View绘制三大流程源码分析
在上篇博文DecorView绘制流程源码分析中，关于DecorView作为Activity、Window中的顶级View的绘制，我们已经作了一个详细的分析。但在具体说到View的绘制的时候，我们没有详细说明，所以本篇博文将会对View的绘制原理作深度分析。
在开始分析之前，我们需要了解一些概念，如：
- View：是所有UI组件的基类,是Android平台中用户界面体现的基础单位。
- ViewGroup:是容纳UI组件的容器,它本身也是View的子类。
- ViewRootImpl:是View的绘制的辅助类，所有View的绘制都离不开ViewRootImpl。
- MeasureSpec： View的内部类，主要就是View测量模式的工具类
一、View绘制三大流程分析
在DecorView的具体绘制中，我们涉及了View绘制的三大流程，具体分别为measure(测量)、layout(布局)和draw(绘制)。下面我们就来一一分析：
1.performMeasure(测量)
我们知道ViewRootImpl是View绘制的辅助类，View的绘制都是在ViewRootImpl的帮助下完成的，所以要了解View的measure(测量)，我们就必须看看ViewRootImpl中的performMeasure()方法
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
try {
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}在DecorView绘制流程源码分析中，我们知道mView就是DecorView,而DecorView继承于FrameLayout,FrameLayout又继承于ViewGroup，ViewGroup又继承于View，根据他们之间的关系，我们知道，mView.measure()是调用了父类View的measure()方法(因为只有View有measure方法)，所以来分析View的measure()方法
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
if ((mPrivateFlags & FORCE_LAYOUT) == FORCE_LAYOUT ||
widthMeasureSpec != mOldWidthMeasureSpec ||
........
onMeasure(widthMeasureSpec, heightMeasureSpec);//核心方法
........
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
}此onMeasure()方法，在DecorView，Framelayout和View中都有定义，并且DecorView和FrameLayout重载了此方法，根据调用关系，这里调用了DecorView的onMeasure(),我们来看看此方法
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
final DisplayMetrics metrics = getContext().getResources().getDisplayMetrics();
final boolean isPortrait = metrics.widthPixels < metrics.heightPixels;
final int widthMode = getMode(widthMeasureSpec);
final int heightMode = getMode(heightMeasureSpec);
......
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
......
}关于测量模式，这里我们先不说，后面我们会说到，这里调用了父类FrameLayout中的onMeasure()方法，这里我们来看一下源码
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
........//计算top,left,bottom,right的margin值，从而确定FrameLayout的宽高
//设置宽高
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
count = mMatchParentChildren.size();
if (count > 1) {
for (int i = 0; i < count; i++) {
final View child = mMatchParentChildren.get(i);//获取子View
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();//子View配置参数
int childWidthMeasureSpec;
int childHeightMeasureSpec;
if (lp.width == LayoutParams.MATCH_PARENT) {
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth() -
getPaddingLeftWithForeground() - getPaddingRightWithForeground() -
lp.leftMargin - lp.rightMargin,
MeasureSpec.EXACTLY);
} else {
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
lp.leftMargin + lp.rightMargin,
lp.width);
}
if (lp.height == LayoutParams.MATCH_PARENT) {
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredHeight() -
getPaddingTopWithForeground() - getPaddingBottomWithForeground() -
lp.topMargin - lp.bottomMargin,
MeasureSpec.EXACTLY);
} else {
childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
lp.topMargin + lp.bottomMargin,
lp.height);
}
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}由于FrameLayout是一个容器，可以装载其他的View，所以这里需要进行遍历其中的子View，并一一进行measure(测量)。
在具体说此方法前，我们先要了解一下View的测量模式及MeasureSpec类，具体我们先来看看MeasureSpec类的源码
public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
/**
* Measure specification mode: The parent has not imposed any constraint
* on the child. It can be whatever size it wants.
* 父View不对子View有任何限制，子View需要多大就多大
*/
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
/**
* Measure specification mode: The parent has determined an exact size
* for the child. The child is going to be given those bounds regardless
* of how big it wants to be.
* 父View已经测量出子View所需要的精确大小，这时候View的最终大小就是SpecSize所指定的值。对应于match_parent和精确数* 值这两种模式
*/
public static final int EXACTLY = 1 << MODE_SHIFT;
/**
* Measure specification mode: The child can be as large as it wants up
* to the specified size.
* 子View的最终大小是父View指定的SpecSize值，并且子View的大小不能大于这个值，即对应wrap_content这种模式。
*/
public static final int AT_MOST = 2 << MODE_SHIFT;
/**
* 用实际值和测量模式组装成measureSpec测量规格
* 将size和mode打包成一个32位的int型数值
* 高2位表示SpecMode，测量模式，低30位表示SpecSize，某种测量模式下的规格大小
*/
public static int makeMeasureSpec(int size, int mode) {
return size + mode;
}
//将32位的MeasureSpec解包，返回SpecMode,测量模式(EXACTLY、UNSPECIFIED或AT_MOST）
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK);
}
//将32位的MeasureSpec解包，返回SpecSize
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
}这里主要通过位运算，来实现View的三种测量模式UNSPECIFIED、EXACTLY和AT_MOST。相关定义如下：
- UNSPECIFIED：父View不对子View有任何限制，子View需要多大就多大
- EXACTLY：父View已经测量出子View所需要的精确大小，这时候View的最终大小就是SpecSize所指定的值。对应于match_parent和精确数值这两种模式
- AT_MOST： 子View的最终大小是父View指定的SpecSize值，并且子View的大小不能大于这个值，即对应wrap_content这种模式。
我们继续上面FrameLayout的onMeasure()方法继续分析，可以发现，此方法主要就是组装子View宽高的测量规格MeasureSpec，然后作为参数传给子View的measure()方法。这里我们只来看一个组装就好，我们来看宽的组装的测量规格MeasureSpec，我们来看看相关代码
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
........
final View child = mMatchParentChildren.get(i);//获取子View
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();//子View配置参数
int childWidthMeasureSpec;
int childHeightMeasureSpec;
if (lp.width == LayoutParams.MATCH_PARENT) {
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth() -
getPaddingLeftWithForeground() - getPaddingRightWithForeground() -
lp.leftMargin - lp.rightMargin,
MeasureSpec.EXACTLY);
} else {
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
lp.leftMargin + lp.rightMargin,
lp.width);
}
........
}当子View的布局参数lp.width为LayoutParams.MATCH_PARENT时，生成后的测量规格MeasureSpec是以测量模式为MeasureSpec.EXACTLY的值。当lp.width不为LayoutParams.MATCH_PARENT时，这是调用了ViewGroup中的getChildMeasureSpec()方法，来生成相关值，这里我们来看此方法
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size, but our size is not fixed.
// Constrain child to not be bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent asked to see how big we want to be
case MeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {
// Child wants a specific size... let him have it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
resultSize = 0;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = 0;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}这里主要传入了FrameLayout的测量规格MeasureSpec，根据FrameLayout的测量规格和子View具体的padding和childDimension值，从而决定子view宽的测量规格MeasureSpec。具体创建细节，这里就不说。回到FrameLayout的onMeasure()方法，这样当子View的宽高的测量规格都计算出来之后，就会调用子View的measure()方法。如果子View不再是ViewGroup，那样就会调用子View(或自定义View)的onMeasure()方法，从而完成View的测量；如果子View还是ViewGroup，那就会按我们说的逻辑再走一遍measure方法。
2.performLayout(布局)
说完View绘制的measure(测量)，我们来看看View绘制的layout(布局)。同样的，我们先来看ViewRootImpl中的performLayout()方法
private void performLayout() {
mLayoutRequested = false;
mScrollMayChange = true;
final View host = mView;
if (DEBUG_ORIENTATION || DEBUG_LAYOUT) {
Log.v(TAG, "Laying out " + host + " to (" +
host.getMeasuredWidth() + ", " + host.getMeasuredHeight() + ")");
}
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout");
try {
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}由相关继承类的关系，我们知道，这里调用的是View的layout()方法
public void layout(int l, int t, int r, int b) {
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
boolean changed = setFrame(l, t, r, b);
if (changed || (mPrivateFlags & LAYOUT_REQUIRED) == LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);//核心方法
mPrivateFlags &= ~LAYOUT_REQUIRED;
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLayoutChangeListeners != null) {
ArrayList listenersCopy =
(ArrayList)li.mOnLayoutChangeListeners.clone();
int numListeners = listenersCopy.size();
for (int i = 0; i < numListeners; ++i) {
listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
}
}
}
mPrivateFlags &= ~FORCE_LAYOUT;
} 通过源码我们知道，ViewGroup是一个抽象的View的子类，而FrameLayout是ViewGroup的实现类，所以这里onLayout()是FrameLayout中的方法，我们来看一下此方法
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
final int count = getChildCount();
final int parentLeft = getPaddingLeftWithForeground();
final int parentRight = right - left - getPaddingRightWithForeground();
final int parentTop = getPaddingTopWithForeground();
final int parentBottom = bottom - top - getPaddingBottomWithForeground();
mForegroundBoundsChanged = true;
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final int width = child.getMeasuredWidth();
final int height = child.getMeasuredHeight();
int childLeft;
int childTop;
int gravity = lp.gravity;
if (gravity == -1) {
gravity = DEFAULT_CHILD_GRAVITY;
}
final int layoutDirection = getResolvedLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.LEFT:
childLeft = parentLeft + lp.leftMargin;
break;
case Gravity.CENTER_HORIZONTAL:
childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
childLeft = parentRight - width - lp.rightMargin;
break;
default:
childLeft = parentLeft + lp.leftMargin;
}
switch (verticalGravity) {
case Gravity.TOP:
childTop = parentTop + lp.topMargin;
break;
case Gravity.CENTER_VERTICAL:
childTop = parentTop + (parentBottom - parentTop - height) / 2 +
lp.topMargin - lp.bottomMargin;
break;
case Gravity.BOTTOM:
childTop = parentBottom - height - lp.bottomMargin;
break;
default:
childTop = parentTop + lp.topMargin;
}
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}这里主要就是通过padding和margin算出子View的top,left,bottom,right四个顶点的值，从而再调其子View的layout方法。如果子View child不是ViewGroup，最后直接调用子View(或自定义View)的onLayout()方法，如果child是ViewGroup，那就再走一遍流程。
3.performDraw(绘制)
从DecorView绘制流程源码分析中，我们知道performDraw()绘制有两种方式，分别为Hardware渲染(硬件加速)和Software渲染，因为两种绘制方式最后也都走到调用View的draw()方法，所以这里我们来看看software渲染方式的绘制
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int yoff,
boolean scalingRequired, Rect dirty) {
// Draw with software renderer.
Canvas canvas;
try {
.......
canvas = mSurface.lockCanvas(dirty);
......
} catch (Surface.OutOfResourcesException e) {
.....
} catch (IllegalArgumentException e) {
.....
}
try {
.......
try {
......
mView.draw(canvas);//核心方法
drawAccessibilityFocusedDrawableIfNeeded(canvas);
} finally {
if (!attachInfo.mSetIgnoreDirtyState) {
// Only clear the flag if it was not set during the mView.draw() call
attachInfo.mIgnoreDirtyState = false;
}
}
} finally {
.....
}
return true;
}同上，通过分析知，这里mView.draw(canvas)其实是调用View.draw(canvas)方法，让我们来看看此方法
public void draw(Canvas canvas) {
final int privateFlags = mPrivateFlags;
final boolean dirtyOpaque = (privateFlags & DIRTY_MASK) == DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~DIRTY_MASK) | DRAWN;
/*
* Draw traversal performs several drawing steps which must be executed
* in the appropriate order:
*
* 1. Draw the background
* 2. If necessary, save the canvas' layers to prepare for fading
* 3. Draw view's content
* 4. Draw children
* 5. If necessary, draw the fading edges and restore layers
* 6. Draw decorations (scrollbars for instance)
*/
// 第一步,如果有背景，绘制背景
int saveCount;
if (!dirtyOpaque) {
final Drawable background = mBackground;
if (background != null) {
final int scrollX = mScrollX;
final int scrollY = mScrollY;
if (mBackgroundSizeChanged) {
background.setBounds(0, 0, mRight - mLeft, mBottom - mTop);
mBackgroundSizeChanged = false;
}
if ((scrollX | scrollY) == 0) {
background.draw(canvas);
} else {
canvas.translate(scrollX, scrollY);
background.draw(canvas);
canvas.translate(-scrollX, -scrollY);
}
}
}
........
//第二步,保存画布的层级
int paddingLeft = mPaddingLeft;
final boolean offsetRequired = isPaddingOffsetRequired();
if (offsetRequired) {
paddingLeft += getLeftPaddingOffset();
}
int left = mScrollX + paddingLeft;
int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
int top = mScrollY + getFadeTop(offsetRequired);
int bottom = top + getFadeHeight(offsetRequired);
if (offsetRequired) {
right += getRightPaddingOffset();
bottom += getBottomPaddingOffset();
}
final ScrollabilityCache scrollabilityCache = mScrollCache;
final float fadeHeight = scrollabilityCache.fadingEdgeLength;
int length = (int) fadeHeight;
// clip the fade length if top and bottom fades overlap
// overlapping fades produce odd-looking artifacts
if (verticalEdges && (top + length > bottom - length)) {
length = (bottom - top) / 2;
}
// also clip horizontal fades if necessary
if (horizontalEdges && (left + length > right - length)) {
length = (right - left) / 2;
}
if (verticalEdges) {
topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
drawTop = topFadeStrength * fadeHeight > 1.0f;
bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
}
if (horizontalEdges) {
leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
drawLeft = leftFadeStrength * fadeHeight > 1.0f;
rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
drawRight = rightFadeStrength * fadeHeight > 1.0f;
}
saveCount = canvas.getSaveCount();
int solidColor = getSolidColor();
if (solidColor == 0) {
final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;
if (drawTop) {
canvas.saveLayer(left, top, right, top + length, null, flags);
}
if (drawBottom) {
canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
}
if (drawLeft) {
canvas.saveLayer(left, top, left + length, bottom, null, flags);
}
if (drawRight) {
canvas.saveLayer(right - length, top, right, bottom, null, flags);
}
} else {
scrollabilityCache.setFadeColor(solidColor);
}
// 第三步，绘制内容
if (!dirtyOpaque)
onDraw(canvas);//核心方法
//第四步，分发绘制子View
dispatchDraw(canvas);
//第五步，绘制fade效果和restore Layers
final Paint p = scrollabilityCache.paint;
final Matrix matrix = scrollabilityCache.matrix;
final Shader fade = scrollabilityCache.shader;
if (drawTop) {
matrix.setScale(1, fadeHeight * topFadeStrength);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
canvas.drawRect(left, top, right, top + length, p);
}
if (drawBottom) {
matrix.setScale(1, fadeHeight * bottomFadeStrength);
matrix.postRotate(180);
matrix.postTranslate(left, bottom);
fade.setLocalMatrix(matrix);
canvas.drawRect(left, bottom - length, right, bottom, p);
}
if (drawLeft) {
matrix.setScale(1, fadeHeight * leftFadeStrength);
matrix.postRotate(-90);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
canvas.drawRect(left, top, left + length, bottom, p);
}
if (drawRight) {
matrix.setScale(1, fadeHeight * rightFadeStrength);
matrix.postRotate(90);
matrix.postTranslate(right, top);
fade.setLocalMatrix(matrix);
canvas.drawRect(right - length, top, right, bottom, p);
}
canvas.restoreToCount(saveCount);
// Step 6, draw decorations (scrollbars)
onDrawScrollBars(canvas);
}从此方法，我们知道View的draw()分五步,分别为：
- 第一步，如果有背景，绘制背景
- 第二步，保存画布的层级
- 第三步，绘制内容
- 第四步，分发绘制子View
- 第五步，绘制fade效果和restore Layers
由于我们的DecorView是FrameLayout,是ViewGroup，所以我们来看一下第四步，分发绘制子View，来看ViewGroup中dispatchDraw()方法(此方法主要是ViewGroup中实现)
protected void dispatchDraw(Canvas canvas) {
final int count = mChildrenCount;
final View[] children = mChildren;
.......
if ((flags & FLAG_USE_CHILD_DRAWING_ORDER) == 0) {
for (int i = 0; i < count; i++) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
more |= drawChild(canvas, child, drawingTime);
}
}
} else {
for (int i = 0; i < count; i++) {
final View child = children[getChildDrawingOrder(count, i)];
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
more |= drawChild(canvas, child, drawingTime);
}
}
}
// Draw any disappearing views that have animations
if (mDisappearingChildren != null) {
final ArrayList disappearingChildren = mDisappearingChildren;
final int disappearingCount = disappearingChildren.size() - 1;
// Go backwards -- we may delete as animations finish
for (int i = disappearingCount; i >= 0; i--) {
final View child = disappearingChildren.get(i);
more |= drawChild(canvas, child, drawingTime);
}
}
.......
}
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
return child.draw(canvas, this, drawingTime);
} 通过遍历ViewGroup中的子View，然后在调用子View的draw方法，这里draw()方法和我们前面的view的draw()有点不一样，因为是三个参数的，我们再来看此方法
/**
* This method is called by ViewGroup.drawChild() to have each child view draw itself.
* This draw() method is an implementation detail and is not intended to be overridden or
* to be called from anywhere else other than ViewGroup.drawChild().
*/
boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
......
if (hasNoCache) {
.......
if (!layerRendered) {
if (!hasDisplayList) {
// Fast path for layouts with no backgrounds
if ((mPrivateFlags & SKIP_DRAW) == SKIP_DRAW) {
mPrivateFlags &= ~DIRTY_MASK;
dispatchDraw(canvas);
} else {
draw(canvas);
}
} else {
mPrivateFlags &= ~DIRTY_MASK;
((HardwareCanvas) canvas).drawDisplayList(displayList, null, flags);
}
}
} else if (cache != null) {
.....
}
......
return more;
}可以发现，最后还是调用回了View的draw(canvas)方法。所以对于View的draw(绘制)，其实也和measure(测量)和layout(布局)一样，如果View是ViewGroup，就是在draw的时候会进行分发绘制子View，如果view就是View,那就会调用View(或自定义View)的onDraw()方法，绘制内容。
到这里，我们View的三大绘制原理就分析完了。
注：源码采用android-4.1.1_r1版本，建议下载源码然后自己走一遍流程，这样更能加深理解。
三、参考文档
Android View 测量流程(Measure)完全解析
View的绘制原理