Fragment.OnPause的事情
我们知道Fragment的生命周期依附于相应Activity的生命周期，如果activity A调用了onPause，则A里面的fragment也会相应收到onPause回调，这里以support27.1.1版本的源码来说明Fragment生命周期onPause的事情。
当activity执行onPause时，进入如下的代码：
/**
* Dispatch onPause() to fragments.
*/
@Override
protected void onPause() {
super.onPause();
mResumed = false;
if (mHandler.hasMessages(MSG_RESUME_PENDING)) {
mHandler.removeMessages(MSG_RESUME_PENDING);
onResumeFragments();
}
mFragments.dispatchPause();//关注这里
}mFragments.dispatchPause()这里进入到Fragment的生命周期分发流程中，mFragments其实是一个FragmentController对象，dispatchPause相应代码如下：
/**
* Moves all Fragments managed by the controller's FragmentManager
* into the pause state.
* Call when Fragments should be paused.
*
* @see Fragment#onPause()
*/
public void dispatchPause() {
mHost.mFragmentManager.dispatchPause();
}
这里也是简单的调用转发，进入到FragmentManager里面的dispatchPause方法，如下：
public void dispatchCreate() {
mStateSaved = false;
mStopped = false;
dispatchStateChange(Fragment.CREATED);
}
public void dispatchActivityCreated() {
mStateSaved = false;
mStopped = false;
dispatchStateChange(Fragment.ACTIVITY_CREATED);
}
public void dispatchStart() {
mStateSaved = false;
mStopped = false;
dispatchStateChange(Fragment.STARTED);
}
public void dispatchResume() {
mStateSaved = false;
mStopped = false;
dispatchStateChange(Fragment.RESUMED);
}
public void dispatchPause() {
dispatchStateChange(Fragment.STARTED);
}
public void dispatchStop() {
mStopped = true;
dispatchStateChange(Fragment.STOPPED);
}
public void dispatchReallyStop() {
dispatchStateChange(Fragment.ACTIVITY_CREATED);
}
public void dispatchDestroyView() {
dispatchStateChange(Fragment.CREATED);
}
public void dispatchDestroy() {
mDestroyed = true;
execPendingActions();
dispatchStateChange(Fragment.INITIALIZING);
mHost = null;
mContainer = null;
mParent = null;
}
private void dispatchStateChange(int nextState) {
try {
mExecutingActions = true;
moveToState(nextState, false);
} finally {
mExecutingActions = false;
}
execPendingActions();
}这里有一堆的生命周期事件分发入口，说明Activity的相应生命周期事件都会分发到这里对应的方法；当前的onPause会进入dispatchPause方法，然后调用dispatchStateChange方法，注意调用参数为Fragment.STARTED（值为4），这里看看Fragment的几个状态定义如下：
static final int INITIALIZING = 0; // Not yet created.
static final int CREATED = 1; // Created.
static final int ACTIVITY_CREATED = 2; // The activity has finished its creation.
static final int STOPPED = 3; // Fully created, not started.
static final int STARTED = 4; // Created and started, not resumed.
static final int RESUMED = 5; // Created started and resumed.
int mState = INITIALIZING;接着进入moveToState方法，重点来了，即将进入Fragment的状态转换了，传递给moveToState的参数nextState为Fragment.STARTED（值为4），always为false，相应代码如下：
/**
* Changes the state of the fragment manager to {@code newState}. If the fragment manager
* changes state or {@code always} is {@code true}, any fragments within it have their
* states updated as well.
*
* @param newState The new state for the fragment manager
* @param always If {@code true}, all fragments update their state, even
* if {@code newState} matches the current fragment manager's state.
*/
void moveToState(int newState, boolean always) {
if (mHost == null && newState != Fragment.INITIALIZING) {
throw new IllegalStateException("No activity");
}
if (!always && newState == mCurState) {
return;
}
mCurState = newState;//1关注这里
if (mActive != null) {
// Must add them in the proper order. mActive fragments may be out of order
final int numAdded = mAdded.size();
for (int i = 0; i < numAdded; i++) {
Fragment f = mAdded.get(i);
moveFragmentToExpectedState(f);//2关注这里
}
// Now iterate through all active fragments. These will include those that are removed
// and detached.
final int numActive = mActive.size();
for (int i = 0; i < numActive; i++) {
Fragment f = mActive.valueAt(i);
if (f != null && (f.mRemoving || f.mDetached) && !f.mIsNewlyAdded) {
moveFragmentToExpectedState(f);
}
}
startPendingDeferredFragments();
if (mNeedMenuInvalidate && mHost != null && mCurState == Fragment.RESUMED) {
mHost.onSupportInvalidateOptionsMenu();
mNeedMenuInvalidate = false;
}
}
}
moveToState中关注两点，首先是将状态newState保存到mCurState（为STARTED，4）中，其次遍历集合mAdded并调用moveFragmentToExpectedState改变相应fragment的生命周期状态。
mAdded：保存当前activity中已经添加的fragment实例。
moveFragmentToExpectedState代码如下：
/**
* Moves a fragment to its expected final state or the fragment manager's state, depending
* on whether the fragment manager's state is raised properly.
*
* @param f The fragment to change.
*/
void moveFragmentToExpectedState(Fragment f) {
if (f == null) {
return;
}
int nextState = mCurState;
if (f.mRemoving) {
if (f.isInBackStack()) {
nextState = Math.min(nextState, Fragment.CREATED);
} else {
nextState = Math.min(nextState, Fragment.INITIALIZING);
}
}
//状态转换
moveToState(f, nextState, f.getNextTransition(), f.getNextTransitionStyle(), false);
if (f.mView != null) {
// Move the view if it is out of order
Fragment underFragment = findFragmentUnder(f);
if (underFragment != null) {
final View underView = underFragment.mView;
// make sure this fragment is in the right order.
final ViewGroup container = f.mContainer;
int underIndex = container.indexOfChild(underView);
int viewIndex = container.indexOfChild(f.mView);
if (viewIndex < underIndex) {
container.removeViewAt(viewIndex);
container.addView(f.mView, underIndex);
}
}
if (f.mIsNewlyAdded && f.mContainer != null) {
// Make it visible and run the animations
if (f.mPostponedAlpha > 0f) {
f.mView.setAlpha(f.mPostponedAlpha);
}
f.mPostponedAlpha = 0f;
f.mIsNewlyAdded = false;
// run animations:
AnimationOrAnimator anim = loadAnimation(f, f.getNextTransition(), true,
f.getNextTransitionStyle());
if (anim != null) {
setHWLayerAnimListenerIfAlpha(f.mView, anim);
if (anim.animation != null) {
f.mView.startAnimation(anim.animation);
} else {
anim.animator.setTarget(f.mView);
anim.animator.start();
}
}
}
}
if (f.mHiddenChanged) {
completeShowHideFragment(f);
}
}
该方法主要调用moveToState执行Fragment状态转换，这里将执行真正的生命周期方法，代码如下：
@SuppressWarnings("ReferenceEquality")
void moveToState(Fragment f, int newState, int transit, int transitionStyle,
boolean keepActive) {
// Fragments that are not currently added will sit in the onCreate() state.
if ((!f.mAdded || f.mDetached) && newState > Fragment.CREATED) {
newState = Fragment.CREATED;
}
if (f.mRemoving && newState > f.mState) {
if (f.mState == Fragment.INITIALIZING && f.isInBackStack()) {
// Allow the fragment to be created so that it can be saved later.
newState = Fragment.CREATED;
} else {
// While removing a fragment, we can't change it to a higher state.
newState = f.mState;
}
}
// Defer start if requested; don't allow it to move to STARTED or higher
// if it's not already started.
if (f.mDeferStart && f.mState < Fragment.STARTED && newState > Fragment.STOPPED) {
newState = Fragment.STOPPED;
}
if (f.mState <= newState) {
// For fragments that are created from a layout, when restoring from
// state we don't want to allow them to be created until they are
// being reloaded from the layout.
//该分支表示生命周期转换 create -> start -> resume
} else if (f.mState > newState) {
//该分支表示生命周期转换 pause -> stop -> destoryView -> destory -> detach
switch (f.mState) {
case Fragment.RESUMED:
if (newState < Fragment.RESUMED) {
if (DEBUG) Log.v(TAG, "movefrom RESUMED: " + f);
f.performPause();
dispatchOnFragmentPaused(f, false);
}
// fall through
case Fragment.STARTED:
if (newState < Fragment.STARTED) {
if (DEBUG) Log.v(TAG, "movefrom STARTED: " + f);
f.performStop();
dispatchOnFragmentStopped(f, false);
}
// fall through
case Fragment.STOPPED:
if (newState < Fragment.STOPPED) {
if (DEBUG) Log.v(TAG, "movefrom STOPPED: " + f);
f.performReallyStop();
}
// fall through
case Fragment.ACTIVITY_CREATED:
if (newState < Fragment.ACTIVITY_CREATED) {
if (DEBUG) Log.v(TAG, "movefrom ACTIVITY_CREATED: " + f);
if (f.mView != null) {
// Need to save the current view state if not
// done already.
if (mHost.onShouldSaveFragmentState(f) && f.mSavedViewState == null) {
saveFragmentViewState(f);
}
}
f.performDestroyView();
dispatchOnFragmentViewDestroyed(f, false);
if (f.mView != null && f.mContainer != null) {
// Stop any current animations:
f.mContainer.endViewTransition(f.mView);
f.mView.clearAnimation();
AnimationOrAnimator anim = null;
if (mCurState > Fragment.INITIALIZING && !mDestroyed
&& f.mView.getVisibility() == View.VISIBLE
&& f.mPostponedAlpha >= 0) {
anim = loadAnimation(f, transit, false,
transitionStyle);
}
f.mPostponedAlpha = 0;
if (anim != null) {
animateRemoveFragment(f, anim, newState);
}
f.mContainer.removeView(f.mView);
}
f.mContainer = null;
f.mView = null;
f.mInnerView = null;
f.mInLayout = false;
}
// fall through
case Fragment.CREATED:
if (newState < Fragment.CREATED) {
if (mDestroyed) {
// The fragment's containing activity is
// being destroyed, but this fragment is
// currently animating away. Stop the
// animation right now -- it is not needed,
// and we can't wait any more on destroying
// the fragment.
if (f.getAnimatingAway() != null) {
View v = f.getAnimatingAway();
f.setAnimatingAway(null);
v.clearAnimation();
} else if (f.getAnimator() != null) {
Animator animator = f.getAnimator();
f.setAnimator(null);
animator.cancel();
}
}
if (f.getAnimatingAway() != null || f.getAnimator() != null) {
// We are waiting for the fragment's view to finish
// animating away. Just make a note of the state
// the fragment now should move to once the animation
// is done.
f.setStateAfterAnimating(newState);
newState = Fragment.CREATED;
} else {
if (DEBUG) Log.v(TAG, "movefrom CREATED: " + f);
if (!f.mRetaining) {
f.performDestroy();
dispatchOnFragmentDestroyed(f, false);
} else {
f.mState = Fragment.INITIALIZING;
}
f.performDetach();
dispatchOnFragmentDetached(f, false);
if (!keepActive) {
if (!f.mRetaining) {
makeInactive(f);
} else {
f.mHost = null;
f.mParentFragment = null;
f.mFragmentManager = null;
}
}
}
}
}
}
if (f.mState != newState) {
Log.w(TAG, "moveToState: Fragment state for " + f + " not updated inline; "
+ "expected state " + newState + " found " + f.mState);
f.mState = newState;
}
}
moveToState包含两大分支逻辑，分别是创建与销毁，由于我们这里分析的是onPause，所以创建的分支省略了，这里newState值为Fragment.STARTED，f.mSate为Fragment.RESUMED，因此进入到Fragment.RESUMED对应的销毁分支，即执行如下代码：
case Fragment.RESUMED:
if (newState < Fragment.RESUMED) {
if (DEBUG) Log.v(TAG, "movefrom RESUMED: " + f);
f.performPause();
dispatchOnFragmentPaused(f, false);
}由于newState为Fragment.STARTED，其值为4小于Fragment.RESUMED(5)，因此调用f.performPause，代码如下：
void performPause() {
mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE);
if (mChildFragmentManager != null) {
mChildFragmentManager.dispatchPause();
}
mState = STARTED;
mCalled = false;
onPause();
if (!mCalled) {
throw new SuperNotCalledException("Fragment " + this
+ " did not call through to super.onPause()");
}
}这里首先处理了生命周期变化handleLifecycleEvent，然后如果当前fragment还有子fragment的话，子fragment的生命周期会通过mChildFragmentManager.dispatchPause()来分发，这里还将当前fragment的mState变量赋值为STARTED(4)，最后调用生命周期方法onPause，到这里算是走完了onPause的生命周期流程，为了逻辑的完整性，我们继续分析。
上一步继续调用dispatchOnFragmentPaused方法，代码如下：
void dispatchOnFragmentPaused(Fragment f, boolean onlyRecursive) {
if (mParent != null) {
FragmentManager parentManager = mParent.getFragmentManager();
if (parentManager instanceof FragmentManagerImpl) {
((FragmentManagerImpl) parentManager)
.dispatchOnFragmentPaused(f, true);
}
}
for (Pair p : mLifecycleCallbacks) {
if (!onlyRecursive || p.second) {
p.first.onFragmentPaused(this, f);
}
}
} 首先判断有没有父fragment，有的话同样进行生命周期分发，其次遍历mLifecycleCallbacks并进行生命周期分发，这里的mLifecycleCallbacks其实是我们通过activity的FragmentManager调用registerFragmentLifecycleCallbacks方法注册的生命周期回调函数。
不难看出，onPause对应的Fragment.RESUMED分支，其实只是处理生命周期相关的回调而已。
onPause分支总结：
1 处理mLifecycleRegistry中的回调。
2 通过mChildFragmentManager.dispatchPause()分发其子fragment的生命周期回调。
3 调用onPause执行自己的生命周期方法（同时mState = STARTED）。
4 如果有父framnet，则分发父frament的生命周期回调。
5 执行保存在mLifecycleCallbacks中的生命周期回调（比如LeakCanary的内存泄漏判断就是此时触发的）。
onPause执行完成，注意到moveToState中的分支都不带break，因此会继续执行到下一分支，代码如下：
case Fragment.RESUMED:
if (newState < Fragment.RESUMED) {
if (DEBUG) Log.v(TAG, "movefrom RESUMED: " + f);
f.performPause();
dispatchOnFragmentPaused(f, false);
}
// fall through
case Fragment.STARTED://执行到这里
if (newState < Fragment.STARTED) {
if (DEBUG) Log.v(TAG, "movefrom STARTED: " + f);
f.performStop();
dispatchOnFragmentStopped(f, false);
}
// fall through
case Fragment.STOPPED:
if (newState < Fragment.STOPPED) {
if (DEBUG) Log.v(TAG, "movefrom STOPPED: " + f);
f.performReallyStop();
}穿透到Fragment.STARTED这里，但是注意这里的代码执行时有条件的，此时newState=Fragment.STARTED，条件不满足因此不会继续执行分支的代码，之后的分支也不满足条件，因此最后跳出整个大的switch块。
自此Fragment的onPause生命周期切换完成。
执行完onPause之后，Fragment的生命周期状态mState为Fragment.STARTED。