View的繪制是從ViewRootImpl的setView開始的，
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView,
int userId) {
。。。
requestLayout();
。。。
setView方法調(diào)用了requestLayout
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
requestLayout（）中執(zhí)行了scheduleTraversals（Traversals的意思是遍歷）
@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中又執(zhí)行了mTraversalRunnable；
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
mTraversalRunnable執(zhí)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方法中又三個(gè)比較重要的方法
performMeasure---測量
performLayout---布局
performDraw---繪制
performMeasure流程
首先跟據(jù)屏幕的寬高和DecorView的LayoutParams計(jì)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.
// 精確模式，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，大小不得超過窗口大小
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break;
default:
// Window wants to be an exact size. Force root view to be that size.
// 精確大小，大小即為rootDimension
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}
分析都在上面代碼的注釋中；
由此，DecorView的MeasureSpec已經(jīng)生成，下面就會(huì)調(diào)用DecorView的Measure方法
為啥說此處的mView就是DecorView，是在調(diào)用setView方法的時(shí)候傳入的，此處不在贅述；
需要去看WindowManager.addView的流程；
由于DecorView本身是一個(gè)FramLayout,所以他的Measure過程肯定會(huì)遵循View，ViewGroup和FramLayout的measure過程；
由于ViewGroup和FrameLayout都沒有實(shí)現(xiàn)measure，所以只會(huì)執(zhí)行View的measure方法；
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
onMeasure(widthMeasureSpec, heightMeasureSpec);
}
View的默認(rèn)onMeasure方法實(shí)現(xiàn)：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
但是FrameLayout會(huì)復(fù)習(xí)這個(gè)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) {
// 注釋1調(diào)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);
}
}
}
因此方法會(huì)直接調(diào)用FrameLayout里面的onMeasure方法，并不會(huì)走到View里面的onMeasure方法；
FrameLayout首先執(zhí)行了一個(gè)for循環(huán)，讓他的每個(gè)子view都去執(zhí)行measureChildWithMargin；
注釋1處調(diào)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);
// 注釋1，又開始調(diào)用view的measure方法
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
從注釋1可以看出，這個(gè)方法是viewGroup遞歸的開始，也就是
view.measure會(huì)調(diào)用view的onMeasure方法
---》如果view是一個(gè)ViewGroup的具體實(shí)現(xiàn)類，則會(huì)調(diào)用實(shí)現(xiàn)類的onMeasure，比如framlayout方法
---》實(shí)現(xiàn)類一般會(huì)使用for循環(huán)，去遍歷子view
---》子view如果依舊是一個(gè)ViewGroup的實(shí)現(xiàn)類，則會(huì)繼續(xù)調(diào)用子VIew的onMeasure，繼續(xù)遍歷子View的measure
---》直到子VIew不是一個(gè)ViewGroup類型，則調(diào)用子VIew的onMeasure方法完成測量
上面的measureChildWithMargins總共分成了2步；
1.通過父容器的MeasureSpec和子元素本身的LayoutParam決定子View的MeasureSpec
2.調(diào)用子view的measure方法；
通過這個(gè)遞歸，就會(huì)得到所有FramLayout所有子View的測量寬高，也就是mMeasuredWidth和mMeasuredHeight
最后就是比大小，由于FrameLayout的寬高是由所有子VIew中的最大寬和所有子view中的最大高決定的（當(dāng)然還要考慮一下margin）；
就得的了frameLayout自己的寬高，也就是DecorView的寬高；
在討論ViewGroup的測量過程，是不能直接看ViewGroup的，因?yàn)閂iewGroup畢竟是個(gè)抽象的，他不會(huì)直接實(shí)現(xiàn)onMeasure方法，他是把這個(gè)過程交給子類去實(shí)現(xiàn)的，由子類的for循環(huán)+viewgroup的measureChildWithMargins完成整個(gè)View樹的測量，
為啥ViewGroup不自己實(shí)現(xiàn)呢，因?yàn)樗膊恢雷约旱牟季质巧稑友剑?/p>
整個(gè)測量過程一定是父容器的MeasureSpec+自身的layoutParam一起，決定了自身的MeasureSpec，等自身的MeasureSpec確定了，就可以通過onMeasure方法去完成最終的測量，所有頂端的View就是DecorView，那么DecorView的父容器就只能從他的Window中去獲??；
完成測量后，整個(gè)view樹上的所有的view都會(huì)有兩個(gè)測量值，mMeasuredWidth和mMeasuredHeight，通過setMeasuredDimensionRaw方法設(shè)置；這也是measure階段的產(chǎn)出，供給layout使用的；
performLayout布局
private void performTraversals() {
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);//1
...
performLayout(lp, mWidth, mHeight); //2
...
performDraw();//3
}
傳入的三個(gè)參數(shù)：
lp：window的layoutParam
mWidth：
mHeight：
此處的host就是DecorView
此處先通過setFrame設(shè)置了自身的屬性；mLeft，mTop，mRight，mBottom
然后在調(diào)用view的onLayout方法，
可以看到這邊是一個(gè)空實(shí)現(xiàn)，為啥會(huì)是空的呢，因?yàn)関iew自己的屬性值剛剛已經(jīng)通過setFrame方法自己設(shè)置了??；
由于DecorView是一個(gè)FrameLayout，所有雖然不會(huì)View的onLayout為空，但是會(huì)執(zhí)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);
}
}
}
此處又是通過一個(gè)for循環(huán)，完成遞歸，可以看出來的是，每一個(gè)子view都是通過先確定子view的left和top，再根據(jù)測量寬高mMeasuredWidth和mMeasuredHeight，計(jì)算出他的right和bottom；
還是很容易理解的；
mMeasuredWidth:View的測量寬
mMeasuredHeight：View的測量高
getWidth（）：實(shí)際寬
getHeight（）：實(shí)際高
所有實(shí)際寬高是在layout產(chǎn)生的；
layout階段作用根據(jù)測量階段的寬高算出他的四個(gè)值，
mLeft，mTop，mRight，mBottom
而這四個(gè)值，又會(huì)對draw階段產(chǎn)生實(shí)際影響
performDraw流程
明天繼續(xù)
