前言
之前簡單分析了setContentView的一個執(zhí)行流程，但是我們還是不清楚我們的布局是怎么添加到Activity窗口當中的，要解決這個問題，我們必須從Activity啟動的源碼入手。
流程分析
- ActivityThread.java
public static void main(String[] args) {
// 代碼省略
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
Looper.loop();
}
很多曾經(jīng)從事Java，現(xiàn)在從事Android的人都好奇為什么Android沒有main方法，其實是有的。通過閱讀相關文檔，我們知道ActivityThread是應用程序真真的入口，每個應用程序有且只有一個ActivityThread，由它來負責進行所在進程的主線程消息循環(huán)處理。在ActivityThread的main方法中，我們看到它初始化了主線程handler，開啟了消息循環(huán)，那么它的handler里面循環(huán)處理什么工作呢？
- ActivityThread.java
public void handleMessage(Message msg) {
switch (msg.what) {
case LAUNCH_ACTIVITY: {
handleLaunchActivity(r, null);
} break;
// 代碼省略...
}
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {
// 代碼省略...
Activity a = performLaunchActivity(r, customIntent);
if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
Bundle oldState = r.state;
handleResumeActivity(r.token, false, r.isForward,
!r.activity.mFinished && !r.startsNotResumed);
}
// 代碼省略...
}
handleMessage里面處理的事情基本上是與四大組件相關，比如開啟暫停停止一個Activity或Service等，這里我們只用關心LAUNCH_ACTIVITY，看看它里面做了什么。
我們發(fā)現(xiàn)在LAUNCH_ACTIVITY里面，它調用了handleLaunchActivity,而handleLaunchActivity里面又調用了handleResumeActivity。
- ActivityThread.java
final void handleResumeActivity(IBinder token,
// 代碼省略
ActivityClientRecord r = performResumeActivity(token, clearHide);
if (r != null) {
final Activity a = r.activity;
if (r.window == null && !a.mFinished && willBeVisible) {
r.window = r.activity.getWindow();
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
ViewManager wm = a.getWindowManager();
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
l.softInputMode |= forwardBit;
if (a.mVisibleFromClient) {
a.mWindowAdded = true;
wm.addView(decor, l);
}
}
// 代碼省略
在handleResumeActivity里面，首先是通過調用performResumeActivity方法獲取了當前Activity的標識（ActivityClientRecord持有當前Activity以及相關的各種狀態(tài)信息），然后再來獲取當前Activity關聯(lián)的Window對象以及WindowManager對象，最后將DecorView添加到WindowManager中。那么WindowManager中是怎么實現(xiàn)的呢？
- WindowManager.java
public interface WindowManager extends ViewManager {
//代碼省略
}
可以發(fā)現(xiàn)，WindowManager只是一個接口，那么它的實現(xiàn)類在什么地方呢，從handleResumeActivity方法中我們可以看到，WindowManager是通過Activity獲取的，所以我們還是要從Activity的代碼中去尋找它的實現(xiàn)類。
- ActivityThread.java
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
//代碼省略
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);
//代碼省略
activity.mCalled = false;
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
//代碼省略
return activity;
}
- Activity.java
public class Activity extends ContextThemeWrapper
implements LayoutInflater.Factory2,
Window.Callback, KeyEvent.Callback,
OnCreateContextMenuListener, ComponentCallbacks2,
Window.OnWindowDismissedCallback {
public WindowManager getWindowManager() {
return mWindowManager;
}
final void attach(Context context, ActivityThread aThread..)
//省略
mWindow.setWindowManager(
(WindowManager)context.getSystemService(Context.WINDOW_SERVICE),
mToken, mComponent.flattenToString(),
(info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);
if (mParent != null) {
mWindow.setContainer(mParent.getWindow());
}
mWindowManager = mWindow.getWindowManager();
mCurrentConfig = config;
}
}
- Window.java
public void setWindowManager(WindowManager wm, IBinder appToken, String appName,
boolean hardwareAccelerated) {
mAppToken = appToken;
mAppName = appName;
mHardwareAccelerated = hardwareAccelerated
|| SystemProperties.getBoolean(PROPERTY_HARDWARE_UI, false);
if (wm == null) {
wm = (WindowManager)mContext.getSystemService(Context.WINDOW_SERVICE);
}
mWindowManager = ((WindowManagerImpl)wm).createLocalWindowManager(this);
}
我們知道，當一個Activity啟動，會調用ActivityThread的performLaunchActivity，然后在performLaunchActivity里面會首先調用Activity的attach方法，再調用onCreate方法。而在Activity的attach方法里面可以看到，Activity的WindowManager對象實際上是來自于Window，而Window的setWindowManager方法里面揭示出WindowManager的實現(xiàn)類是WindowManagerImpl。
- WindowManagerImpl.java
private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();
@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.addView(view, params, mDisplay, mParentWindow);
}
- WindowManagerGlobal.java
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
//代碼省略
ViewRootImpl root;
View panelParentView = null;
}
if (wparams.type >= WindowManager.LayoutParams.FIRST_SUB_WINDOW &&
wparams.type <= WindowManager.LayoutParams.LAST_SUB_WINDOW) {
//代碼省略
root = new ViewRootImpl(view.getContext(), display);
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
}
root.setView(view, wparams, panelParentView);
//代碼省略
*ViewRootImpl.java
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
//代碼省略
requestLayout(); // 調用View的繪制
if ((mWindowAttributes.inputFeatures
& WindowManager.LayoutParams.INPUT_FEATURE_NO_INPUT_CHANNEL) == 0) {
mInputChannel = new InputChannel();
}
try {
mOrigWindowType = mWindowAttributes.type;
mAttachInfo.mRecomputeGlobalAttributes = true;
collectViewAttributes();
// 最終的添加顯示
res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
getHostVisibility(), mDisplay.getDisplayId(),
mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,
mAttachInfo.mOutsets, mInputChannel);
}
}
在WindowManagerImpl的addView方法中，我們發(fā)現(xiàn)它又是調用的WindowManagerGlobal的addView方法，而在WindowManagerGlobal的addView方法中，通過層層傳遞，它會把DecorView傳遞給ViewRootImpl的setView，然后在ViewRootImpl中進行最終的繪制工作。
在ViewRootImpl中，有二個地方需要我們注意，一個是requestLayout()方法，一個是mWindowSession.addToDisplay方法，mWindowSession是系統(tǒng)服務WindowManagerService在應用程序中創(chuàng)建的代理對象，它最終會通過Binder跨進程調用WindowManagerService的addWindow()方法，然后增加窗口界面。
而requestLayout()方法里面則是進行具體界面繪制的關鍵方法。
*ViewRootImpl.java
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}
void doTraversal() {
if (mTraversalScheduled) {
mTraversalScheduled = false;
mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
if (mProfile) {
Debug.startMethodTracing("ViewAncestor");
}
performTraversals();
if (mProfile) {
Debug.stopMethodTracing();
mProfile = false;
}
}
}
//真正執(zhí)行UI繪制的遍歷過程
private void performTraversals() {
// 代碼省略
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
performLayout(lp, desiredWindowWidth, desiredWindowHeight);
performDraw();
}
在requestLayout里面調用了scheduleTraversals（）方法，在scheduleTraversals（）方法里面開啟了一個叫做TraversalRunnable的線程進行執(zhí)行遍歷，而在執(zhí)行遍歷的doTraversal（）方法里面，它最終調用了performTraversals（）方法，這個方法是最終的界面繪制的方法，這個方法實際上是調用了DecorView的measure、layout、draw方法進行View樹繪制遍歷。
總結
最后我們來總結下布局添加到Activity窗口的具體流程。
