在Activity的onCreated执行
@Override
protected void onCreate(Bundle savedInstanceState) {
super(savedInstanceState);
setContentView(R.layout.content_view);
getSupportFragmentManager().beginTransaction().add(R.id.container, new Fragment()).commit();
Looper.getMainLooper().dump(new LogPrinter(android.util.Log.DEBUG, "TAG"), "");
}
@Override
protected void onStart() {
super.onStart();
Looper.getMainLooper().dump(new LogPrinter(android.util.Log.DEBUG, "TAG"), "");
}
@Override
protected void onResume() {
super.onResume();
Looper.getMainLooper().dump(new LogPrinter(android.util.Log.DEBUG, "TAG"), "");
}
根据日志如下：
06-11 15:45:50.933 8193-8193/com.demo D/HandlerDebugManager_TAG: Looper (main, tid 1) {125caec}
06-11 15:45:50.933 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 0: { when=-4h58m8s221ms callback=com.demo.debug.HandlerDebugManager$InnerActivityLifecycleCallbacks$1 target=android.os.Handler }
06-11 15:45:50.933 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 1: { when=-20ms callback=androidx.core.content.res.ResourcesCompat$FontCallback$2 target=android.os.Handler }
06-11 15:45:50.933 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 2: { when=-13ms callback=android.view.View$UnsetPressedState target=android.view.ViewRootImpl$ViewRootHandler }
06-11 15:45:50.933 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 3: { when=0 callback=androidx.fragment.app.FragmentManagerImpl$1 target=androidx.fragment.app.FragmentActivity$1 }
06-11 15:45:50.933 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 4: { when=+654ms callback=androidx.lifecycle.ProcessLifecycleOwner$1 target=android.os.Handler }
06-11 15:45:50.933 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 5: { when=+4m49s725ms what=2 target=org.chromium.base.SystemMessageHandler }
06-11 15:45:50.933 8193-8193/com.demo D/HandlerDebugManager_TAG: (Total messages: 6, polling=false, quitting=false)
06-11 15:45:50.936 8193-8193/com.demo D/HandlerDebugManager_TAG: onActivityStarted --
06-11 15:45:50.936 8193-8193/com.demo D/HandlerDebugManager_TAG: Looper (main, tid 1) {125caec}
06-11 15:45:50.936 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 0: { when=-4h58m8s223ms callback=com.demo.debug.HandlerDebugManager$InnerActivityLifecycleCallbacks$1 target=android.os.Handler }
06-11 15:45:50.936 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 1: { when=-22ms callback=androidx.core.content.res.ResourcesCompat$FontCallback$2 target=android.os.Handler }
06-11 15:45:50.936 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 2: { when=-15ms callback=android.view.View$UnsetPressedState target=android.view.ViewRootImpl$ViewRootHandler }
06-11 15:45:50.936 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 3: { when=-2ms callback=androidx.fragment.app.FragmentManagerImpl$1 target=androidx.fragment.app.FragmentActivity$1 }
06-11 15:45:50.936 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 4: { when=+652ms callback=androidx.lifecycle.ProcessLifecycleOwner$1 target=android.os.Handler }
06-11 15:45:50.936 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 5: { when=+4m49s723ms what=2 target=org.chromium.base.SystemMessageHandler }
06-11 15:45:50.936 8193-8193/com.demo D/HandlerDebugManager_TAG: (Total messages: 6, polling=false, quitting=false)
06-11 15:45:50.987 8193-8193/com.demo D/HandlerDebugManager_TAG: onActivityResumed --
06-11 15:45:50.987 8193-8193/com.demo D/HandlerDebugManager_TAG: Looper (main, tid 1) {125caec}
06-11 15:45:50.987 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 0: { when=-4h58m8s275ms callback=com.demo.debug.HandlerDebugManager$InnerActivityLifecycleCallbacks$1 target=android.os.Handler }
06-11 15:45:50.987 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 1: { when=-74ms callback=androidx.core.content.res.ResourcesCompat$FontCallback$2 target=android.os.Handler }
06-11 15:45:50.987 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 2: { when=-67ms callback=android.view.View$UnsetPressedState target=android.view.ViewRootImpl$ViewRootHandler }
06-11 15:45:50.987 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 3: { when=-49ms callback=com.alibaba.wireless.security.open.edgecomputing.ECMiscInfo$1 target=android.os.Handler }
06-11 15:45:50.987 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 4: { when=-39ms callback=androidx.core.content.res.ResourcesCompat$FontCallback$2 target=android.os.Handler }
06-11 15:45:50.987 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 5: { when=-36ms callback=androidx.core.content.res.ResourcesCompat$FontCallback$2 target=android.os.Handler }
06-11 15:45:50.987 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 6: { when=-34ms callback=androidx.core.content.res.ResourcesCompat$FontCallback$2 target=android.os.Handler }
06-11 15:45:50.987 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 7: { when=-34ms callback=androidx.core.content.res.ResourcesCompat$FontCallback$2 target=android.os.Handler }
06-11 15:45:50.987 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 8: { when=-33ms callback=androidx.core.content.res.ResourcesCompat$FontCallback$2 target=android.os.Handler }
06-11 15:45:50.987 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 9: { when=-1ms callback=android.app.LoadedApk$ReceiverDispatcher$Args target=android.app.ActivityThread$H }
06-11 15:45:50.987 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 10: { when=+600ms callback=androidx.lifecycle.ProcessLifecycleOwner$1 target=android.os.Handler }
06-11 15:45:50.987 8193-8193/com.demo D/HandlerDebugManager_TAG: Message 11: { when=+4m49s671ms what=2 target=org.chromium.base.SystemMessageHandler }
06-11 15:45:50.987 8193-8193/com.demo D/HandlerDebugManager_TAG: (Total messages: 12, polling=false, quitting=false)
日志中出现了一条message3
Message 3: { when=0 callback=androidx.fragment.app.FragmentManagerImpl$1 target=androidx.fragment.app.FragmentActivity$1 }
可以推测出activity在新加fragment时加入了handler.post 来完成fragment的添加，但是为何在onResume中该message消失了，而排在该message之前的message任然存在呢？
解答
FragmentActivity源码如下
@Override
protected void onStart() {
super.onStart();
mStopped = false;
if (!mCreated) {
mCreated = true;
mFragments.dispatchActivityCreated();
}
mFragments.noteStateNotSaved();
mFragments.execPendingActions();
// NOTE: HC onStart goes here.
mFragments.dispatchStart(); // -->dispatchStart()?
}
FragmentController源码
public void dispatchStart() {
mHost.mFragmentManager.dispatchStart(); // -->dispatchStart()?
}
FragmentManager源码
public void dispatchStart() {
mStateSaved = false;
mStopped = false;
dispatchStateChange(Fragment.STARTED); // -->dispatchStateChange()?
}
private void dispatchStateChange(int nextState) {
try {
mExecutingActions = true;
moveToState(nextState, false);
} finally {
mExecutingActions = false;
}
execPendingActions(); // -->execPendingActions()?
}
public boolean execPendingActions() {
ensureExecReady(true);
boolean didSomething = false;
while (generateOpsForPendingActions(mTmpRecords, mTmpIsPop)) { // -->generateOpsForPendingActions()?
mExecutingActions = true;
try {
removeRedundantOperationsAndExecute(mTmpRecords, mTmpIsPop);
} finally {
cleanupExec();
}
didSomething = true;
}
doPendingDeferredStart();
burpActive();
return didSomething;
}
private boolean generateOpsForPendingActions(ArrayList<BackStackRecord> records,
ArrayList<Boolean> isPop) {
boolean didSomething = false;
synchronized (this) {
if (mPendingActions == null || mPendingActions.size() == 0) {
return false;
}
final int numActions = mPendingActions.size();
for (int i = 0; i < numActions; i++) {
didSomething |= mPendingActions.get(i).generateOps(records, isPop);
}
mPendingActions.clear();
mHost.getHandler().removeCallbacks(mExecCommit); // -- removeCallbacks ?
}
return didSomething;
}
最终定位在generateOpsForPendingActions()方法中将该Message移除出messagequeue，同时执行了Fragment的初始化
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