NestedScrollView滚动到顶部固定子View悬停挂靠粘在顶端

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网上有一个StickyScrollView，称之为粘性ScrollView，比如一个垂直方向的布局，依次摆放几个子View，当某一个子View滚到到顶端时候要停靠在顶部，悬停在顶部的位置不动。这很像联系人的pinned，假设联系人按照Z分组后，若滚动到顶部，Z就悬停靠在顶端不动，下面到的联系人继续滚到。当用户下拉时候，又恢复正常。

StickyScrollView原先是继承自ScrollView，然而现在ScrollView已经被新型的NestedScrollView逐渐取代了，现在我把它改写成继承自NestedScrollView后，整理出来：

package zhangphil.test;
 
import java.util.ArrayList;
 
import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.drawable.Drawable;
import android.os.Build;
import android.support.v4.widget.NestedScrollView;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
import android.view.ViewGroup;
import android.view.animation.AlphaAnimation;
 
public class StickyScrollView extends NestedScrollView {
 
    /**
     * Tag for views that should stick and have constant drawing. e.g. TextViews, ImageViews etc
     */
    public static final String STICKY_TAG = "sticky";
 
    /**
     * Flag for views that should stick and have non-constant drawing. e.g. Buttons, ProgressBars etc
     */
    public static final String FLAG_NONCONSTANT = "-nonconstant";
 
    /**
     * Flag for views that have aren't fully opaque
     */
    public static final String FLAG_HASTRANSPARANCY = "-hastransparancy";
 
    /**
     * Default height of the shadow peeking out below the stuck view.
     */
    private static final int DEFAULT_SHADOW_HEIGHT = 10; // dp;
 
    private ArrayList stickyViews;
    private View currentlyStickingView;
    private float stickyViewTopOffset;
    private int stickyViewLeftOffset;
    private boolean redirectTouchesToStickyView;
    private boolean clippingToPadding;
    private boolean clipToPaddingHasBeenSet;
 
    private int mShadowHeight;
    private Drawable mShadowDrawable;
 
    private final Runnable invalidateRunnable = new Runnable() {
 
        @Override
        public void run() {
            if (currentlyStickingView != null) {
                int l = getLeftForViewRelativeOnlyChild(currentlyStickingView);
                int t = getBottomForViewRelativeOnlyChild(currentlyStickingView);
                int r = getRightForViewRelativeOnlyChild(currentlyStickingView);
                int b = (int) (getScrollY() + (currentlyStickingView.getHeight() + stickyViewTopOffset));
                invalidate(l, t, r, b);
            }
            postDelayed(this, 16);
        }
    };
 
    public StickyScrollView(Context context) {
        this(context, null);
    }
 
    public StickyScrollView(Context context, AttributeSet attrs) {
        this(context, attrs, android.R.attr.scrollViewStyle);
    }
 
    public StickyScrollView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        setup();
 
        TypedArray a = context.obtainStyledAttributes(attrs,
                R.styleable.StickyScrollView, defStyle, 0);
 
        final float density = context.getResources().getDisplayMetrics().density;
        int defaultShadowHeightInPix = (int) (DEFAULT_SHADOW_HEIGHT * density + 0.5f);
 
        mShadowHeight = a.getDimensionPixelSize(
                R.styleable.StickyScrollView_stuckShadowHeight,
                defaultShadowHeightInPix);
 
        int shadowDrawableRes = a.getResourceId(
                R.styleable.StickyScrollView_stuckShadowDrawable, -1);
 
        if (shadowDrawableRes != -1) {
            mShadowDrawable = context.getResources().getDrawable(
                    shadowDrawableRes);
        }
 
        a.recycle();
    }
 
    /**
     * Sets the height of the shadow drawable in pixels.
     *
     * @param height
     */
    public void setShadowHeight(int height) {
        mShadowHeight = height;
    }
 
 
    public void setup() {
        stickyViews = new ArrayList();
    }
 
    private int getLeftForViewRelativeOnlyChild(View v) {
        int left = v.getLeft();
        while (v.getParent() != getChildAt(0)) {
            v = (View) v.getParent();
            left += v.getLeft();
        }
        return left;
    }
 
    private int getTopForViewRelativeOnlyChild(View v) {
        int top = v.getTop();
        while (v.getParent() != getChildAt(0)) {
            v = (View) v.getParent();
            top += v.getTop();
        }
        return top;
    }
 
    private int getRightForViewRelativeOnlyChild(View v) {
        int right = v.getRight();
        while (v.getParent() != getChildAt(0)) {
            v = (View) v.getParent();
            right += v.getRight();
        }
        return right;
    }
 
    private int getBottomForViewRelativeOnlyChild(View v) {
        int bottom = v.getBottom();
        while (v.getParent() != getChildAt(0)) {
            v = (View) v.getParent();
            bottom += v.getBottom();
        }
        return bottom;
    }
 
    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        super.onLayout(changed, l, t, r, b);
        if (!clipToPaddingHasBeenSet) {
            clippingToPadding = true;
        }
        notifyHierarchyChanged();
    }
 
    @Override
    public void setClipToPadding(boolean clipToPadding) {
        super.setClipToPadding(clipToPadding);
        clippingToPadding = clipToPadding;
        clipToPaddingHasBeenSet = true;
    }
 
    @Override
    public void addView(View child) {
        super.addView(child);
        findStickyViews(child);
    }
 
    @Override
    public void addView(View child, int index) {
        super.addView(child, index);
        findStickyViews(child);
    }
 
    @Override
    public void addView(View child, int index, android.view.ViewGroup.LayoutParams params) {
        super.addView(child, index, params);
        findStickyViews(child);
    }
 
    @Override
    public void addView(View child, int width, int height) {
        super.addView(child, width, height);
        findStickyViews(child);
    }
 
    @Override
    public void addView(View child, android.view.ViewGroup.LayoutParams params) {
        super.addView(child, params);
        findStickyViews(child);
    }
 
    @Override
    protected void dispatchDraw(Canvas canvas) {
        super.dispatchDraw(canvas);
        if (currentlyStickingView != null) {
            canvas.save();
            canvas.translate(getPaddingLeft() + stickyViewLeftOffset, getScrollY() + stickyViewTopOffset + (clippingToPadding ? getPaddingTop() : 0));
 
            canvas.clipRect(0, (clippingToPadding ? -stickyViewTopOffset : 0),
                    getWidth() - stickyViewLeftOffset,
                    currentlyStickingView.getHeight() + mShadowHeight + 1);
 
            if (mShadowDrawable != null) {
                int left = 0;
                int right = currentlyStickingView.getWidth();
                int top = currentlyStickingView.getHeight();
                int bottom = currentlyStickingView.getHeight() + mShadowHeight;
                mShadowDrawable.setBounds(left, top, right, bottom);
                mShadowDrawable.draw(canvas);
            }
 
            canvas.clipRect(0, (clippingToPadding ? -stickyViewTopOffset : 0), getWidth(), currentlyStickingView.getHeight());
            if (getStringTagForView(currentlyStickingView).contains(FLAG_HASTRANSPARANCY)) {
                showView(currentlyStickingView);
                currentlyStickingView.draw(canvas);
                hideView(currentlyStickingView);
            } else {
                currentlyStickingView.draw(canvas);
            }
            canvas.restore();
        }
    }
 
    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            redirectTouchesToStickyView = true;
        }
 
        if (redirectTouchesToStickyView) {
            redirectTouchesToStickyView = currentlyStickingView != null;
            if (redirectTouchesToStickyView) {
                redirectTouchesToStickyView =
                        ev.getY() <= (currentlyStickingView.getHeight() + stickyViewTopOffset) &&
                                ev.getX() >= getLeftForViewRelativeOnlyChild(currentlyStickingView) &&
                                ev.getX() <= getRightForViewRelativeOnlyChild(currentlyStickingView);
            }
        } else if (currentlyStickingView == null) {
            redirectTouchesToStickyView = false;
        }
        if (redirectTouchesToStickyView) {
            ev.offsetLocation(0, -1 * ((getScrollY() + stickyViewTopOffset) - getTopForViewRelativeOnlyChild(currentlyStickingView)));
        }
        return super.dispatchTouchEvent(ev);
    }
 
    private boolean hasNotDoneActionDown = true;
 
    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        if (redirectTouchesToStickyView) {
            ev.offsetLocation(0, ((getScrollY() + stickyViewTopOffset) - getTopForViewRelativeOnlyChild(currentlyStickingView)));
        }
 
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            hasNotDoneActionDown = false;
        }
 
        if (hasNotDoneActionDown) {
            MotionEvent down = MotionEvent.obtain(ev);
            down.setAction(MotionEvent.ACTION_DOWN);
            super.onTouchEvent(down);
            hasNotDoneActionDown = false;
        }
 
        if (ev.getAction() == MotionEvent.ACTION_UP || ev.getAction() == MotionEvent.ACTION_CANCEL) {
            hasNotDoneActionDown = true;
        }
 
        return super.onTouchEvent(ev);
    }
 
    @Override
    protected void onScrollChanged(int l, int t, int oldl, int oldt) {
        super.onScrollChanged(l, t, oldl, oldt);
        doTheStickyThing();
    }
 
    private void doTheStickyThing() {
        View viewThatShouldStick = null;
        View approachingView = null;
        for (View v : stickyViews) {
            int viewTop = getTopForViewRelativeOnlyChild(v) - getScrollY() + (clippingToPadding ? 0 : getPaddingTop());
            if (viewTop <= 0) {
                if (viewThatShouldStick == null || viewTop > (getTopForViewRelativeOnlyChild(viewThatShouldStick) - getScrollY() + (clippingToPadding ? 0 : getPaddingTop()))) {
                    viewThatShouldStick = v;
                }
            } else {
                if (approachingView == null || viewTop < (getTopForViewRelativeOnlyChild(approachingView) - getScrollY() + (clippingToPadding ? 0 : getPaddingTop()))) {
                    approachingView = v;
                }
            }
        }
        if (viewThatShouldStick != null) {
            stickyViewTopOffset = approachingView == null ? 0 : Math.min(0, getTopForViewRelativeOnlyChild(approachingView) - getScrollY() + (clippingToPadding ? 0 : getPaddingTop()) - viewThatShouldStick.getHeight());
            if (viewThatShouldStick != currentlyStickingView) {
                if (currentlyStickingView != null) {
                    stopStickingCurrentlyStickingView();
                }
                // only compute the left offset when we start sticking.
                stickyViewLeftOffset = getLeftForViewRelativeOnlyChild(viewThatShouldStick);
                startStickingView(viewThatShouldStick);
            }
        } else if (currentlyStickingView != null) {
            stopStickingCurrentlyStickingView();
        }
    }
 
    private void startStickingView(View viewThatShouldStick) {
        currentlyStickingView = viewThatShouldStick;
        if (getStringTagForView(currentlyStickingView).contains(FLAG_HASTRANSPARANCY)) {
            hideView(currentlyStickingView);
        }
        if (((String) currentlyStickingView.getTag()).contains(FLAG_NONCONSTANT)) {
            post(invalidateRunnable);
        }
    }
 
    private void stopStickingCurrentlyStickingView() {
        if (getStringTagForView(currentlyStickingView).contains(FLAG_HASTRANSPARANCY)) {
            showView(currentlyStickingView);
        }
        currentlyStickingView = null;
        removeCallbacks(invalidateRunnable);
    }
 
    /**
     * Notify that the sticky attribute has been added or removed from one or more views in the View hierarchy
     */
    public void notifyStickyAttributeChanged() {
        notifyHierarchyChanged();
    }
 
    private void notifyHierarchyChanged() {
        if (currentlyStickingView != null) {
            stopStickingCurrentlyStickingView();
        }
        stickyViews.clear();
        findStickyViews(getChildAt(0));
        doTheStickyThing();
        invalidate();
    }
 
    private void findStickyViews(View v) {
        if (v instanceof ViewGroup) {
            ViewGroup vg = (ViewGroup) v;
            for (int i = 0; i < vg.getChildCount(); i++) {
                String tag = getStringTagForView(vg.getChildAt(i));
                if (tag != null && tag.contains(STICKY_TAG)) {
                    stickyViews.add(vg.getChildAt(i));
                } else if (vg.getChildAt(i) instanceof ViewGroup) {
                    findStickyViews(vg.getChildAt(i));
                }
            }
        } else {
            String tag = (String) v.getTag();
            if (tag != null && tag.contains(STICKY_TAG)) {
                stickyViews.add(v);
            }
        }
    }
 
    private String getStringTagForView(View v) {
        Object tagObject = v.getTag();
        return String.valueOf(tagObject);
    }
 
    private void hideView(View v) {
        if (Build.VERSION.SDK_INT >= 11) {
            v.setAlpha(0);
        } else {
            AlphaAnimation anim = new AlphaAnimation(1, 0);
            anim.setDuration(0);
            anim.setFillAfter(true);
            v.startAnimation(anim);
        }
    }
 
    private void showView(View v) {
        if (Build.VERSION.SDK_INT >= 11) {
            v.setAlpha(1);
        } else {
            AlphaAnimation anim = new AlphaAnimation(0, 1);
            anim.setDuration(0);
            anim.setFillAfter(true);
            v.startAnimation(anim);
        }
    }
}

同时需要在res/values/attr.xml加入定义的属性：

使用起来很简单，在xml布局中把某一个子View增加一个属性：

android:tag="sticky"

即可实现该子View在NestedScrollView中，当滚动到顶部后悬停挂靠在顶部不动，直到下一个粘性子View滚动上来把它顶飞，或者用户下拉，拉出上面的View。如xml代码：

ViewPager与NestedScrollView套嵌会引起冲突，滑动不了的问题
https://blog.csdn.net/wjr1949/article/details/72479583?utm_source=itdadao&utm_medium=referral