Android进阶 - View 工作原理探究

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对象被回收时，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（8W多行代码），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工作流程示意图

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：
1. 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。
2. 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：
1. 取出子元素的LayoutParams
2. 通过getChildMeasureSpec离开创建子元素的MeasureSpec
3. 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：

1. 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();        }    })}

1. 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：
1. setFrame方法设定View四个顶点位置
2. 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：
1. 绘制背景(background.draw(canvas))
2. 绘制自己(onDraw)
3. 绘制children(dispatchDraw)
4. 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。