在Android知识体系中，View扮演了很重要的角色；它是Android在视觉上的呈现，Android本身提供了一套GUI库，但是我们的需求不止于系统自带的GUI，因此我们还需要自定义View。而自定义View过程中我们势必要对View的底层工作原理有所了解。这篇文章就记录一下View的测量、布局、绘制三大流程。
ViewRoot对应于ViewRootImpl类，连接WindowManager和DecorView的纽带；View的三大流程基于RootView来完成的；View的绘制流程是从ViewRoot的performTraversals方法开始的。
DecorView顶级View,内部一般为一个竖直方向的LinearLayout，分为2部分：titlebar以及android.R.id.content的FrameLayout。 结构图如下：
字面意思为“测量规格”；
它代表一个32位int值，高2位代表SpecMode(测量模式)，低30位代表SpecSize(某种测量模式下的规格大小)。
SpecMode一共有3类：
EXACTLY:match_parent和具体数值，父容器已经检测出View的精确大小AT_MOST:父容器制定一个可用大小，View的大小不能大于它UNSPECIFIED: 父容器不对View有任何限制一般来说，View的MeasureSpec由本身的LayoutParams与父容器的MeasureSpec共同决定。DecorView的MeasureSpec由LayoutParams决定。父容器通过measure传递MeasureSpec给子View.
它在View的底层原理中起到的作用下面的篇幅在做具体的介绍。源码如下：所在包import android.view.View.MeasureSpec;
/** * A MeasureSpec encapsulates the layout requirements passed from parent to child. * Each MeasureSpec represents a requirement for either the width or the height. * A MeasureSpec is comprised of a size and a mode. There are three possible * modes: * <dl> * <dt>UNSPECIFIED</dt> * <dd> * The parent has not imposed any constraint on the child. It can be whatever size * it wants. * </dd> * * <dt>EXACTLY</dt> * <dd> * 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. * </dd> * * <dt>AT_MOST</dt> * <dd> * The child can be as large as it wants up to the specified size. * </dd> * </dl> * * MeasureSpecs are implemented as ints to reduce object allocation. This class * is provided to pack and unpack the <size, mode> tuple into the int. */ public static class MeasureSpec { private static final int MODE_SHIFT = 30; private static final int MODE_MASK = 0x3 << MODE_SHIFT; /** @hide */ @IntDef({UNSPECIFIED, EXACTLY, AT_MOST}) @Retention(RetentionPolicy.SOURCE) public @interface MeasureSpecMode {} public static final int UNSPECIFIED = 0 << MODE_SHIFT; public static final int EXACTLY = 1 << MODE_SHIFT; public static final int AT_MOST = 2 << MODE_SHIFT; public static int makeMeasureSpec(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size, @MeasureSpecMode int mode) { if (sUseBrokenMakeMeasureSpec) { return size + mode; } else { return (size & ~MODE_MASK) | (mode & MODE_MASK); } } public static int makeSafeMeasureSpec(int size, int mode) { if (sUseZeroUnspecifiedMeasureSpec && mode == UNSPECIFIED) { return 0; } return makeMeasureSpec(size, mode); } public static int getMode(int measureSpec) { //noinspection ResourceType return (measureSpec & MODE_MASK); } public static int getSize(int measureSpec) { return (measureSpec & ~MODE_MASK); } static int adjust(int measureSpec, int delta) { final int mode = getMode(measureSpec); int size = getSize(measureSpec); if (mode == UNSPECIFIED) { // No need to adjust size for UNSPECIFIED mode. return makeMeasureSpec(size, UNSPECIFIED); } size += delta; if (size < 0) { Log.e(VIEW_LOG_TAG, "MeasureSpec.adjust: new size would be negative! (" + size + ") spec: " + toString(measureSpec) + " delta: " + delta); size = 0; } return makeMeasureSpec(size, mode); } public static String toString(int measureSpec) { int mode = getMode(measureSpec); int size = getSize(measureSpec); StringBuilder sb = new StringBuilder("MeasureSpec: "); if (mode == UNSPECIFIED) sb.append("UNSPECIFIED "); else if (mode == EXACTLY) sb.append("EXACTLY "); else if (mode == AT_MOST) sb.append("AT_MOST "); else sb.append(mode).append(" "); sb.append(size); return sb.toString(); } }上面介绍了View底层工作原理所必备的一些知识后，现在重点介绍View的工作原理：
View三大流程的入口（通过Log可以得知）：是从onResume()之后开始的。跟踪ActivityThread的源码，在handleResumeActivity()中开始.
,注意注释 // in addView->ViewRootImpl#setView. If we are instead reusing最终调用ViewRootImpl.setView();
进入requestLayout()方法。
核心方法：View的工作主要流程就在此方法中。
归纳上方的流程为下图：
measure 测量，决定了测量后的宽高layout 布局，确定四个定点坐标以及View的实际宽高draw 绘制，决定View的显示依次调用performMeasure、performLayout、performDraw方法，完成顶级View的measure、layout、draw；而在measure、layout、draw又会调用oMeasure、onLayout、onDraw对其子元素重复递归上过程，直到view不为ViewGroup时。
一系列的方法会设置View宽高的测量值。其中getSuggestedMinimumWidth()会根据是存在背景来返回最小宽度。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); } 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 = specSize; break; } return result; } protected int getSuggestedMinimumWidth() { return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth()); }从getDefault方法中可得结论： 直接继承View的自定义控件需重写onMeasure方法并设置wrap_content时的自身大小，否则使用wrap_content就相当于使用match_content。
ViewGroup的measure而ViewGroup需要完成本身的measure过程外，还要遍历所有子元素的measure方法，递归执行。本身未重写onMeasure()方法，提供measureChildren()方法遍历测量View树。具体继承自它的类重写onMeasure()对子View进行测量。
首先要确定DecorView的MeasureSpec用来传递给子View进行生成MeasureSpec。DecorView通过getRootMeasureSpec确定本身测量模式以及大小。其中windowSize就是屏幕的宽高。
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; }其次，通过measureChildren(int widthMeasureSpec, int heightMeasureSpec)遍历测量所有的View树。
// 遍历测量View树 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); } } } // 根据LayoutParams 以及 父View传递过来的MeasureSpec 创建自身的测量模式/规格 protected void measureChild(View child, int parentWidthMeasureSpec, int parentHeightMeasureSpec) { // 自身布局参数 final LayoutParams lp = child.getLayoutParams(); // 获取当前所需测量child的宽度的测量模式 final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec, mPaddingLeft + mPaddingRight, lp.width); // 获取当前所需测量child的高度的测量模式 final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec, mPaddingTop + mPaddingBottom, lp.height); // 最终调用measure，判断是View还是ViewGroup child.measure(childWidthMeasureSpec, childHeightMeasureSpec); }其中最重要的源码部分如下：此部分即可以归纳为上面引入的图片。
// spec 父Spec padding值 childDimension为布局文件中设置的参数 public static int getChildMeasureSpec(int spec, int padding, int childDimension) { // 父specMode int specMode = MeasureSpec.getMode(spec); // 父specSize int specSize = MeasureSpec.getSize(spec); // 父实际可用的空间 int size = Math.max(0, specSize - padding); // 最终size int resultSize = 0; // 最终模式 int resultMode = 0; // 结合父测量模式以及自身LayoutParams来创建child的测量模式 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; } //noinspection ResourceType return MeasureSpec.makeMeasureSpec(resultSize, resultMode); }Layout的作用是确定子元素的位置，layout方法确定本身所在位置，onLayout确定所有子元素的位置。跟踪ViewRootImpl中的performLayout方法。
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth, int desiredWindowHeight) { ...... // host 即为DecorView final View host = mView; if (host == null) { return; } ...... host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); ...... }首先初始化LTRB，其中l,t,r,b分别为View的四个顶点的位置。接着调用onLayout确定子元素的位置。onLayout与onMeasure相似，具体的实现在具体的View中重写即可。onLayout中遍历调用setChildFrame()，内部调用的是 child.layout(left, top, left + width, top + height);通过递归完成View树的布局过程。
/** * <p>Derived classes should not override this method. * Derived classes with children should override * onLayout. In that method, they should * call layout on each of their children.</p> * * @param l Left position, relative to parent * @param t Top position, relative to parent * @param r Right position, relative to parent * @param b Bottom position, relative to parent */ @SuppressWarnings({"unchecked"}) 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; } 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); // 设置四个顶点的坐标。 if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) { // 调用onLayout确定所有子元素的位置 onLayout(changed, l, t, r, b); if (shouldDrawRoundScrollbar()) { if(mRoundScrollbarRenderer == null) { mRoundScrollbarRenderer = new RoundScrollbarRenderer(this); } } else { mRoundScrollbarRenderer = null; } 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); } } } ...... ...... }把LinearLayout中onLayout的具体实现做分析：
@Override protected void onLayout(boolean changed, int l, int t, int r, int b) { if (mOrientation == VERTICAL) { layoutVertical(l, t, r, b); } else { layoutHorizontal(l, t, r, b); } } // 具体的实现方法 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); // 测量宽高 final int childWidth = child.getMeasuredWidth(); final int childHeight = child.getMeasuredHeight(); ...... // 为子元素指定对应的位置 // 设置的就是测量的宽高 setChildFrame(child, childLeft, childTop + getLocationOffset(child), childWidth, childHeight); ...... } } } private void setChildFrame(View child, int left, int top, int width, int height) { child.layout(left, top, left + width, top + height); }Draw过程相对比较简单，即将View绘制到屏幕上。从RootViewImpl中跟踪。观察源码：
private void performDraw() { if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) { return; } else if (mView == null) { return; } ...... boolean canUseAsync = draw(fullRedrawNeeded); ...... } private boolean draw(boolean fullRedrawNeeded) { ...... if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty, surfaceInsets)) { return false; } ...... return useAsyncReport; } private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff, boolean scalingRequired, Rect dirty, Rect surfaceInsets) { ...... mView.draw(canvas); ...... }接着会调用到View的draw方法，其源码如下：
@CallSuper 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); // Step 6, draw decorations (foreground, scrollbars) onDrawForeground(canvas); // Step 7, draw the default focus highlight drawDefaultFocusHighlight(canvas); if (debugDraw()) { debugDrawFocus(canvas); } // we're done... return; } // 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; // Step 6, draw decorations (foreground, scrollbars) onDrawForeground(canvas); }注释已经解释的很清楚了，主要过程遵循如下几步：
绘制背景 drawBackground(canvas);绘制内容 onDraw(canvas);绘制children dispatchDraw(canvas);绘制装饰 onDrawForeground(canvas);View绘制过程的传递是通过dispatchDraw()来实现的，它会遍历调用所有子元素的draw方法。
感谢： https://blog.csdn.net/xfhy_/article/details/90270630#comments
参考：《Android 开发艺术探索》
