目錄：
目錄
一魏宽、問(wèn)題：
首先還是來(lái)看一種情況，我們?cè)贏ctivity的三個(gè)地方查看View的measureHeight屬性决乎，因?yàn)橹挥衜easure后的View才具有measureHeight屬性队询，否則為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())
}
我們?cè)倏匆幌耹og
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
這里顯示构诚，只有View.post()里面蚌斩，才打印出了View的measureHeight屬性，這就說(shuō)明范嘱，在onCreate方法和onResume方法執(zhí)行時(shí)送膳，我們的View都還沒(méi)被measure
二员魏、onCreate()方法、onResume()方法
之前我們分析過(guò)肠缨，setContentView的源碼逆趋，為了看仔細(xì)一點(diǎn)，我們選擇看繼承Avtivity的setContentView的源碼晒奕，而Activity里面的setContentView是調(diào)用的PhoneWindow的setContentView闻书，所以PhoneWindow的setContentView
public void setContentView(int layoutResID) {
if (mContentParent == null) {
//創(chuàng)建DecorView和mContentParent
//并將我們系統(tǒng)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);
}
//...
}
這里的setContent只是創(chuàng)建了DecorView并把我們的布局添加到了DecorView上，但是還沒(méi)有調(diào)用onMeasure方法脑慧。
我們?cè)購(gòu)腁ctivity的啟動(dòng)流程入手魄眉，對(duì)于ActivityThread里面的performLaunchActivity方法，它里面通過(guò)反射調(diào)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方法闷袒，他里面調(diào)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里面調(diào)用了performResume坑律，這里才去執(zhí)行了我們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方法里面調(diào)用完performResumeActivity之后囊骤，才調(diào)用的wm.addView(decor, l);晃择，，他是從這里才把DecorView加載到ViewManager里面也物，這時(shí)才開(kāi)始View的繪制流程：measure() -> layout() -> draw()
所以宫屠，這里就解釋了，為什么我們?cè)趏nCreate()和onResume()里面獲取不到View的measureHeight屬性滑蚯，就是因?yàn)閛nCreate和obResume方法執(zhí)行的時(shí)候浪蹂，View的繪制還沒(méi)開(kāi)始。而View.post()里面為什么就可以了呢告材？
三坤次、View.post()方法
我們看一下View.post的源碼
public boolean post(Runnable action) {
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
return attachInfo.mHandler.post(action);
}
getRunQueue().post(action);
return true;
}
這里可以看到，post進(jìn)來(lái)之后斥赋，他是調(diào)用的HandlerActionQueue.java類里面的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];
}
//入隊(duì)列
mActions = GrowingArrayUtils.append(mActions, mCount, handlerAction);
mCount++;
}
}
post進(jìn)來(lái)之后的cation會(huì)保存進(jìn)隊(duì)列中缰猴，但是沒(méi)有執(zhí)行，而是在View的dispatchAttachedToWindow方法中執(zhí)行疤剑，而dispatchAttachedToWindow方法就是在測(cè)量完畢之后才調(diào)用的
void dispatchAttachedToWindow(AttachInfo info, int visibility) {
//...
// Transfer all pending runnables.
if (mRunQueue != null) {
//里面是個(gè)for循環(huán)洛波，去執(zhí)行隊(duì)列中的全部Action
mRunQueue.executeActions(info.mHandler);
mRunQueue = null;
}
}
所以，到了這里骚露，我們也就基本解釋清楚了蹬挤，為什么onCreate、onResume和View.post里面獲取View的measureHeight屬性出現(xiàn)的情況了棘幸。
四焰扳、總結(jié)（過(guò)程展示）
執(zhí)行過(guò)程：
image-20201013162709179.png
五、WindowManagerImpl
當(dāng)我們調(diào)用Activity的startActivity()時(shí)，最終會(huì)調(diào)用ActivityThread的handleLaunchActivity()方法來(lái)創(chuàng)建Activity的
public Activity handleLaunchActivity(ActivityClientRecord r,
PendingTransactionActions pendingActions, Intent customIntent) {
//...
//創(chuàng)建Activity吨悍，這里面會(huì)調(diào)用到Activity的onCreate()方法（通過(guò)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) {
//創(chuàng)建Activity
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
//初始化時(shí)已經(jīng)創(chuàng)建好了扫茅，這里只是獲得
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
//調(diào)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);
//調(diào)用Activity的onCreate()方法
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
}
執(zhí)行完handleLaunchActivity()，就會(huì)繼續(xù)去執(zhí)行handleResumeActivity()方法
我們?cè)贏ctivityThread的handleResumeActivity()方法中育瓜，看到了他內(nèi)部調(diào)用了ViewManager的addView()和updateViewLayout()的方法葫隙，而ViewManager的這兩個(gè)方法是在他的實(shí)現(xiàn)類WindowManagerImpl里面實(shí)現(xiàn)的
public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
String reason) {
//...
//這里面回去調(diào)用Activity的onResume()方法
final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
//...
ViewManager wm = a.getWindowManager();
//把decorView傳進(jìn)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) {
//調(diào)用Activity的onResume()方法
r.activity.performResume(r.startsNotResumed, reason);
}
我們進(jìn)到WindowManagerImpl類中，他調(diào)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);
//此時(shí)的view(DecorView繼續(xù)被傳遞)
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);
}
再往里面進(jìn)去
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow, int userId) {
//...
ViewRootImpl root;
View panelParentView = null;
//實(shí)例化了ViewRootImpl的對(duì)象root
root = new ViewRootImpl(view.getContext(), display);
//這里的View就是外面?zhèn)鬟M(jìn)來(lái)的DecorView躏仇，
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 {
//**重點(diǎn)**恋脚，這里把DecorView加到了ViewRootImpl上，即把DecorView加載到了PhoneWindow上
root.setView(view, wparams, panelParentView, userId);
} catch (RuntimeException e) {
// BadTokenException or InvalidDisplayException, clean up.
if (index >= 0) {
removeViewLocked(index, true);
}
throw e;
}
}
從這里我們發(fā)現(xiàn)一個(gè)事情焰手，我們以前一直以為DecorView就是PhoneWindow下面的根布局糟描，但是從這里，我們可以看到书妻，DecorView也是加到ViewRootImpl上面的root.setView(view, wparams, panelParentView, userId);
雖然DecorView被加載到了PhoneWindow上船响，但是此時(shí)界面仍然沒(méi)有什么顯示，因?yàn)閂iew的工作流程還沒(méi)有執(zhí)行完躲履，還需要經(jīng)過(guò)measure见间、layout、draw過(guò)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();
}
}
//上面這個(gè)類的實(shí)例化對(duì)象
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
void doTraversal() {
//...
performTraversals();
//...
}
終于工猜，我們追到了performTraversals()方法米诉，網(wǎng)上很多文章，也就是從這里開(kāi)始講的
六域慷、重點(diǎn)：View的繪制流程
//root.setView(view, wparams, panelParentView); 使DecorView加載到了PhoneWindow上荒辕，但是不可見(jiàn)
//performTraversals() 這個(gè)方法使ViewTree開(kāi)始View的工作流程汗销，讓DecorView上面的東西開(kāi)始展示出來(lái)
private void performTraversals() {
getRunQueue().executeActions(mAttachInfo.mHandler);
//...
//根據(jù)DecorView自身的LayoutParams犹褒，和WindowSize去獲取DecorView的MeasureSpec，然后傳給performMeasure()方法
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
//...
performLayout(lp, mWidth, mHeight);
//...
performDraw();
}
//第一個(gè)參數(shù)是window的大小
//所以弛针，對(duì)于DecorView來(lái)說(shuō)叠骑，她的Measure是由他自身的LayoutParams和Window的大小決定的。這和一般的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;
}
思考削茁？
MeasureSpec是收自身的LayoutParams和父容器的MeasureSpec共同影響的宙枷。那么，作為頂層View的DecorView來(lái)所茧跋，其并沒(méi)有父容器慰丛，那么他的MeasureSpec是怎么得來(lái)的呢？瘾杭？诅病？？
對(duì)于DecorView來(lái)說(shuō)，她的Measure是由他自身的LayoutParams和Window的大小決定的贤笆。這和一般的View不同
1. 繪制流程第一步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
//調(diào)用View里面的onMeasure方法蝇棉，測(cè)量開(kāi)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;
}
}
這個(gè)方法里面調(diào)用了onMeasure方法，而此時(shí)onMeasure測(cè)量的是我們的根布局DecorView方法芥永，測(cè)量了DecorView后篡殷，我們又會(huì)用onMeasure測(cè)量我們布局的根布局
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?zhèn)鬟M(jìn)來(lái)的getDefaultSize()
case MeasureSpec.UNSPECIFIED:
result = size;
break;
//說(shuō)明wrap_content和match_parent屬性的效果是一樣的
//因此，在實(shí)現(xiàn)自定義View的warp_content時(shí)埋涧，需要重寫onMeasure
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
//根據(jù)是否設(shè)置背景來(lái)確定返回值
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
//Drawable.java
public int getMinimumWidth() {
//得到Drawable的固有寬度
final int intrinsicWidth = getIntrinsicWidth();
//寬度大于0則返回板辽，否則返回0
return intrinsicWidth > 0 ? intrinsicWidth : 0;
}
在View的onMeasure方法里面，調(diào)用了setMeasuredDimension()飞袋，它里面主要是用來(lái)設(shè)置View的寬高的戳气。
而在傳參數(shù)到setMeasuredDimension()時(shí)，還調(diào)用了getDefaultSize()方法對(duì)其進(jìn)行處理巧鸭，而它里面則是根據(jù)MeasureSpec的Mode來(lái)返回不同的值瓶您。
對(duì)于UNSPECIFIED模式，直接返回外面?zhèn)鬟M(jìn)來(lái)的返回外面?zhèn)鬟M(jìn)來(lái)的getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec)值纲仍。而getSuggestedMinimumWidth()方法是根據(jù)用戶是否設(shè)置了View的background來(lái)確定其返回值的呀袱。如果沒(méi)有背景，則返回mMinWidth值郑叠，mMinWidth是可以在外部設(shè)置的夜赵，可以是Android:minWidth也可以是view.setMinimumWidth()來(lái)設(shè)置。如果設(shè)置了背景乡革，則取mMinWidth和mBackground.getMinimumWidth()的最大值作為返回值寇僧，由于mBackground是Drawable對(duì)象，所以在Drawable的源碼里面查看getMinimumWidth()的實(shí)現(xiàn)沸版。
由于AT_MOST模式和EXACTLY是一樣的嘁傀，所以在實(shí)現(xiàn)自定義View的warp_content時(shí)候，需要重寫onMeasure视粮，下面是模板代碼：
private int measureWidth(int measureSpec){
int result = 0;
int mode = MeasureSpec.getMode(measureSpec);
int size = MeasureSpec.getSize(measureSpec);
if (mode == MeasureSpec.EXACTLY){//精確模式
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
（這里以LinearLayout為例）
LinearLayout.java
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//首先判斷的是orientation
if (mOrientation == VERTICAL) {
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
measureHorizontal(widthMeasureSpec, heightMeasureSpec);
}
}
void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
//記錄垂直方向的高度
mTotalLength = 0;
//通過(guò)for循環(huán)去拿我們里面的子布局
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child == null) {
mTotalLength += measureNullChild(i);
continue;
}
//把每個(gè)子View的高度加起來(lái)并賦值給mTotalLength
mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
measureChildBeforeLayout(child, i, widthMeasureSpec, 0,
heightMeasureSpec, usedHeight);
}
//測(cè)完子View细办，還得加上LinearLayout的Padding屬性
mTotalLength += mPaddingTop + mPaddingBottom;
//測(cè)量完之后，接著調(diào)用這個(gè)方法蕾殴，我們的布局才有了真正的寬和高笑撞，這里進(jìn)去是給measuredWidth和measuredHeight賦值
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);
//通過(guò)調(diào)用子View的onMeasure方法，去測(cè)量子View的寬高钓觉，這里的兩個(gè)參數(shù)childXXXMeasureSpec都是父布局已經(jīng)給我們測(cè)量好了的
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
ViewGroup.java里面查看測(cè)量的方式getChildMeasureSpec()方法
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
//獲取模式和大小
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
//根據(jù)父布局的模式去給子View的resultSize茴肥、resultMode賦值
switch (specMode) {
// Parent has imposed an exact size on us
//當(dāng)父布局是match_parent、fill_parent(Deprecated)荡灾、明確的值
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {//當(dāng)ViewGroup中的子view是設(shè)置了具體大小的值時(shí)( X dp)
//子View的大小就等于設(shè)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外面設(shè)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
//當(dāng)父布局是warp_content時(shí)
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension;
//子View被設(shè)置成了明確的值瓤狐，那么他的模式直接就是EXACTLY堕虹，不受父布局的影響
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;
//當(dāng)父布局是AT_MOST模式時(shí)，即使子View是MATCH_PARENT芬首，他的模式赴捞，最終還是AT_MOST，所以郁稍，子View的mode是父布局的mode和自己一起決定的
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】：
當(dāng)父布局是AT_MOST模式時(shí)赦政，即使子View是MATCH_PARENT，他的模式耀怜，最終還是AT_MOST恢着，所以，子View的mode是父布局的mode和自己一起決定的
調(diào)用完measure方法之后财破，就會(huì)調(diào)用setMeasuredDimension()方法掰派，這個(gè)時(shí)候我們的布局才真正指定寬度和高度，measuredWidth和measuredHeight才真正有值
void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
//通過(guò)for循環(huán)去拿我們里面的子布局
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (childWeight > 0) {
//記錄所有子View的高左痢，方便計(jì)算自己的高靡羡，這里我們看的是Vertical方向的LinearLayout，所以他只是計(jì)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);
//去測(cè)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);
}
//測(cè)量完之后俊性，接著調(diào)用這個(gè)方法略步，我們的布局才有了真正的寬和高，這里進(jìn)去是給measuredWidth和measuredHeight賦值
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
heightSizeAndState);
}
總結(jié)：
測(cè)量都是從外往里遞歸定页，他先會(huì)把ViewRootImpl的測(cè)量模式傳到DecorView趟薄，然后DecorView再把測(cè)量模式傳到我們的根布局，依次向底部傳典徊，然后杭煎，最底層的View會(huì)拿子View的寬高來(lái)計(jì)算自己的寬高，依次向外傳
image-20201013215655174.png
2. 繪制流程第二步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是一個(gè)空方法卒落，因?yàn)闆](méi)有一個(gè)統(tǒng)一的樣式羡铲，所以留給自定義View自己去實(shí)現(xiàn)
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
2）ViewGroup的onLayout()方法
走到這里，基本的步驟流程差不多定好了导绷，我們?nèi)タ淳唧w的View中是怎么實(shí)現(xiàn)的犀勒，還是用LinearLayout舉例
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
//計(jì)算父窗口推薦的子View寬度
final int width = right - left;
//計(jì)算父窗口推薦的子View右側(cè)位置
int childRight = width - mPaddingRight;
// Space available for child
//child可使用空間大小
int childSpace = width - paddingLeft - mPaddingRight;
//通過(guò)ViewGroup的getChildCount方法獲取ViewGroup的子View個(gè)數(shù)
final int count = getVirtualChildCount();
//獲取Gravity屬性設(shè)置
final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
//依據(jù)majorGravity計(jì)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;
}
//重點(diǎn)Ｊ浩Ｍ浊！開(kāi)始遍歷
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child == null) {
childTop += measureNullChild(i);
} else if (child.getVisibility() != GONE) {
//LinearLayout中其子視圖顯示的寬和高由measure過(guò)程來(lái)決定的钦购，因此measure過(guò)程的意義就是為layout過(guò)程提供視圖顯示范圍的參考值
final int childWidth = child.getMeasuredWidth();
final int childHeight = child.getMeasuredHeight();
//獲取子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);
//依據(jù)不同的absoluteGravity計(jì)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;
//通過(guò)垂直排列計(jì)算調(diào)運(yùn)child的layout設(shè)置child的位置
setChildFrame(child, childLeft, childTop + getLocationOffset(child),
childWidth, childHeight);
childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
i += getChildrenSkipCount(child, i);
}
}
}
總結(jié)：
layout也是從頂層父View向子View的遞歸調(diào)用view.layout方法的過(guò)程檐盟，即父View根據(jù)第一步performMeasure，來(lái)獲取子View所的布局大小和布局參數(shù)押桃，將子View放在合適的位置上葵萎，==不過(guò)這個(gè)方法沒(méi)有再往外走，只是不斷的往里面走==。
3.繪制流程第三步performDraw
1）View的draw()方法
View的draw流程很簡(jiǎn)單羡忘，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:如果需要谎痢，則繪制背景
* 2. If necessary, save the canvas' layers to prepare for fading：保存當(dāng)前camvas層
* 3. Draw view's content：繪制View的內(nèi)容
* 4. Draw children：繪制子View
* 5. If necessary, draw the fading edges and restore layers：如果需要，則繪制View的褪色邊緣卷雕，這類似于陰影效果
* 6. Draw decorations (scrollbars for instance)：繪制裝飾节猿，比如滾動(dòng)條
*/
2）ViewGroup的draw()方法
performDraw和上面的流程類似
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的繪制流程其實(shí)就三個(gè)步驟：onMeasure(測(cè)量) -> onLayout(擺放) -> onDraw(繪制)
