View的工作原理
View的绘制是从ViewRootImpl的setView开始的，
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView,
int userId) {
。。。
requestLayout();
。。。
setView方法调用了requestLayout
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
requestLayout（）中执行了scheduleTraversals（Traversals的意思是遍历）
@UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, trackingBug = 170729553)
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
scheduleTraversals中又执行了mTraversalRunnable；
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
mTraversalRunnable执行了doTraversal（）方法
void doTraversal() {
if (mTraversalScheduled) {
mTraversalScheduled = false;
mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
if (mProfile) {
Debug.startMethodTracing("ViewAncestor");
}
performTraversals();
if (mProfile) {
Debug.stopMethodTracing();
mProfile = false;
}
}
}
最后走到了PerformTraversals（）
performTraversals方法中又三个比较重要的方法
performMeasure---测量
performLayout---布局
performDraw---绘制
performMeasure流程
image.png
首先跟据屏幕的宽高和DecorView的LayoutParams计算出DecorView的MeasureSpec；
那么DecorView的LayoutParams在哪呢？---待分析
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.
// 精确模式，大小是窗口大小
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
break;
case ViewGroup.LayoutParams.WRAP_CONTENT:
// Window can resize. Set max size for root view.
// 最大模式，大小不得超过窗口大小
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break;
default:
// Window wants to be an exact size. Force root view to be that size.
// 精确大小，大小即为rootDimension
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}
分析都在上面代码的注释中；
由此，DecorView的MeasureSpec已经生成，下面就会调用DecorView的Measure方法
image.png
为啥说此处的mView就是DecorView，是在调用setView方法的时候传入的，此处不在赘述；
需要去看WindowManager.addView的流程；
image.png
由于DecorView本身是一个FramLayout,所以他的Measure过程肯定会遵循View，ViewGroup和FramLayout的measure过程；
由于ViewGroup和FrameLayout都没有实现measure，所以只会执行View的measure方法；
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
onMeasure(widthMeasureSpec, heightMeasureSpec);
}
View的默认onMeasure方法实现：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
但是FrameLayout会复习这个方法：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
mMatchParentChildren.clear();
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
// 注释1调用ViewGroup的方法
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
maxWidth = Math.max(maxWidth,
child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
maxHeight = Math.max(maxHeight,
child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
childState = combineMeasuredStates(childState, child.getMeasuredState());
if (measureMatchParentChildren) {
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
mMatchParentChildren.add(child);
}
}
}
}
// Account for padding too
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
// Check against our minimum height and width
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
// Check against our foreground's minimum height and width
final Drawable drawable = getForeground();
if (drawable != null) {
maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
}
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
count = mMatchParentChildren.size();
if (count > 1) {
for (int i = 0; i < count; i++) {
final View child = mMatchParentChildren.get(i);
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec;
if (lp.width == LayoutParams.MATCH_PARENT) {
final int width = Math.max(0, getMeasuredWidth()
- getPaddingLeftWithForeground() - getPaddingRightWithForeground()
- lp.leftMargin - lp.rightMargin);
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
width, MeasureSpec.EXACTLY);
} else {
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
lp.leftMargin + lp.rightMargin,
lp.width);
}
final int childHeightMeasureSpec;
if (lp.height == LayoutParams.MATCH_PARENT) {
final int height = Math.max(0, getMeasuredHeight()
- getPaddingTopWithForeground() - getPaddingBottomWithForeground()
- lp.topMargin - lp.bottomMargin);
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
height, MeasureSpec.EXACTLY);
} else {
childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
lp.topMargin + lp.bottomMargin,
lp.height);
}
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}
因此方法会直接调用FrameLayout里面的onMeasure方法，并不会走到View里面的onMeasure方法；
FrameLayout首先执行了一个for循环，让他的每个子view都去执行measureChildWithMargin；
注释1处调用了ViewGroup的measureChildWithMargin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);
// 注释1，又开始调用view的measure方法
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
从注释1可以看出，这个方法是viewGroup递归的开始，也就是
view.measure会调用view的onMeasure方法
---》如果view是一个ViewGroup的具体实现类，则会调用实现类的onMeasure，比如framlayout方法
---》实现类一般会使用for循环，去遍历子view
---》子view如果依旧是一个ViewGroup的实现类，则会继续调用子VIew的onMeasure，继续遍历子View的measure
---》直到子VIew不是一个ViewGroup类型，则调用子VIew的onMeasure方法完成测量
上面的measureChildWithMargins总共分成了2步；
1.通过父容器的MeasureSpec和子元素本身的LayoutParam决定子View的MeasureSpec
2.调用子view的measure方法；
通过这个递归，就会得到所有FramLayout所有子View的测量宽高，也就是mMeasuredWidth和mMeasuredHeight
最后就是比大小，由于FrameLayout的宽高是由所有子VIew中的最大宽和所有子view中的最大高决定的（当然还要考虑一下margin）；
就得的了frameLayout自己的宽高，也就是DecorView的宽高；
在讨论ViewGroup的测量过程，是不能直接看ViewGroup的，因为ViewGroup毕竟是个抽象的，他不会直接实现onMeasure方法，他是把这个过程交给子类去实现的，由子类的for循环+viewgroup的measureChildWithMargins完成整个View树的测量，
为啥ViewGroup不自己实现呢，因为他也不知道自己的布局是啥样呀；
整个测量过程一定是父容器的MeasureSpec+自身的layoutParam一起，决定了自身的MeasureSpec，等自身的MeasureSpec确定了，就可以通过onMeasure方法去完成最终的测量，所有顶端的View就是DecorView，那么DecorView的父容器就只能从他的Window中去获取；
完成测量后，整个view树上的所有的view都会有两个测量值，mMeasuredWidth和mMeasuredHeight，通过setMeasuredDimensionRaw方法设置；这也是measure阶段的产出，供给layout使用的；
performLayout布局
private void performTraversals() {
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);//1
...
performLayout(lp, mWidth, mHeight); //2
...
performDraw();//3
}
传入的三个参数：
lp：window的layoutParam
mWidth：
mHeight：
image.png
此处的host就是DecorView
image.png
此处先通过setFrame设置了自身的属性；mLeft，mTop，mRight，mBottom
然后在调用view的onLayout方法，
image.png
可以看到这边是一个空实现，为啥会是空的呢，因为view自己的属性值刚刚已经通过setFrame方法自己设置了啊；
由于DecorView是一个FrameLayout，所有虽然不会View的onLayout为空，但是会执行
FrameLayout的onLayout方法；
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
final int count = getChildCount();
final int parentLeft = getPaddingLeftWithForeground();
final int parentRight = right - left - getPaddingRightWithForeground();
final int parentTop = getPaddingTopWithForeground();
final int parentBottom = bottom - top - getPaddingBottomWithForeground();
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final int width = child.getMeasuredWidth();
final int height = child.getMeasuredHeight();
int childLeft;
int childTop;
int gravity = lp.gravity;
if (gravity == -1) {
gravity = DEFAULT_CHILD_GRAVITY;
}
final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
if (!forceLeftGravity) {
childLeft = parentRight - width - lp.rightMargin;
break;
}
case Gravity.LEFT:
default:
childLeft = parentLeft + lp.leftMargin;
}
switch (verticalGravity) {
case Gravity.TOP:
childTop = parentTop + lp.topMargin;
break;
case Gravity.CENTER_VERTICAL:
childTop = parentTop + (parentBottom - parentTop - height) / 2 +
lp.topMargin - lp.bottomMargin;
break;
case Gravity.BOTTOM:
childTop = parentBottom - height - lp.bottomMargin;
break;
default:
childTop = parentTop + lp.topMargin;
}
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
此处又是通过一个for循环，完成递归，可以看出来的是，每一个子view都是通过先确定子view的left和top，再根据测量宽高mMeasuredWidth和mMeasuredHeight，计算出他的right和bottom；
还是很容易理解的；
mMeasuredWidth:View的测量宽
mMeasuredHeight：View的测量高
getWidth（）：实际宽
getHeight（）：实际高
image.png
所有实际宽高是在layout产生的；
layout阶段作用根据测量阶段的宽高算出他的四个值，
mLeft，mTop，mRight，mBottom
而这四个值，又会对draw阶段产生实际影响
performDraw流程
明天继续
