpl extends HCViewCompatImpl {
        static Field mAccessibilityDelegateField;
        static boolean accessibilityDelegateCheckFailed = false;
        @Override
        public boolean canScrollHorizontally(View v, int direction) {
            return ViewCompatICS.canScrollHorizontally(v, direction);
        }
        @Override
        public boolean canScrollVertically(View v, int direction) {
            return ViewCompatICS.canScrollVertically(v, direction);
        }
        @Override
        public void onPopulateAccessibilityEvent(View v, AccessibilityEvent event) {
            ViewCompatICS.onPopulateAccessibilityEvent(v, event);
        }
        @Override
        public void onInitializeAccessibilityEvent(View v, AccessibilityEvent event) {
            ViewCompatICS.onInitializeAccessibilityEvent(v, event);
        }
        @Override
        public void onInitializeAccessibilityNodeInfo(View v, AccessibilityNodeInfoCompat info) {
            ViewCompatICS.onInitializeAccessibilityNodeInfo(v, info.getInfo());
        }
        @Override
        public void setAccessibilityDelegate(View v,
                @Nullable AccessibilityDelegateCompat delegate) {
            ViewCompatICS.setAccessibilityDelegate(v,
                    delegate == null ? null : delegate.getBridge());
        }

        @Override
        public boolean hasAccessibilityDelegate(View v) {
            if (accessibilityDelegateCheckFailed) {
                return false; // View implementation might have changed.
            }
            if (mAccessibilityDelegateField == null) {
                try {
                    mAccessibilityDelegateField = View.class
                            .getDeclaredField("mAccessibilityDelegate");
                    mAccessibilityDelegateField.setAccessible(true);
                } catch (Throwable t) {
                    accessibilityDelegateCheckFailed = true;
                    return false;
                }
            }
            try {
                return mAccessibilityDelegateField.get(v) != null;
            } catch (Throwable t) {
                accessibilityDelegateCheckFailed = true;
                return false;
            }
        }

        @Override
        public ViewPropertyAnimatorCompat animate(View view) {
            if (mViewPropertyAnimatorCompatMap == null) {
                mViewPropertyAnimatorCompatMap =
                        new WeakHashMap();
            }
            ViewPropertyAnimatorCompat vpa = mViewPropertyAnimatorCompatMap.get(view);
            if (vpa == null) {
                vpa = new ViewPropertyAnimatorCompat(view);
                mViewPropertyAnimatorCompatMap.put(view, vpa);
            }
            return vpa;
        }

        @Override
        public void setFitsSystemWindows(View view, boolean fitSystemWindows) {
            ViewCompatICS.setFitsSystemWindows(view, fitSystemWindows);
        }
    }

    static class ICSMr1ViewCompatImpl extends ICSViewCompatImpl {
        @Override
        public boolean hasOnClickListeners(View view) {
            return ViewCompatICSMr1.hasOnClickListeners(view);
        }
    }

    static class JBViewCompatImpl extends ICSMr1ViewCompatImpl {
        @Override
        public boolean hasTransientState(View view) {
            return ViewCompatJB.hasTransientState(view);
        }
        @Override
        public void setHasTransientState(View view, boolean hasTransientState) {
            ViewCompatJB.setHasTransientState(view, hasTransientState);
        }
        @Override
        public void postInvalidateOnAnimation(View view) {
            ViewCompatJB.postInvalidateOnAnimation(view);
        }
        @Override
        public void postInvalidateOnAnimation(View view, int left, int top, int right, int bottom) {
            ViewCompatJB.postInvalidateOnAnimation(view, left, top, right, bottom);
        }
        @Override
        public void postOnAnimation(View view, Runnable action) {
            ViewCompatJB.postOnAnimation(view, action);
        }
        @Override
        public void postOnAnimationDelayed(View view, Runnable action, long delayMillis) {
            ViewCompatJB.postOnAnimationDelayed(view, action, delayMillis);
        }
        @Override
        public int getImportantForAccessibility(View view) {
            return ViewCompatJB.getImportantForAccessibility(view);
        }
        @Override
        public void setImportantForAccessibility(View view, int mode) {
            // IMPORTANT_FOR_ACCESSIBILITY_NO_HIDE_DESCENDANTS is not available
            // on this platform so replace with IMPORTANT_FOR_ACCESSIBILITY_NO
            // which is closer semantically.
            if (mode == IMPORTANT_FOR_ACCESSIBILITY_NO_HIDE_DESCENDANTS) {
                mode = IMPORTANT_FOR_ACCESSIBILITY_NO;
            }
            ViewCompatJB.setImportantForAccessibility(view, mode);
        }
        @Override
        public boolean performAccessibilityAction(View view, int action, Bundle arguments) {
            return ViewCompatJB.performAccessibilityAction(view, action, arguments);
        }
        @Override
        public AccessibilityNodeProviderCompat getAccessibilityNodeProvider(View view) {
            Object compat = ViewCompatJB.getAccessibilityNodeProvider(view);
            if (compat != null) {
                return new AccessibilityNodeProviderCompat(compat);
            }
            return null;
        }

        @Override
        public ViewParent getParentForAccessibility(View view) {
            return ViewCompatJB.getParentForAccessibility(view);
        }

        @Override
        public int getMinimumWidth(View view) {
            return ViewCompatJB.getMinimumWidth(view);
        }

        @Override
        public int getMinimumHeight(View view) {
            return ViewCompatJB.getMinimumHeight(view);
        }

        @Override
        public void requestApplyInsets(View view) {
            ViewCompatJB.requestApplyInsets(view);
        }

        @Override
        public boolean getFitsSystemWindows(View view) {
            return ViewCompatJB.getFitsSystemWindows(view);
        }

        @Override
        public boolean hasOverlappingRendering(View view) {
            return ViewCompatJB.hasOverlappingRendering(view);
        }
    }

    static class JbMr1ViewCompatImpl extends JBViewCompatImpl {

        @Override
        public int getLabelFor(View view) {
            return ViewCompatJellybeanMr1.getLabelFor(view);
        }

        @Override
        public void setLabelFor(View view, int id) {
            ViewCompatJellybeanMr1.setLabelFor(view, id);
        }

        @Override
        public void setLayerPaint(View view, Paint paint) {
            ViewCompatJellybeanMr1.setLayerPaint(view, paint);
        }

        @Override
        public int getLayoutDirection(View view) {
            return ViewCompatJellybeanMr1.getLayoutDirection(view);
        }

        @Override
        public void setLayoutDirection(View view, int layoutDirection) {
            ViewCompatJellybeanMr1.setLayoutDirection(view, layoutDirection);
        }

        @Override
        public int getPaddingStart(View view) {
            return ViewCompatJellybeanMr1.getPaddingStart(view);
        }

        @Override
        public int getPaddingEnd(View view) {
            return ViewCompatJellybeanMr1.getPaddingEnd(view);
        }

        @Override
        public void setPaddingRelative(View view, int start, int top, int end, int bottom) {
            ViewCompatJellybeanMr1.setPaddingRelative(view, start, top, end, bottom);
        }

        @Override
        public int getWindowSystemUiVisibility(View view) {
            return ViewCompatJellybeanMr1.getWindowSystemUiVisibility(view);
        }

        @Override
        public boolean isPaddingRelative(View view) {
            return ViewCompatJellybeanMr1.isPaddingRelative(view);
        }
    }

    static class JbMr2ViewCompatImpl extends JbMr1ViewCompatImpl {
        @Override
        public void setClipBounds(View view, Rect clipBounds) {
            ViewCompatJellybeanMr2.setClipBounds(view, clipBounds);
        }

        @Override
        public Rect getClipBounds(View view) {
            return ViewCompatJellybeanMr2.getClipBounds(view);
        }
    }

    static class KitKatViewCompatImpl extends JbMr2ViewCompatImpl {
        @Override
        public int getAccessibilityLiveRegion(View view) {
            return ViewCompatKitKat.getAccessibilityLiveRegion(view);
        }

        @Override
        public void setAccessibilityLiveRegion(View view, int mode) {
            ViewCompatKitKat.setAccessibilityLiveRegion(view, mode);
        }

        @Override
        public void setImportantForAccessibility(View view, int mode) {
            ViewCompatJB.setImportantForAccessibility(view, mode);
        }

        @Override
        public boolean isLaidOut(View view) {
            return ViewCompatKitKat.isLaidOut(view);
        }

        @Override
        public boolean isAttachedToWindow(View view) {
            return ViewCompatKitKat.isAttachedToWindow(view);
        }
    }

    static class LollipopViewCompatImpl extends KitKatViewCompatImpl {
        @Override
        public void setTransitionName(View view, String transitionName) {
            ViewCompatLollipop.setTransitionName(view, transitionName);
        }

        @Override
        public String getTransitionName(View view) {
            return ViewCompatLollipop.getTransitionName(view);
        }

        @Override
        public void requestApplyInsets(View view) {
            ViewCompatLollipop.requestApplyInsets(view);
        }

        @Override
        public void setElevation(View view, float elevation) {
            ViewCompatLollipop.setElevation(view, elevation);
        }

        @Override
        public float getElevation(View view) {
            return ViewCompatLollipop.getElevation(view);
        }

        @Override
        public void setTranslationZ(View view, float translationZ) {
            ViewCompatLollipop.setTranslationZ(view, translationZ);
        }

        @Override
        public float getTranslationZ(View view) {
            return ViewCompatLollipop.getTranslationZ(view);
        }

        @Override
        public void setOnApplyWindowInsetsListener(View view, OnApplyWindowInsetsListener listener) {
            ViewCompatLollipop.setOnApplyWindowInsetsListener(view, listener);
        }

        @Override
        public void setNestedScrollingEnabled(View view, boolean enabled) {
            ViewCompatLollipop.setNestedScrollingEnabled(view, enabled);
        }

        @Override
        public boolean isNestedScrollingEnabled(View view) {
            return ViewCompatLollipop.isNestedScrollingEnabled(view);
        }

        @Override
        public boolean startNestedScroll(View view, int axes) {
            return ViewCompatLollipop.startNestedScroll(view, axes);
        }

        @Override
        public void stopNestedScroll(View view) {
            ViewCompatLollipop.stopNestedScroll(view);
        }

        @Override
        public boolean hasNestedScrollingParent(View view) {
            return ViewCompatLollipop.hasNestedScrollingParent(view);
        }

        @Override
        public boolean dispatchNestedScroll(View view, int dxConsumed, int dyConsumed,
                int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow) {
            return ViewCompatLollipop.dispatchNestedScroll(view, dxConsumed, dyConsumed,
                    dxUnconsumed, dyUnconsumed, offsetInWindow);
        }

        @Override
        public boolean dispatchNestedPreScroll(View view, int dx, int dy,
                int[] consumed, int[] offsetInWindow) {
            return ViewCompatLollipop.dispatchNestedPreScroll(view, dx, dy, consumed,
                    offsetInWindow);
        }

        @Override
        public boolean dispatchNestedFling(View view, float velocityX, float velocityY,
                boolean consumed) {
            return ViewCompatLollipop.dispatchNestedFling(view, velocityX, velocityY, consumed);
        }

        @Override
        public boolean dispatchNestedPreFling(View view, float velocityX, float velocityY) {
            return ViewCompatLollipop.dispatchNestedPreFling(view, velocityX, velocityY);
        }

        @Override
        public boolean isImportantForAccessibility(View view) {
            return ViewCompatLollipop.isImportantForAccessibility(view);
        }

        @Override
        public ColorStateList getBackgroundTintList(View view) {
            return ViewCompatLollipop.getBackgroundTintList(view);
        }

        @Override
        public void setBackgroundTintList(View view, ColorStateList tintList) {
            ViewCompatLollipop.setBackgroundTintList(view, tintList);
        }

        @Override
        public void setBackgroundTintMode(View view, PorterDuff.Mode mode) {
            ViewCompatLollipop.setBackgroundTintMode(view, mode);
        }

        @Override
        public PorterDuff.Mode getBackgroundTintMode(View view) {
            return ViewCompatLollipop.getBackgroundTintMode(view);
        }

        @Override
        public WindowInsetsCompat onApplyWindowInsets(View v, WindowInsetsCompat insets) {
            return ViewCompatLollipop.onApplyWindowInsets(v, insets);
        }

        @Override
        public WindowInsetsCompat dispatchApplyWindowInsets(View v, WindowInsetsCompat insets) {
            return ViewCompatLollipop.dispatchApplyWindowInsets(v, insets);
        }

        @Override
        public float getZ(View view) {
            return ViewCompatLollipop.getZ(view);
        }
    }

    static class MarshmallowViewCompatImpl extends LollipopViewCompatImpl {
        @Override
        public void setScrollIndicators(View view, int indicators) {
            ViewCompatMarshmallow.setScrollIndicators(view, indicators);
        }

        @Override
        public void setScrollIndicators(View view, int indicators, int mask) {
            ViewCompatMarshmallow.setScrollIndicators(view, indicators, mask);
        }

        @Override
        public int getScrollIndicators(View view) {
            return ViewCompatMarshmallow.getScrollIndicators(view);
        }
    }

    static final ViewCompatImpl IMPL;
    static {
        final int version = android.os.Build.VERSION.SDK_INT;
        if (version >= 23) {
            IMPL = new MarshmallowViewCompatImpl();
        } else if (version >= 21) {
            IMPL = new LollipopViewCompatImpl();
        } else if (version >= 19) {
            IMPL = new KitKatViewCompatImpl();
        } else if (version >= 17) {
            IMPL = new JbMr1ViewCompatImpl();
        } else if (version >= 16) {
            IMPL = new JBViewCompatImpl();
        } else if (version >= 15) {
            IMPL = new ICSMr1ViewCompatImpl();
        } else if (version >= 14) {
            IMPL = new ICSViewCompatImpl();
        } else if (version >= 11) {
            IMPL = new HCViewCompatImpl();
        } else if (version >= 9) {
            IMPL = new GBViewCompatImpl();
        } else if (version >= 7) {
            IMPL = new EclairMr1ViewCompatImpl();
        } else {
            IMPL = new BaseViewCompatImpl();
        }
    }

    /**
     * Check if this view can be scrolled horizontally in a certain direction.
     *
     * @param v The View against which to invoke the method.
     * @param direction Negative to check scrolling left, positive to check scrolling right.
     * @return true if this view can be scrolled in the specified direction, false otherwise.
     */
    public static boolean canScrollHorizontally(View v, int direction) {
        return IMPL.canScrollHorizontally(v, direction);
    }

    /**
     * Check if this view can be scrolled vertically in a certain direction.
     *
     * @param v The View against which to invoke the method.
     * @param direction Negative to check scrolling up, positive to check scrolling down.
     * @return true if this view can be scrolled in the specified direction, false otherwise.
     */
    public static boolean canScrollVertically(View v, int direction) {
        return IMPL.canScrollVertically(v, direction);
    }

    /**
     * Returns the over-scroll mode for this view. The result will be
     * one of {@link #OVER_SCROLL_ALWAYS} (default), {@link #OVER_SCROLL_IF_CONTENT_SCROLLS}
     * (allow over-scrolling only if the view content is larger than the container),
     * or {@link #OVER_SCROLL_NEVER}.
     *
     * @param v The View against which to invoke the method.
     * @return This view's over-scroll mode.
     */
    @OverScroll
    public static int getOverScrollMode(View v) {
        return IMPL.getOverScrollMode(v);
    }

    //略，下面方法类似，都是interface ViewCompatImpl的实现
}

注意到了没，此类除了interface ViewCompatImpl及其中的方法实现外，还有以下几个内部类，注意各个内部类继承的类：

static class BaseViewCompatImpl implements ViewCompatImpl
static class EclairMr1ViewCompatImpl extends BaseViewCompatImpl
static class GBViewCompatImpl extends EclairMr1ViewCompatImpl 
static class HCViewCompatImpl extends GBViewCompatImpl
static class ICSViewCompatImpl extends HCViewCompatImpl
static class ICSMr1ViewCompatImpl extends ICSViewCompatImpl
static class JBViewCompatImpl extends ICSMr1ViewCompatImpl
static class JbMr1ViewCompatImpl extends JBViewCompatImpl
static class JbMr2ViewCompatImpl extends JbMr1ViewCompatImpl
static class KitKatViewCompatImpl extends JbMr2ViewCompatImpl
static class LollipopViewCompatImpl extends KitKatViewCompatImpl
static class MarshmallowViewCompatImpl extends LollipopViewCompatImpl

另外，还有一个初始化值为选择的常量

static final ViewCompatImpl IMPL;
    static {
        final int version = android.os.Build.VERSION.SDK_INT;
        if (version >= 23) {
            IMPL = new MarshmallowViewCompatImpl();
        } else if (version >= 21) {
            IMPL = new LollipopViewCompatImpl();
        } else if (version >= 19) {
            IMPL = new KitKatViewCompatImpl();
        } else if (version >= 17) {
            IMPL = new JbMr1ViewCompatImpl();
        } else if (version >= 16) {
            IMPL = new JBViewCompatImpl();
        } else if (version >= 15) {
            IMPL = new ICSMr1ViewCompatImpl();
        } else if (version >= 14) {
            IMPL = new ICSViewCompatImpl();
        } else if (version >= 11) {
            IMPL = new HCViewCompatImpl();
        } else if (version >= 9) {
            IMPL = new GBViewCompatImpl();
        } else if (version >= 7) {
            IMPL = new EclairMr1ViewCompatImpl();
        } else {
            IMPL = new BaseViewCompatImpl();
        }
    }

另外，ViewCompat的方法的结构都类似于：

@OverScroll
    public static int getOverScrollMode(View v) {
        return IMPL.getOverScrollMode(v);
    }

类结构分析，ViewCompat先是在接口ViewCompatImpl中列出由于版本原因不兼容的方法，并且定义一个常量，初始值if得到的API版本，然后在本类中实现此方法，最后是内部类层层继承。

ViewCompat通过API判断，封装了版本不兼容的方法在不同版本上的实现。值得注意的是，它仅仅让你调用此方法的app不崩溃, 并不保证你调用的结果在不同版本的机器上一致.
使用：通过ViewCompat调用其方法就可以了。
另外，此类的代码组织方式也很值得我们借鉴。

如果还有不明白的可以参考：
https://www.zhihu.com/question/27775923/answer/65086423