概述
ViewTree的绘制，起点是ViewRootImpl的performTraversals方法
private void performTraversals() {
...
windowSizeMayChange |= measureHierarchy(host, lp, res,desiredWindowWidth, desiredWindowHeight);
...
performLayout(lp, desiredWindowWidth, desiredWindowHeight);
...
performDraw();
...
}
View系统的绘制流程会从ViewRootImpl的performTraversals()方法中开始, 每一个视图的绘制过程都必须经历三个最主要的阶段onMeasure()、onLayout()和onDraw()。
measure
measure函数的作用是为整个View树计算实际的大小, 设置每个View对象的布局大小（“窗口”大小）。实际对应属性就是View中的mMeasuredHeight（高）和mMeasureWidth（宽）。方法中参数widthMeasureSpec和heightMeasureSpec, 这两个值分别用于确定视图的宽度和高度的规格和大小。
MeasureSpec的值由specSize和specMode共同组成的, 其中specSize记录的是大小, specMode记录的是规格。
| specMode | 含义 |
|---|---|
| EXACTLY | 表示父视图希望子视图的大小应该是由specSize的值来决定的。子元素将被限定在给定的边界里而忽略它本身大小 |
| AT_MOST | 表示子视图最多只能是specSize中指定的大小, 开发人员应该尽可能小得去设置这个视图, 并且保证不会超过specSize |
| UNSPECIFIED | 表示开发人员可以将视图按照自己的意愿设置成任意的大小, 没有任何限制。这种情况比较少见, 不太会用到 |
View的measure方法代码：
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
...
final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
final boolean isExactly = MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY &&
MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY;
final boolean matchingSize = isExactly &&
getMeasuredWidth() == MeasureSpec.getSize(widthMeasureSpec) &&
getMeasuredHeight() == MeasureSpec.getSize(heightMeasureSpec);
if (forceLayout || !matchingSize &&
(widthMeasureSpec != mOldWidthMeasureSpec ||
heightMeasureSpec != mOldHeightMeasureSpec)) {
// first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded();
int cacheIndex = forceLayout ? -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 {
...
}
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
...
}
View的onMeasure默认实现：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
子View不可重写的setMeasureDimension方法
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
...
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
...
}
measure()这个方法是final的, 因此我们无法在子类中去重写这个方法, 说明Android是不允许我们改变View的measure框架的。measure方法中会调用onMeasure方法。
FrameLayout的onMeasure实现：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
...
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
...
}
}
...
}
ViewGroup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);
}
layout
private void performLayout(WindowManager.LayoutParams lp,
int desiredWindowWidth, int desiredWindowHeight) {
...
final View host = mView;
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
...
}
传入测量出的宽度、高度。
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);
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 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 &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
layout()方法接收四个参数, 分别代表着左、上、右、下的坐标, 当然这些坐标是相对于当前视图的父视图而言的。在layout()方法中, 首先会调用setFrame()方法来判断视图的大小是否发生过变化, 以确定有没有必要对当前的视图进行重绘, 同时还会在这里把传递过来的四个参数分别赋值给mLeft、mTop、mRight和mBottom这几个变量。View中的onLayout()方法就是一个空方法, 因为onLayout()过程是为了确定视图在布局中所在的位置, 而这个操作应该是由布局来完成的, 即父视图决定子视图的显示位置。
ViewGroup的onLayout:
protected abstract void onLayout(boolean changed,
int l, int t, int r, int b);
所有ViewGroup的子类都必须重写这个方法。在onLayout()过程结束后, 我们就可以调用getWidth()方法和getHeight()方法来获取视图的宽高了。
getWidth()方法和getMeasureWidth()方法到底有什么区别
getMeasureWidth()方法在measure()过程结束后就可以获取到了, 而getWidth()方法要在layout()过程结束后才能获取到。另外, getMeasureWidth()方法中的值是通过setMeasuredDimension()方法来进行设置的, 而getWidth()方法中的值则是通过视图右边的坐标减去左边的坐标计算出来的。
draw
ViewRootImpl首先会判断是否要硬件加速。没有硬件加速，走drawSoftware方法：
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {
...
// Draw with software renderer.
final Canvas canvas;
...//设置canvas
mView.draw(canvas);
...
}
View中的draw方法：
public void draw(Canvas canvas) {
...
/*
* 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 (scrollbars)
onDrawScrollBars(canvas);
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// we're done...
return;
}
...
}
protected void onDraw(Canvas canvas) {
}
/**
* Called by draw to draw the child views. This may be overridden
* by derived classes to gain control just before its children are drawn
* (but after its own view has been drawn).
*/
protected void dispatchDraw(Canvas canvas) {
}
View的onDraw为空方法, 因为每个视图的内容部分肯定都是各不相同的, 这部分的功能需交给子类去实现。dispatchDraw这一步的作用是对当前视图的所有子视图进行绘制。但如果当前的视图没有子视图, 那么也就不需要进行绘制了。因此你会发现View中的dispatchDraw()方法又是一个空方法，而ViewGroup的dispatchDraw()方法中就会有具体的绘制代码。onDrawScrollBars 是对视图的滚动条进行绘制。
其他
窗口的UI最终是需要通过SurfaceFlinger服务来统一渲染的, 而SurfaceFlinger服务在渲染窗口的UI之前, 需要计算基于各个窗口的Z轴位置来计算它们的可见区域。而WindowManagerService服务就是负责计算好每一个窗口的Z轴位置之后, 还需要将它们设置到SurfaceFlinger服务中去, 以便SurfaceFlinger服务可以正确地渲染每一个窗口的UI。