目录：
目录
一、问题：
首先还是来看一种情况，我们在Activity的三个地方查看View的measureHeight属性，因为只有measure后的View才具有measureHeight属性，否则为0，
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
mHandle = Handler(Looper.getMainLooper())
Log.d("MainActivity", "onCreate btn_view.measuredHeight ==> " + btn_view.measuredHeight.toString())
btn_view.post {
Log.d("MainActivity", "post btn_view.measuredHeight ==> " + btn_view.measuredHeight.toString())
}
}
override fun onResume() {
super.onResume()
Log.d("MainActivity", "onResume btn_view.measuredHeight ==> " + btn_view.measuredHeight.toString())
}
我们再看一下log
2020-10-13 10:22:26.067 22953-22953/? D/MainActivity: onCreate btn_view.measuredHeight ==> 0
2020-10-13 10:22:26.074 22953-22953/? D/MainActivity: onResume btn_view.measuredHeight ==> 0
2020-10-13 10:22:26.233 22953-22953/? D/MainActivity: post btn_view.measuredHeight ==> 144
这里显示，只有View.post()里面，才打印出了View的measureHeight属性，这就说明，在onCreate方法和onResume方法执行时，我们的View都还没被measure
二、onCreate()方法、onResume()方法
之前我们分析过，setContentView的源码，为了看仔细一点，我们选择看继承Avtivity的setContentView的源码，而Activity里面的setContentView是调用的PhoneWindow的setContentView，所以PhoneWindow的setContentView
public void setContentView(int layoutResID) {
if (mContentParent == null) {
//创建DecorView和mContentParent
//并将我们系统的布局android.R.layout.content添加到了mDecor上
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
} else {
mLayoutInflater.inflate(layoutResID, mContentParent);
}
//...
}
这里的setContent只是创建了DecorView并把我们的布局添加到了DecorView上，但是还没有调用onMeasure方法。
我们再从Activity的启动流程入手，对于ActivityThread里面的performLaunchActivity方法，它里面通过反射调用了Activity的onCreate方法
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
//...
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
//...
}
而ActivityThread的handleResumeActivity方法，他里面调用了performResumeActivity方法
public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
String reason) {
//...
final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
//...
if (r.window == null && !a.mFinished && willBeVisible) {
ViewManager wm = a.getWindowManager();
//...
View decor = r.window.getDecorView();
if (a.mVisibleFromClient) {
if (!a.mWindowAdded) {
wm.addView(decor, l);
}
}
}
if (!r.activity.mFinished && willBeVisible && r.activity.mDecor != null && !r.hideForNow) {
//...
if ((l.softInputMode
& WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION)
!= forwardBit) {
if (r.activity.mVisibleFromClient) {
ViewManager wm = a.getWindowManager();
View decor = r.window.getDecorView();
wm.updateViewLayout(decor, l);
}
}
if (r.activity.mVisibleFromClient) {
r.activity.makeVisible();
}
}
}
performResumeActivity里面调用了performResume，这里才去执行了我们Activity的onResume方法，
public ActivityClientRecord performResumeActivity(IBinder token, boolean finalStateRequest,
String reason) {
try {
r.activity.performResume(r.startsNotResumed, reason);
}catch (Exception e) {
//...
}
}
handleResumeActivity方法里面调用完performResumeActivity之后，才调用的wm.addView(decor, l);，，他是从这里才把DecorView加载到ViewManager里面，这时才开始View的绘制流程：measure() -> layout() -> draw()
所以，这里就解释了，为什么我们在onCreate()和onResume()里面获取不到View的measureHeight属性，就是因为onCreate和obResume方法执行的时候，View的绘制还没开始。而View.post()里面为什么就可以了呢？
三、View.post()方法
我们看一下View.post的源码
public boolean post(Runnable action) {
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
return attachInfo.mHandler.post(action);
}
getRunQueue().post(action);
return true;
}
这里可以看到，post进来之后，他是调用的HandlerActionQueue.java类里面的post方法
public void post(Runnable action) {
postDelayed(action, 0);
}
public void postDelayed(Runnable action, long delayMillis) {
final HandlerAction handlerAction = new HandlerAction(action, delayMillis);
synchronized (this) {
if (mActions == null) {
mActions = new HandlerAction[4];
}
//入队列
mActions = GrowingArrayUtils.append(mActions, mCount, handlerAction);
mCount++;
}
}
post进来之后的cation会保存进队列中，但是没有执行，而是在View的dispatchAttachedToWindow方法中执行，而dispatchAttachedToWindow方法就是在测量完毕之后才调用的
void dispatchAttachedToWindow(AttachInfo info, int visibility) {
//...
// Transfer all pending runnables.
if (mRunQueue != null) {
//里面是个for循环，去执行队列中的全部Action
mRunQueue.executeActions(info.mHandler);
mRunQueue = null;
}
}
所以，到了这里，我们也就基本解释清楚了，为什么onCreate、onResume和View.post里面获取View的measureHeight属性出现的情况了。
四、总结（过程展示）
执行过程：
image-20201013162709179.png
五、WindowManagerImpl
当我们调用Activity的startActivity()时，最终会调用ActivityThread的handleLaunchActivity()方法来创建Activity的
public Activity handleLaunchActivity(ActivityClientRecord r,
PendingTransactionActions pendingActions, Intent customIntent) {
//...
//创建Activity，这里面会调用到Activity的onCreate()方法（通过反射）
final Activity a = performLaunchActivity(r, customIntent);
if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
reportSizeConfigurations(r);
if (!r.activity.mFinished && pendingActions != null) {
pendingActions.setOldState(r.state);
pendingActions.setRestoreInstanceState(true);
pendingActions.setCallOnPostCreate(true);
}
}
//...
return a;
}
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
//创建Activity
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
//初始化时已经创建好了，这里只是获得
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
//调用Activity的attch方法
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config,
r.referrer, r.voiceInteractor, window, r.configCallback,
r.assistToken);
//调用Activity的onCreate()方法
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
}
执行完handleLaunchActivity()，就会继续去执行handleResumeActivity()方法
我们在ActivityThread的handleResumeActivity()方法中，看到了他内部调用了ViewManager的addView()和updateViewLayout()的方法，而ViewManager的这两个方法是在他的实现类WindowManagerImpl里面实现的
public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
String reason) {
//...
//这里面回去调用Activity的onResume()方法
final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
//...
ViewManager wm = a.getWindowManager();
//把decorView传进了addView和updateViewLayout里面了
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
ViewManager wm = a.getWindowManager();
if(){
wm.addView(decor, l);
}else{
wm.updateViewLayout(decor, l);
}
}
public ActivityClientRecord performResumeActivity(IBinder token, boolean finalStateRequest,
String reason) {
//调用Activity的onResume()方法
r.activity.performResume(r.startsNotResumed, reason);
}
我们进到WindowManagerImpl类中，他调用的是mGlobal的方法
private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();
@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
//此时的view(DecorView继续被传递)
mGlobal.addView(view, params, mContext.getDisplayNoVerify(), mParentWindow,
mContext.getUserId());
}
@Override
public void updateViewLayout(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.updateViewLayout(view, params);
}
再往里面进去
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow, int userId) {
//...
ViewRootImpl root;
View panelParentView = null;
//实例化了ViewRootImpl的对象root
root = new ViewRootImpl(view.getContext(), display);
//这里的View就是外面传进来的DecorView，
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
// do this last because it fires off messages to start doing things
try {
//**重点**，这里把DecorView加到了ViewRootImpl上，即把DecorView加载到了PhoneWindow上
root.setView(view, wparams, panelParentView, userId);
} catch (RuntimeException e) {
// BadTokenException or InvalidDisplayException, clean up.
if (index >= 0) {
removeViewLocked(index, true);
}
throw e;
}
}
从这里我们发现一个事情，我们以前一直以为DecorView就是PhoneWindow下面的根布局，但是从这里，我们可以看到，DecorView也是加到ViewRootImpl上面的root.setView(view, wparams, panelParentView, userId);
虽然DecorView被加载到了PhoneWindow上，但是此时界面仍然没有什么显示，因为View的工作流程还没有执行完，还需要经过measure、layout、draw过程
ViewRootImpl.java
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView,
int userId) {
//...
requestLayout();
//...
}
public void requestLayout() {
//...
scheduleTraversals();
}
void scheduleTraversals() {
//...
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
//...
}
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}
//上面这个类的实例化对象
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
void doTraversal() {
//...
performTraversals();
//...
}
终于，我们追到了performTraversals()方法，网上很多文章，也就是从这里开始讲的
六、重点：View的绘制流程
//root.setView(view, wparams, panelParentView); 使DecorView加载到了PhoneWindow上，但是不可见
//performTraversals() 这个方法使ViewTree开始View的工作流程，让DecorView上面的东西开始展示出来
private void performTraversals() {
getRunQueue().executeActions(mAttachInfo.mHandler);
//...
//根据DecorView自身的LayoutParams，和WindowSize去获取DecorView的MeasureSpec，然后传给performMeasure()方法
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
//...
performLayout(lp, mWidth, mHeight);
//...
performDraw();
}
//第一个参数是window的大小
//所以，对于DecorView来说，她的Measure是由他自身的LayoutParams和Window的大小决定的。这和一般的View不同
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;
}
思考？
MeasureSpec是收自身的LayoutParams和父容器的MeasureSpec共同影响的。那么，作为顶层View的DecorView来所，其并没有父容器，那么他的MeasureSpec是怎么得来的呢？？？？
对于DecorView来说，她的Measure是由他自身的LayoutParams和Window的大小决定的。这和一般的View不同
1. 绘制流程第一步performMeasure
ViewRootImpl.java
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
try {
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
在View.java中查看measure方法
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
if (cacheIndex < 0 || sIgnoreMeasureCache) {
// measure ourselves, this should set the measured dimension flag back
//调用View里面的onMeasure方法，测量开始
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;
}
}
这个方法里面调用了onMeasure方法，而此时onMeasure测量的是我们的根布局DecorView方法，测量了DecorView后，我们又会用onMeasure测量我们布局的根布局
1）View的onMeasure()方法
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) {
//返回外面传进来的getDefaultSize()
case MeasureSpec.UNSPECIFIED:
result = size;
break;
//说明wrap_content和match_parent属性的效果是一样的
//因此，在实现自定义View的warp_content时，需要重写onMeasure
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
//根据是否设置背景来确定返回值
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
//Drawable.java
public int getMinimumWidth() {
//得到Drawable的固有宽度
final int intrinsicWidth = getIntrinsicWidth();
//宽度大于0则返回，否则返回0
return intrinsicWidth > 0 ? intrinsicWidth : 0;
}
在View的onMeasure方法里面，调用了setMeasuredDimension()，它里面主要是用来设置View的宽高的。
而在传参数到setMeasuredDimension()时，还调用了getDefaultSize()方法对其进行处理，而它里面则是根据MeasureSpec的Mode来返回不同的值。
对于UNSPECIFIED模式，直接返回外面传进来的返回外面传进来的getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec)值。而getSuggestedMinimumWidth()方法是根据用户是否设置了View的background来确定其返回值的。如果没有背景，则返回mMinWidth值，mMinWidth是可以在外部设置的，可以是Android:minWidth也可以是view.setMinimumWidth()来设置。如果设置了背景，则取mMinWidth和mBackground.getMinimumWidth()的最大值作为返回值，由于mBackground是Drawable对象，所以在Drawable的源码里面查看getMinimumWidth()的实现。
由于AT_MOST模式和EXACTLY是一样的，所以在实现自定义View的warp_content时候，需要重写onMeasure，下面是模板代码：
private int measureWidth(int measureSpec){
int result = 0;
int mode = MeasureSpec.getMode(measureSpec);
int size = MeasureSpec.getSize(measureSpec);
if (mode == MeasureSpec.EXACTLY){//精确模式
result = size;
}else{//AT_MOST或UNSPECIFIED
result = 200;
if (mode == MeasureSpec.AT_MOST){//最小值
result = Math.min(result,size);
}
}
return result;
}
2）ViewGroup的onMeasure()方法
（这里以LinearLayout为例）
LinearLayout.java
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//首先判断的是orientation
if (mOrientation == VERTICAL) {
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
measureHorizontal(widthMeasureSpec, heightMeasureSpec);
}
}
void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
//记录垂直方向的高度
mTotalLength = 0;
//通过for循环去拿我们里面的子布局
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child == null) {
mTotalLength += measureNullChild(i);
continue;
}
//把每个子View的高度加起来并赋值给mTotalLength
mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
measureChildBeforeLayout(child, i, widthMeasureSpec, 0,
heightMeasureSpec, usedHeight);
}
//测完子View，还得加上LinearLayout的Padding属性
mTotalLength += mPaddingTop + mPaddingBottom;
//测量完之后，接着调用这个方法，我们的布局才有了真正的宽和高，这里进去是给measuredWidth和measuredHeight赋值
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
heightSizeAndState);
}
void measureChildBeforeLayout(View child, int childIndex,
int widthMeasureSpec, int totalWidth, int heightMeasureSpec,
int totalHeight) {
measureChildWithMargins(child, widthMeasureSpec, totalWidth,
heightMeasureSpec, totalHeight);
}
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);
//通过调用子View的onMeasure方法，去测量子View的宽高，这里的两个参数childXXXMeasureSpec都是父布局已经给我们测量好了的
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
ViewGroup.java里面查看测量的方式getChildMeasureSpec()方法
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
//获取模式和大小
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
//根据父布局的模式去给子View的resultSize、resultMode赋值
switch (specMode) {
// Parent has imposed an exact size on us
//当父布局是match_parent、fill_parent(Deprecated)、明确的值
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {//当ViewGroup中的子view是设置了具体大小的值时( X dp)
//子View的大小就等于设置的子View的大小
resultSize = childDimension;
//mode就是EXACTLY
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {//如果子View外面设置的大小是MATCH_PARENT
// Child wants to be our size. So be it.
//子View的大小就等于父布局的大小
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
//当父布局是warp_content时
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension;
//子View被设置成了明确的值，那么他的模式直接就是EXACTLY，不受父布局的影响
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;
//当父布局是AT_MOST模式时，即使子View是MATCH_PARENT，他的模式，最终还是AT_MOST，所以，子View的mode是父布局的mode和自己一起决定的
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);
}
【注】：
当父布局是AT_MOST模式时，即使子View是MATCH_PARENT，他的模式，最终还是AT_MOST，所以，子View的mode是父布局的mode和自己一起决定的
调用完measure方法之后，就会调用setMeasuredDimension()方法，这个时候我们的布局才真正指定宽度和高度，measuredWidth和measuredHeight才真正有值
void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
//通过for循环去拿我们里面的子布局
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (childWeight > 0) {
//记录所有子View的高，方便计算自己的高，这里我们看的是Vertical方向的LinearLayout，所以他只是计算的Height
if (mUseLargestChild && heightMode != MeasureSpec.EXACTLY) {
childHeight = largestChildHeight;
} else if (lp.height == 0 && (!mAllowInconsistentMeasurement
|| heightMode == MeasureSpec.EXACTLY)) {
childHeight = share;
} else {
childHeight = child.getMeasuredHeight() + share;
}
final int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
Math.max(0, childHeight), MeasureSpec.EXACTLY);
final int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin,
lp.width);
//去测量子View
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
// Child may now not fit in vertical dimension.
childState = combineMeasuredStates(childState, child.getMeasuredState()
& (MEASURED_STATE_MASK>>MEASURED_HEIGHT_STATE_SHIFT));
}
measureChildBeforeLayout(child, i, widthMeasureSpec, 0,
heightMeasureSpec, usedHeight);
}
//测量完之后，接着调用这个方法，我们的布局才有了真正的宽和高，这里进去是给measuredWidth和measuredHeight赋值
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
heightSizeAndState);
}
总结：
测量都是从外往里递归，他先会把ViewRootImpl的测量模式传到DecorView，然后DecorView再把测量模式传到我们的根布局，依次向底部传，然后，最底层的View会拿子View的宽高来计算自己的宽高，依次向外传
image-20201013215655174.png
2. 绘制流程第二步performLayout
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
final View host = mView;
try {
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
}
}
public void layout(int l, int t, int r, int b) {
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
}
}
1）View的onLayout()方法
View的onLayout是一个空方法，因为没有一个统一的样式，所以留给自定义View自己去实现
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
2）ViewGroup的onLayout()方法
走到这里，基本的步骤流程差不多定好了，我们去看具体的View中是怎么实现的，还是用LinearLayout举例
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);
}
}
void layoutVertical(int left, int top, int right, int bottom) {
final int paddingLeft = mPaddingLeft;
int childTop;
int childLeft;
// Where right end of child should go
//计算父窗口推荐的子View宽度
final int width = right - left;
//计算父窗口推荐的子View右侧位置
int childRight = width - mPaddingRight;
// Space available for child
//child可使用空间大小
int childSpace = width - paddingLeft - mPaddingRight;
//通过ViewGroup的getChildCount方法获取ViewGroup的子View个数
final int count = getVirtualChildCount();
//获取Gravity属性设置
final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
//依据majorGravity计算childTop的位置值
switch (majorGravity) {
case Gravity.BOTTOM:
// mTotalLength contains the padding already
childTop = mPaddingTop + bottom - top - mTotalLength;
break;
// mTotalLength contains the padding already
case Gravity.CENTER_VERTICAL:
childTop = mPaddingTop + (bottom - top - mTotalLength) / 2;
break;
case Gravity.TOP:
default:
childTop = mPaddingTop;
break;
}
//重点！！！开始遍历
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child == null) {
childTop += measureNullChild(i);
} else if (child.getVisibility() != GONE) {
//LinearLayout中其子视图显示的宽和高由measure过程来决定的，因此measure过程的意义就是为layout过程提供视图显示范围的参考值
final int childWidth = child.getMeasuredWidth();
final int childHeight = child.getMeasuredHeight();
//获取子View的LayoutParams
final LinearLayout.LayoutParams lp =
(LinearLayout.LayoutParams) child.getLayoutParams();
int gravity = lp.gravity;
if (gravity < 0) {
gravity = minorGravity;
}
final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
//依据不同的absoluteGravity计算childLeft位置
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = paddingLeft + ((childSpace - childWidth) / 2)
+ lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
childLeft = childRight - childWidth - lp.rightMargin;
break;
case Gravity.LEFT:
default:
childLeft = paddingLeft + lp.leftMargin;
break;
}
if (hasDividerBeforeChildAt(i)) {
childTop += mDividerHeight;
}
childTop += lp.topMargin;
//通过垂直排列计算调运child的layout设置child的位置
setChildFrame(child, childLeft, childTop + getLocationOffset(child),
childWidth, childHeight);
childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
i += getChildrenSkipCount(child, i);
}
}
}
总结：
layout也是从顶层父View向子View的递归调用view.layout方法的过程，即父View根据第一步performMeasure，来获取子View所的布局大小和布局参数，将子View放在合适的位置上，==不过这个方法没有再往外走，只是不断的往里面走==。
3.绘制流程第三步performDraw
1）View的draw()方法
View的draw流程很简单，官方指定了6步
/*
* Draw traversal performs several drawing steps which must be executed
* in the appropriate order:
*
* 1. Draw the background:如果需要，则绘制背景
* 2. If necessary, save the canvas' layers to prepare for fading：保存当前camvas层
* 3. Draw view's content：绘制View的内容
* 4. Draw children：绘制子View
* 5. If necessary, draw the fading edges and restore layers：如果需要，则绘制View的褪色边缘，这类似于阴影效果
* 6. Draw decorations (scrollbars for instance)：绘制装饰，比如滚动条
*/
2）ViewGroup的draw()方法
performDraw和上面的流程类似
private void performDraw() {
try {
draw(fullRedrawNeeded);
} finally {
mIsDrawing = false;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
private void draw(boolean fullRedrawNeeded) {
Surface surface = mSurface;
if (!surface.isValid()) {
return;
}
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
return;
}
}
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {
// Draw with software renderer.
final Canvas canvas;
final int left = dirty.left;
final int top = dirty.top;
final int right = dirty.right;
final int bottom = dirty.bottom;
canvas = mSurface.lockCanvas(dirty);
// ... ...
mView.draw(canvas);
}
View的绘制流程其实就三个步骤：onMeasure(测量) -> onLayout(摆放) -> onDraw(绘制)
