前言
探究分析了View绘制的总体流程：onMeasure、onLayout、onDraw三大方法。
知识准备
ViewRoot
ViewRoot对应ViewRootImpl类，是连接WindowManager与DecorView的纽带。View的三大流程都是通过ViewRoot完成的。ActivityThread中，Activity对象被创建时，会将DecorView添加到Window中，同时创建ViewRootImpl对象，并将ViewRootImpl对象和DecorView对象建立关联。
代码示例：
//创建ViewRootImpl对象
root = new ViewRootImpl(view.getContext(),display);
//添加关联
root.setView(view,wparams,panelparentView);
View绘制流程：从ViewRoot的performTraversals开始，经过measure、layout、draw三大流程后才将View绘制出来。
performTraversals方法，会依次调用performMeasure、performLayout、performDraw方法（这三个方法分别完成顶级View - DecorView 的measure、layout、draw方法）
其中performMeasure方法会调用measure方法，在measure方法中又会调用onMeasure方法，onMeasure方法则会对所有子元素进行measure，从而达到measure流程从父容器传递到子元素中的目的。接着子元素会重复父容器的measure过程，如此反复完成整个View树的遍历。
最后，perfromLayout、performDraw的传递过程也是类似的，唯一不同，而performDraw的传递过程在draw方法中通过dispatchDraw来实现。
DecorView - 继承FrameLayout
DecorView作为顶级View，一般情况下会包含一个LinearLayout，该LinearLayout分为上下两部分，titlebar部分、android.R.id.content部分（所以，setContentView方法其实就是将布局添加到id为content的FrameLayout中）正如小标题，DecorView本质是FrameLayout，View层时间都必先经过DecorView然后在传递给其中的View。
MeasureSpec - 很大程度决定View的尺寸规格
MeasureSpec代表一个32位int值，高2位代表SpecMode（测量模式），低30位代表SpecSize（规格大小）
小白科普：Java中int为4个字节，Android使用第一个高位字节存储Mode，剩下三个字节存储Size
MeasureSpec内部实现原理
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
//下面是三种测量模式
/**
* 父控件不对子控件施加任何约束，一般用于系统内部
*/
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
/**
* 父控件已为子控件指定精确大小，对应于LayoutParams中的 match_parenet和具体数值这两种模式
*/
public static final int EXACTLY = 1 << MODE_SHIFT;
/**
* 子控件可随意大小，但不可大于父控件大小，对应于LayoutParams中的 wrap_content
*/
public static final int AT_MOST = 2 << MODE_SHIFT;
/**
* 根据提供的测量模式以及大小创建 measure specification
*/
public static int makeMeasureSpec(int size, int mode) {
if (sUseBrokenMakeMeasureSpec) {
return size + mode;
} else {
return (size & ~MODE_MASK) | (mode & MODE_MASK);
}
}
public static int getMode(int measureSpec) {
return (measureSpec & MODE_MASK);
}
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
小白科普：<< 是移位运算，3<<30表示的是首先把3变成二进制的11然后右边补30个0所组成的一个二进制的数。
MearsureSpec和LayoutParams对应关系
系统内部是通过MeasureSpec进行View测量的，但正常情况下，都是使View指定MeasureSpec。View测量时候系统会将LayoutParams在父容器约束下转换成对应的MeasureSpec，然后再根据这个MeasureSpec来确定View测量后的宽高。因此，Measure需要由LayoutParams和父容器一起决定。
另外，对于顶级View（DecorView），其MeasureSpec由窗口尺寸和其自身的LayoutParams共同决定。Measure一旦决定后，onMeasure中in个即可获得View的测量宽高。
/**
* DecorView中，MeasureSpec产生过程
* 根据LayoutParams划分，并产生MeasureSpec
*/
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
int measureSpec;
switch (rootDimension) {
case ViewGroup.LayoutParams.MATCH_PARENT:
// Window can't resize. Force root view to be windowSize.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
break;
case ViewGroup.LayoutParams.WRAP_CONTENT:
// Window can resize. Set max size for root view.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break;
default:
// Window wants to be an exact size. Force root view to be that size.
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}
对于普通View（布局中的View），View的measure方法需要由ViewGroup传递过来
再看看ViewGroup中的measureChildWithMargins方法
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
+ widthUsed, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
+ heightUsed, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
上述方法，对子元素进行measure，调哟in个子元素measure之前会获取子元素的MeasureSpec。显然，子元素MeasureSpec的创建与父容器的MeasureSpec和子元素本身的LayoutParams有关，还与View的margin以及padding有关（具体需要研究ViewGroup的getChildMeasureSpec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 = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
上述方法，主要根据父容器的MeasureSpec同时结合View本身的LayoutParams来确定子元素的MeasureSpec。
另外注意，子元素可用大小为父容器尺寸减去padding。
View工作流程
measure 确定View宽高
layout确定View最终宽高和四个顶点位置
draw将View绘制到屏幕
View的生命周期与工作流程
View生命周期示意图
View生命周期
View工作流程示意图
View工作流程
探究Measure过程
两种情况，若只是一个原始的View，通过measure方法就完成了测量过程；如果是ViewGroup，除了完成自身的measure过程，还需要遍历子元素的measure方法，各个子元素递归去执行这个部分。（如上面的示意图所述）
View的measure方法是一个final类型的方法 - 意味着子类不能重写该方法,因此仔细研究onMeasure方法的实现效果会更好。
这里贴出View中Measure方法，有部分注释，供有兴趣的读者阅读研究。
/**
* View中的Measure方法
*/
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
boolean optical = isLayoutModeOptical(this);
//先判断当前Mode是不是特例LAYOUT_MODE_OPTICAL_BOUNDS
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int oWidth = insets.left + insets.right;
int oHeight = insets.top + insets.bottom;
widthMeasureSpec = MeasureSpec.adjust(widthMeasureSpec, optical ? -oWidth : oWidth);
heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
}
// Suppress sign extension for the low bytes
long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
widthMeasureSpec != mOldWidthMeasureSpec ||
heightMeasureSpec != mOldHeightMeasureSpec) {
// first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded();
int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :
mMeasureCache.indexOfKey(key);
if (cacheIndex < 0 || sIgnoreMeasureCache) {
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
} else {
long value = mMeasureCache.valueAt(cacheIndex);
// Casting a long to int drops the high 32 bits, no mask needed
setMeasuredDimensionRaw((int) (value >> 32), (int) value);
mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
// flag not set, setMeasuredDimension() was not invoked, we raise
// an exception to warn the developer
//如果自定义View重写了onMeasure方法而没有调用setMeasureDimension()方法,将会在这里抛出异常
//判断原理：通过解析状态位mPrivateFlags，setMeasureDimension()方法会将mPrivateFlags设置为已计算状态（PFLAG_MEASURED_DIMENSION_SET），只需要检查mPrivateFlags是否含有PFLAG_MEASURED_DIMENSION_SET即可。
if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
throw new IllegalStateException("View with id " + getId() + ": "
+ getClass().getName() + "#onMeasure() did not set the"
+ " measured dimension by calling"
+ " setMeasuredDimension()");
}
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
//计算出的key作为键，量算结果作为值，将该键值对放入成员变量mMeasureCache中,实现本次计算结果的环缓存
mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}
好啦，接下来，继续研究onMeasure
/**
* View中的onMeasure方法
*/
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
setMeasuredDimension()方法会设置View宽高测量值，接下来进一步深入，研究getDefaultSize()方法。
/**
* View中的getDefaultSize方法
*/
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
case MeasureSpec.UNSPECIFIED://一般是系统内部的测量过程，重点注意另外两种模式
result = size;
break;
case MeasureSpec.AT_MOST://这两种模式都是做一样的事情
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
getDefaultSize方法直接就是根据测量模式返回measureSpec中的specSize，而这个specSize就是View测量后的大小。
注意：View测量后大小 与 View最终大小 需要区分，是两个东西，因为View最终大小是在layout阶段确定的，但两者几乎所有情况都是相等的。
接下来，再继续探究getDefaultSize方法的第一个参数，从onMeasure方法中可知，该参数来源于下面两个方法
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
protected int getSuggestedMinimumHeight() {
return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());
}
上述两个方法实现原理都一致，判断有没有背景，如果有，返回两者较大的宽高，没有则返回自己的宽高（android:minwith这个属性指定的值）。
那么，问题来了，背景最小宽高原理是什么？
public int getMinimumWidth() {
final int intrinsicWidth = getIntrinsicWidth();
return intrinsicWidth > 0 ? intrinsicWidth : 0;
}
上述代码中可见，Drawable的原始宽度，如果没有原始宽度，则返回0。
小白科普：ShapeDrawable无原始宽高，而BimapDrawable有原始宽高（即图片尺寸）
再谈谈ViewGroup的measure过程
主要区别：
- 除了完成自己measure过程还要遍历调用子元素的measure方法，各个子元素再递归执行该过程。
- ViewGroup是抽象类，没有重写View的onMeasure方法，而是提供了一个measureChildren的方法
protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < size; ++i) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
View在measure过程中会对每一个子元素进行measure。
再细说下，measureChild方法的思路：
- 取出子元素的LayoutParams
- 通过getChildMeasureSpec离开创建子元素的MeasureSpec
- 将MeasureSpec传递给View的Measure方法进行测量。
问题：为什么ViewGroup不像View一样对其onMeasure方法做统一实现？
因为不同的ViewGroup子类会有不同的特性，因此其中的onMeasure细节不相同。
获取View宽高方法不当，可能会获取错误。
原因：View的measure过程和Activity生命周期方法执行顺序是不确定的，无法保证Activity执行了onCreate、onStart、onReasume时，View测量完毕。
如果View还没有完成测量，则获取的宽高会是0
给出四种方法解决：
- onWindowFocusChanged - 该方法被调用时候，View已经测量完毕，能够正确获取View宽高。
注意:该方法会被调用多次，Activity窗口得到焦点与失去焦点时均会被调用一次（继续执行，暂停执行）。
public void onWindowFocusChanged(boolean hasWindowFocus) {
super.onWindowFocusChanged(hasWindowFocus);
if(hasWindowFocus){
//获取宽高
int with = view.getMeasuredWidth();
int height = view.getMeasuredHeight();
}
}
- view.post(runnable)
通过post将一个runnable投递到消息队列队尾，等待Looper调用此runnable时，View已初始化完毕。
protected void onStart(){
super.onStart();
view.post(new Runnable(){
@override
public void run(){
//获取宽高
int with = view.getMeasuredWidth();
int height = view.getMeasuredHeight();
}
})
}
- ViewTreeObserver
该类有众多回调接口，其中的OnGlobalLayoutListener接口，当View树状态发生变化或者View树内部的View的可见性发生改变，该方法都会被回调，利用此特性，可获得宽高。
protected void onStart(){
super.onStart();
viewTreeObserver observer = view.getViewTreeObserver();
observer.addOnGlobalLayoutListener(new OnGlobalListener(){
public void onGlobalLayout(){
view.getViewTreeObserver().removeGlobalOnlayoutListener(this);
//获取宽高
int with = view.getMeasuredWidth();
int height = view.getMeasuredHeight();
}
})
}
- view.measure(int widthMeasureSpec,int heightMeasureSpec)
手动对View进行获取，根据View的LayoutParams不同，而采取不同手段。（因不常用，这里就不详细说明）
探究layout过程
layout主要作用是ViewGroup用来确定子元素位置（递归）。
public void layout(int l, int t, int r, int b) {
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
//设定四个顶点位置
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLayoutChangeListeners != null) {
ArrayList<OnLayoutChangeListener> listenersCopy =
(ArrayList<OnLayoutChangeListener>)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 &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}
layout方法流程：
- setFrame方法设定View四个顶点位置
- 调用onLayout方法，父容器确定子元素位置
另外，与onMeasure方法相似，onLayout方法也是各不相同的。
onLayout方法（LinearLayout、RelativeLayout等基本控件可自行尝试研究下）
探究draw过程
draw主要作用是将View绘制到屏幕上
绘制过程：
- 绘制背景(background.draw(canvas))
- 绘制自己(onDraw)
- 绘制children(dispatchDraw)
- 绘制装饰(onDrawScrollBars)
/**
* View中的draw方法
*/
public void draw(Canvas canvas) {
final int privateFlags = mPrivateFlags;
final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_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)
*/
// Step 1, draw the background, if needed
int saveCount;
if (!dirtyOpaque) {
drawBackground(canvas);
}
// skip step 2 & 5 if possible (common case)
final int viewFlags = mViewFlags;
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
if (!verticalEdges && !horizontalEdges) {
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
// Overlay is part of the content and draws beneath Foreground
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
// we're done...
return;
}
/*
* Here we do the full fledged routine...
* (this is an uncommon case where speed matters less,
* this is why we repeat some of the tests that have been
* done above)
*/
boolean drawTop = false;
boolean drawBottom = false;
boolean drawLeft = false;
boolean drawRight = false;
float topFadeStrength = 0.0f;
float bottomFadeStrength = 0.0f;
float leftFadeStrength = 0.0f;
float rightFadeStrength = 0.0f;
// Step 2, save the canvas' layers
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);
}
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
// Step 5, draw the fade effect and 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);
p.setShader(fade);
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);
p.setShader(fade);
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);
p.setShader(fade);
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);
p.setShader(fade);
canvas.drawRect(right - length, top, right, bottom, p);
}
canvas.restoreToCount(saveCount);
// Overlay is part of the content and draws beneath Foreground
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
}
View的绘制过程传递是通过dispatchDraw来实现，dispatchDraw会遍历所有子元素的draw方法，另外，View还有一个特殊的方法setWillNotDraw
public void setWillNotDraw(boolean willNotDraw) {
setFlags(willNotDraw ? WILL_NOT_DRAW : 0, DRAW_MASK);
}
setFlags - 该方法可设置优化标记
如果View不需要绘制任何内容，那么将设置标记为true，系统会相应优化。默认情况下，View不启用这个标记位，但ViewGroup会默认启动该优化标记。
而实际开发意义：当我们自定义控件继承于ViewGroup并且本身不具备绘制功能，则开启标记。而如果明确知道一个ViewGroup需要通过onDraw来绘制内容时候，则需要显式关闭WILL_NOT_DRAW这个标记位。
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