一文彻底搞懂Android(安卓)View的绘制流程

前言

成为一名优秀的Android开发，需要一份完备的知识体系，在这里，让我们一起成长为自己所想的那样~。

上一篇中我们讲到了Android的触摸事件传递机制，除此之外，关于Android View的绘制流程这一块也是View相关的核心知识点。我们都知道，PhoneWindow是Android系统中最基本的窗口系统，每个Activity会创建一个。同时，PhoneWindow也是Activity和View系统交互的接口。DecorView本质上是一个FrameLayout，是Activity中所有View的祖先。

一、开始：DecorView被加载到Window中

从Activity的startActivity开始，最终调用到ActivityThread的handleLaunchActivity方法来创建Activity，相关核心代码如下：

private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {    ....    // 创建Activity，会调用Activity的onCreate方法    // 从而完成DecorView的创建    Activity a = performLaunchActivity(r, customIntent);    if (a != null) {        r.createdConfig = new Configuration(mConfiguration);        Bundle oldState = r.state;        handleResumeActivity(r.tolen, false, r.isForward, !r.activity..mFinished && !r.startsNotResumed);    }}final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward, boolean reallyResume) {    unscheduleGcIdler();    mSomeActivitiesChanged = true;    // 调用Activity的onResume方法    ActivityClientRecord r = performResumeActivity(token, clearHide);    if (r != null) {        final Activity a = r.activity;        ...        if (r.window == null &&& !a.mFinished && willBeVisible) {            r.window = r.activity.getWindow();            // 得到DecorView            View decor = r.window.getDecorView();            decor.setVisibility(View.INVISIBLE);            // 得到了WindowManager，WindowManager是一个接口            // 并且继承了接口ViewManager            ViewManager wm = a.getWindowManager();            WindowManager.LayoutParams l = r.window.getAttributes();            a.mDecor = decor;            l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;            l.softInputMode |= forwardBit;            if (a.mVisibleFromClient) {                a.mWindowAdded = true;                // WindowManager的实现类是WindowManagerImpl，                // 所以实际调用的是WindowManagerImpl的addView方法                wm.addView(decor, l);            }        }    }}public final class WindowManagerImpl implements WindowManager {    private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();    ...        @Override    public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {        applyDefaultToken(params);        mGlobal.addView(view, params, mDisplay, mParentWindow);    }    ...}

在了解View绘制的整体流程之前，我们必须先了解下ViewRoot和DecorView的概念。ViewRoot对应于ViewRootImpl类，它是连接WindowManager和DecorView的纽带，View的三大流程均是通过ViewRoot来完成的。在ActivityThread中，当Activity对象被创建完毕后，会将DecorView添加到Window中，同时会创建ViewRootImpl对象，并将ViewRootImpl对象和DecorView建立关联，相关源码如下所示：

// WindowManagerGlobal的addView方法public void addView(View view, ViewGroup.LayoutParams params, Display display, Window parentWindow) {    ...    ViewRootImpl root;    View pannelParentView = null;    synchronized (mLock) {        ...        // 创建ViewRootImpl实例        root = new ViewRootImpl(view..getContext(), display);        view.setLayoutParams(wparams);        mViews.add(view);        mRoots.add(root);        mParams.add(wparams);    }    try {        // 把DecorView加载到Window中        root.setView(view, wparams, panelParentView);    } catch (RuntimeException e) {        synchronized (mLock) {            final int index = findViewLocked(view, false);            if (index >= 0) {                removeViewLocked(index, true);            }        }        throw e;    }}

二、了解绘制的整体流程

绘制会从根视图ViewRoot的performTraversals()方法开始，从上到下遍历整个视图树，每个View控件负责绘制自己，而ViewGroup还需要负责通知自己的子View进行绘制操作。performTraversals()的核心代码如下。

private void performTraversals() {    ...    int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);    int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);    ...    //执行测量流程    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);    ...    //执行布局流程    performLayout(lp, desiredWindowWidth, desiredWindowHeight);    ...    //执行绘制流程    performDraw();}

performTraversals的大致工作流程图如下所示：

显示不出来的可点击这里查看

注意：

preformLayout和performDraw的传递流程和performMeasure是类似的，唯一不同的是，performDraw的传递过程是在draw方法中通过dispatchDraw来实现的，不过这并没有本质区别。
获取content：

ViewGroup content = (ViewGroup)findViewById(android.R.id.content);

获取设置的View：

content.getChildAt(0);

三、理解MeasureSpec

1.MeasureSpec源码解析

MeasureSpec表示的是一个32位的整形值，它的高2位表示测量模式SpecMode，低30位表示某种测量模式下的规格大小SpecSize。MeasureSpec是View类的一个静态内部类，用来说明应该如何测量这个View。MeasureSpec的核心代码如下。

public static class MeasureSpec {    private static final int MODE_SHIFT = 30;    private static final int MODE_MASK = 0X3 << MODE_SHIFT;        // 不指定测量模式, 父视图没有限制子视图的大小，子视图可以是想要    // 的任何尺寸，通常用于系统内部，应用开发中很少用到。    public static final int UNSPECIFIED = 0 << MODE_SHIFT;        // 精确测量模式，视图宽高指定为match_parent或具体数值时生效，    // 表示父视图已经决定了子视图的精确大小，这种模式下View的测量    // 值就是SpecSize的值。    public static final int EXACTLY = 1 << MODE_SHIFT;        // 最大值测量模式，当视图的宽高指定为wrap_content时生效，此时    // 子视图的尺寸可以是不超过父视图允许的最大尺寸的任何尺寸。    public static final int AT_MOST = 2 << MODE_SHIFT;        // 根据指定的大小和模式创建一个MeasureSpec    public static int makeMeasureSpec(int size, int mode) {        if (sUseBrokenMakeMeasureSpec) {            return size + mode;        } else {            return (size & ~MODE_MASK) | (mode & MODE_MASK);        }    }        // 微调某个MeasureSpec的大小    static int adjust(int measureSpec, int delta) {        final int mode = getMode(measureSpec);        if (mode == UNSPECIFIED) {            // No need to adjust size for UNSPECIFIED mode.            return make MeasureSpec(0, UNSPECIFIED);        }        int size = getSize(measureSpec) + delta;        if (size < 0) {            size = 0;        }        return makeMeasureSpec(size, mode);    }}

MeasureSpec通过将SpecMode和SpecSize打包成一个int值来避免过多的对象内存分配，为了方便操作，其提供了打包和解包的方法，打包方法为上述源码中的makeMeasureSpec，解包方法源码如下：

public static int getMode(int measureSpec) {    return (measureSpec & MODE_MASK);}public static int getSize(int measureSpec) {    return (measureSpec & ~MODE_MASK);}

2.DecorView的MeasureSpec的创建过程：

//desiredWindowWidth和desiredWindowHeight是屏幕的尺寸childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);private static int getRootMeaureSpec(int windowSize, int rootDimension) {    int measureSpec;    switch (rootDimension) {        case ViewGroup.LayoutParams.MATRCH_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;}

3.子元素的MeasureSpec的创建过程

// ViewGroup的measureChildWithMargins方法protected void measureChildWithMargins(View child,int parentWidthMeasureSpec, int widthUsed,int parentHeightMeasureSpec, int heightUsed) {    final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();        // 子元素的MeasureSpec的创建与父容器的MeasureSpec和子元素本身    // 的LayoutParams有关，此外还和View的margin及padding有关    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);}public static int getChildMeasureSpec(int spec, int padding, int childDimesion) {    int specMode = MeasureSpec.getMode(spec);    int specSize = MeasureSpec.getSize(spec);        // padding是指父容器中已占用的空间大小，因此子元素可用的    // 大小为父容器的尺寸减去padding    int size = Math.max(0, specSize - padding);        int resultSize = 0;    int resultMode = 0;        switch (sepcMode) {        // 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 (childDimesion == 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);}

普通View的MeasureSpec的创建规则如下：

注意：UNSPECIFIED模式主要用于系统内部多次Measure的情形，一般不需关注。

结论：对于DecorView而言，它的MeasureSpec由窗口尺寸和其自身的LayoutParams共同决定；对于普通的View，它的MeasureSpec由父视图的MeasureSpec和其自身的LayoutParams共同决定。

四、View绘制流程之Measure

1.Measure的基本流程

由前面的分析可知，页面的测量流程是从performMeasure方法开始的，相关的核心代码流程如下。

private void perormMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {    ...    // 具体的测量操作分发给ViewGroup    mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);    ...}// 在ViewGroup中的measureChildren()方法中遍历测量ViewGroup中所有的Viewprotected 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];        // 当View的可见性处于GONE状态时，不对其进行测量        if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {            measureChild(child, widthMeasureSpec, heightMeasureSpec);        }    }}// 测量某个指定的Viewprotected void measureChild(View child, int parentWidthMeasureSpec, int parentHeightMeasureSpec) {    final LayoutParams lp = child.getLayoutParams();        // 根据父容器的MeasureSpec和子View的LayoutParams等信息计算    // 子View的MeasureSpec    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec, mPaddingLeft + mPaddingRight, lp.width);    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec, mPaddingTop + mPaddingBottom, lp.height);    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);}// View的measure方法public final void measure(int widthMeasureSpec, int heightMeasureSpec) {    ...    // ViewGroup没有定义测量的具体过程，因为ViewGroup是一个    // 抽象类，其测量过程的onMeasure方法需要各个子类去实现    onMeasure(widthMeasureSpec, heightMeasureSpec);    ...}// 不同的ViewGroup子类有不同的布局特性，这导致它们的测量细节各不相同，如果需要自定义测量过程，则子类可以重写这个方法protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {    // setMeasureDimension方法用于设置View的测量宽高    setMeasureDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),     getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));}// 如果View没有重写onMeasure方法，则会默认调用getDefaultSize来获得View的宽高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:            result = size;            break;        case MeasureSpec.AT_MOST:        case MeasureSpec.EXACTLY:            result = sepcSize;            break;    }    return result;}

2.对getSuggestMinimumWidth的分析

protected int getSuggestedMinimumWidth() {    return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinmumWidth());}protected int getSuggestedMinimumHeight() {    return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());}public int getMinimumWidth() {    final int intrinsicWidth = getIntrinsicWidth();    return intrinsicWidth > 0 ? intrinsicWidth : 0;}

如果View没有设置背景，那么返回android:minWidth这个属性所指定的值，这个值可以为0；如果View设置了背景，则返回android:minWidth和背景的最小宽度这两者中的最大值。

3.自定义View时手动处理wrap_content时的情形

直接继承View的控件需要重写onMeasure方法并设置wrap_content时的自身大小，否则在布局中使用wrap_content就相当于使用match_parent。解决方式如下：

protected void onMeasure(int widthMeasureSpec, int height MeasureSpec) {    super.onMeasure(widthMeasureSpec, heightMeasureSpec);    int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);    int widtuhSpecSize = MeasureSpec.getSize(widthMeasureSpec);    int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);    // 在wrap_content的情况下指定内部宽/高(mWidth和mHeight)    int heightSpecSize = MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST) {        setMeasuredDimension(mWidth, mHeight);    } else if (widthSpecMode == MeasureSpec.AT_MOST) {        setMeasureDimension(mWidth, heightSpecSize);    } else if (heightSpecMode == MeasureSpec.AT_MOST) {        setMeasureDimension(widthSpecSize, mHeight);    }}

4.LinearLayout的onMeasure方法实现解析

protected void onMeasure(int widthMeasureSpec, int hegithMeasureSpec) {    if (mOrientation == VERTICAL) {        measureVertical(widthMeasureSpec, heightMeasureSpec);    } else {        measureHorizontal(widthMeasureSpec, heightMeasureSpec);    }}// measureVertical核心源码// See how tall everyone is. Also remember max width.for (int i = 0; i < count; ++i) {    final View child = getVirtualChildAt(i);    ...    // Determine how big this child would like to be. If this or     // previous children have given a weight, then we allow it to     // use all available space (and we will shrink things later     // if need)    measureChildBeforeLayout(            child, i, widthMeasureSpec, 0, heightMeasureSpec,            totalWeight == 0 ? mTotalLength : 0);                if (oldHeight != Integer.MIN_VALUE) {        lp.height = oldHeight;    }        final int childHeight = child.getMeasuredHeight();    final int totalLength = mTotalLength;    mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +     lp.bottomMargin + getNextLocationOffset(child));}

系统会遍历子元素并对每个子元素执行measureChildBeforeLayout方法，这个方法内部会调用子元素的measure方法，这样各个子元素就开始依次进入measure过程，并且系统会通过mTotalLength这个变量来存储LinearLayout在竖直方向的初步高度。每测量一个子元素，mTotalLength就会增加，增加的部分主要包括了子元素的高度以及子元素在竖直方向上的margin等。

// LinearLayout测量自己大小的核心源码// Add in our paddingmTotalLength += mPaddingTop + mPaddingBottom;int heightSize = mTotalLength;// Check against our minimum heightheightSize = Math.max(heightSize, getSuggestedMinimumHeight());// Reconcile our calculated size with the heightMeasureSpecint heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);heightSize = heightSizeAndState & MEASURED_SIZE_MASK；...setMeasuredDimension(resolveSizeAndSize(maxWidth, widthMeasureSpec, childState),heightSizeAndState);public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {    int result = size;    int specMode = MeasureSpec.getMode(measureSpec);    int specSize = MeasureSpec.getSize(measureSpec);    switch (specMode) {        case MeasureSpec.UNSPECIFIED:            result = size;            break;        case MeasureSpec.AT_MOST:            // 高度不能超过父容器的剩余空间            if (specSize < size) {                result = specSize | MEASURED_STATE_TOO_SMALL；            } else {                result = size;            }            break;        case MeasureSpec.EXACTLY:            result = specSize;            break;    }    return result | (childMeasuredState & MEASURED_STATE_MASK);}

5.在Activity中获取某个View的宽高

由于View的measure过程和Activity的生命周期方法不是同步执行的，如果View还没有测量完毕，那么获得的宽/高就是0。所以在onCreate、onStart、onResume中均无法正确得到某个View的宽高信息。解决方式如下：

Activity/View#onWindowFocusChanged

// 此时View已经初始化完毕
// 当Activity的窗口得到焦点和失去焦点时均会被调用一次
// 如果频繁地进行onResume和onPause，那么onWindowFocusChanged也会被频繁地调用
public void onWindowFocusChanged(boolean hasFocus) {
```
super.onWindowFocusChanged(hasFocus);if (hasFocus) {    int width = view.getMeasureWidth();    int height = view.getMeasuredHeight();}
```
}
view.post(runnable)

// 通过post可以将一个runnable投递到消息队列的尾部，// 然后等待Looper调用次runnable的时候，View也已经初// 始化好了protected void onStart() {    super.onStart();    view.post(new Runnable() {                @Override        public void run() {            int width = view.getMeasuredWidth();            int height = view.getMeasuredHeight();        }    });}

ViewTreeObserver

// 当View树的状态发生改变或者View树内部的View的可见// 性发生改变时，onGlobalLayout方法将被回调protected void onStart() {    super.onStart();        ViewTreeObserver observer = view.getViewTreeObserver();    observer.addOnGlobalLayoutListener(new OnGlobalLayoutListener() {                @SuppressWarnings("deprecation")        @Override        public void onGlobalLayout() {            view.getViewTreeObserver().removeGlobalOnLayoutListener(this);            int width = view.getMeasuredWidth();            int height = view.getMeasuredHeight();        }    });}

View.measure(int widthMeasureSpec, int heightMeasureSpec)

五、View的绘制流程之Layout

1.Layout的基本流程

// ViewRootImpl.javaprivate void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth, int desiredWindowHeight) {    ...    host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());    ...}// View.javapublic void layout(int l, int t, int r, int b) {    ...    // 通过setFrame方法来设定View的四个顶点的位置，即View在父容器中的位置    boolean changed = isLayoutModeOptical(mParent) ?     set OpticalFrame(l, t, r, b) : setFrame(l, t, r, b);        ...    onLayout(changed, l, t, r, b);    ...}// 空方法，子类如果是ViewGroup类型，则重写这个方法，实现ViewGroup// 中所有View控件布局流程protected void onLayout(boolean changed, int left, int top, int right, int bottom) {    }

2.LinearLayout的onLayout方法实现解析

protected void onlayout(boolean changed, int l, int t, int r, int b) {    if (mOrientation == VERTICAL) {        layoutVertical(l, t, r, b);    } else {        layoutHorizontal(l,)    }}// layoutVertical核心源码void layoutVertical(int left, int top, int right, int bottom) {    ...    final int count = getVirtualChildCount();    for (int i = 0; i < count; i++) {        final View child = getVirtualChildAt(i);        if (child == null) {            childTop += measureNullChild(i);        } else if (child.getVisibility() != GONE) {            final int childWidth = child.getMeasureWidth();            final int childHeight = child.getMeasuredHeight();                        final LinearLayout.LayoutParams lp =                     (LinearLayout.LayoutParams) child.getLayoutParams();            ...            if (hasDividerBeforeChildAt(i)) {                childTop += mDividerHeight;            }                        childTop += lp.topMargin;            // 为子元素确定对应的位置            setChildFrame(child, childLeft, childTop + getLocationOffset(child), childWidth, childHeight);            // childTop会逐渐增大，意味着后面的子元素会被            // 放置在靠下的位置            childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);                        i += getChildrenSkipCount(child,i)        }    }}private void setChildFrame(View child, int left, int top, int width, int height) {    child.layout(left, top, left + width, top + height);}

注意：在View的默认实现中，View的测量宽/高和最终宽/高是相等的，只不过测量宽/高形成于View的measure过程，而最终宽/高形成于View的layout过程，即两者的赋值时机不同，测量宽/高的赋值时机稍微早一些。在一些特殊的情况下则两者不相等：

重写View的layout方法,使最终宽度总是比测量宽/高大100px

public void layout(int l, int t, int r, int b) {    super.layout(l, t, r + 100, b + 100);}

View需要多长measure才能确定自己的测量宽/高,在前几次测量的过程中，其得出的测量宽/高有可能和最终宽/高不一致，但最终来说，测量宽/高还是和最终宽/高相同

六、View的绘制流程之Draw

1.Draw的基本流程

private void performDraw() {    ...    draw(fullRefrawNeeded);    ...}private void draw(boolean fullRedrawNeeded) {    ...    if (!drawSoftware(surface, mAttachInfo, xOffest, yOffset,     scalingRequired, dirty)) {        return;    }    ...}private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff, boolean scallingRequired, Rect dirty) {    ...    mView.draw(canvas);    ...}// 绘制基本上可以分为六个步骤public void draw(Canvas canvas) {    ...    // 步骤一：绘制View的背景    drawBackground(canvas);        ...    // 步骤二：如果需要的话，保持canvas的图层，为fading做准备    saveCount = canvas.getSaveCount();    ...    canvas.saveLayer(left, top, right, top + length, null, flags);        ...    // 步骤三：绘制View的内容    onDraw(canvas);        ...    // 步骤四：绘制View的子View    dispatchDraw(canvas);        ...    // 步骤五：如果需要的话，绘制View的fading边缘并恢复图层    canvas.drawRect(left, top, right, top + length, p);    ...    canvas.restoreToCount(saveCount);        ...    // 步骤六：绘制View的装饰(例如滚动条等等)    onDrawForeground(canvas)}

2.setWillNotDraw的作用

// 如果一个View不需要绘制任何内容，那么设置这个标记位为true以后，// 系统会进行相应的优化。public void setWillNotDraw(boolean willNotDraw) {    setFlags(willNotDraw ? WILL_NOT_DRAW : 0, DRAW_MASK);}

默认情况下，View没有启用这个优化标记位，但是ViewGroup会默认启用这个优化标记位。
当我们的自定义控件继承于ViewGroup并且本身不具备绘制功能时，就可以开启这个标记位从而便于系统进行后续的优化。
当明确知道一个ViewGroup需要通过onDraw来绘制内容时，我们需要显示地关闭WILL_NOT_DRAW这个标记位。

七、总结

View的绘制流程和事件分发机制都是Android开发中的核心知识点，也是自定义View高手的内功心法。对于一名优秀的Android开发来说，主流三方源码分析和Android核心源码分析可以说是必修课，下一篇，将会带领大家更进一步深入Android。

参考链接：

1、Android开发艺术探索

2、Android进阶之光

3、Android高级进阶

4、Android应用层View绘制流程与源码分析

5、Android中View绘制流程浅析

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