继续承接上一篇文章----Fragment.OnPause的事情，我们知道onPause之后进入的是onStop生命周期方法，那Fragment的onStop方法又干了些什么呢？

        还是从Activity入手，相应的入口代码如下：

//FragmentActivity.java/*** Dispatch onStop() to all fragments.*/@Overrideprotected void onStop() {super.onStop();mStopped = true;markFragmentsCreated();mHandler.sendEmptyMessage(MSG_REALLY_STOPPED);mFragments.dispatchStop();}

        这里主要做了3个事情，首先调用markFragmentsCreated标记相应fragment的LifecycleRegistry状态，这里不是重点，不关注；其次发送了消息MSG_REALLY_STOPPED；最后调用mFragments.dispatchStop()分发activity的生命周期；这里关注dispatchStop，代码如下：

//FragmentManager.java   public void dispatchStop() {mStopped = true;dispatchStateChange(Fragment.STOPPED);}private void dispatchStateChange(int nextState) {try {mExecutingActions = true;moveToState(nextState, false);} finally {mExecutingActions = false;}execPendingActions();}/*** 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;if (mActive != null) {// Must add them in the proper order. mActive fragments may be out of orderfinal int numAdded = mAdded.size();for (int i = 0; i < numAdded; i++) {Fragment f = mAdded.get(i);moveFragmentToExpectedState(f);//这里这里}// 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;}}}/*** 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);}}moveToState(f, nextState, f.getNextTransition(), f.getNextTransitionStyle(), false);if (f.mView != null) {// Move the view if it is out of orderFragment 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 animationsif (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);}}

        调用链与上一篇文章一致，这里关注调用moveToState时的参数，其中newState为Fragment.STOPPED（3），always为false；将newState保存到mCurState中，然后调用moveFragmentToExpectedState进行fragment的状态转移，该方法最后调用5个参数的moveToState，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 -> detachswitch (f.mState) {case Fragment.RESUMED:if (newState < Fragment.RESUMED) {if (DEBUG) Log.v(TAG, "movefrom RESUMED: " + f);f.performPause();dispatchOnFragmentPaused(f, false);}// fall throughcase Fragment.STARTED://走到这里if (newState < Fragment.STARTED) {if (DEBUG) Log.v(TAG, "movefrom STARTED: " + f);f.performStop();dispatchOnFragmentStopped(f, false);}// fall throughcase Fragment.STOPPED:if (newState < Fragment.STOPPED) {if (DEBUG) Log.v(TAG, "movefrom STOPPED: " + f);f.performReallyStop();}// fall throughcase 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 throughcase 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;}}

        这里注意，Fragment执行完onPause之后，mState为STARTED（4），而newState为STOPPED（3），所以进入销毁流程的Fragment.STARTED分支，即执行如下代码：

                case Fragment.STARTED:if (newState < Fragment.STARTED) {if (DEBUG) Log.v(TAG, "movefrom STARTED: " + f);f.performStop();dispatchOnFragmentStopped(f, false);}

        newState < Fragment.STARTED条件为真，因此进入代码块中，performStop代码如下：

void performStop() {mLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_STOP);if (mChildFragmentManager != null) {mChildFragmentManager.dispatchStop();}mState = STOPPED;mCalled = false;onStop();if (!mCalled) {throw new SuperNotCalledException("Fragment " + this+ " did not call through to super.onStop()");}}

        处理LifecycleRegistry中的回调，处理子fragmet的生命周期回调分发，改变当前fragment状态（mState = STOPPED），调用当前fragment的生命周期方法onStop。回到上一步，继续调用方法dispatchOnFragmentStopped，代码如下：

void dispatchOnFragmentStopped(Fragment f, boolean onlyRecursive) {if (mParent != null) {FragmentManager parentManager = mParent.getFragmentManager();if (parentManager instanceof FragmentManagerImpl) {((FragmentManagerImpl) parentManager).dispatchOnFragmentStopped(f, true);}}for (Pair<FragmentLifecycleCallbacks, Boolean> p : mLifecycleCallbacks) {if (!onlyRecursive || p.second) {p.first.onFragmentStopped(this, f);}}}

        处理父fragment生命周期分发，处理mLifecycleCallbacks的注册回调。

onStop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onStop执行自己的生命周期方法（同时mState = STOPPED）。

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）。

        执行完onStop之后，上面的逻辑将穿透到Fragment.STOPPED分支，代码如下：

                case Fragment.STARTED:if (newState < Fragment.STARTED) {if (DEBUG) Log.v(TAG, "movefrom STARTED: " + f);f.performStop();dispatchOnFragmentStopped(f, false);}// fall throughcase Fragment.STOPPED://走到这里if (newState < Fragment.STOPPED) {if (DEBUG) Log.v(TAG, "movefrom STOPPED: " + f);f.performReallyStop();}

        然而由于此时newState=Fragment.STOPPED，因此将会穿透其余的分支最后跳出整个大switch代码块。

        还记得开篇说的MSG_REALLY_STOPPED消息吗，其对应的代码逻辑如下：

    final Handler mHandler = new Handler() {@Overridepublic void handleMessage(Message msg) {switch (msg.what) {case MSG_REALLY_STOPPED:if (mStopped) {doReallyStop(false);}break;case MSG_RESUME_PENDING:onResumeFragments();mFragments.execPendingActions();break;default:super.handleMessage(msg);}}};void doReallyStop(boolean retaining) {if (!mReallyStopped) {mReallyStopped = true;mRetaining = retaining;mHandler.removeMessages(MSG_REALLY_STOPPED);onReallyStop();}}/*** Pre-HC, we didn't have a way to determine whether an activity was* being stopped for a config change or not until we saw* onRetainNonConfigurationInstance() called after onStop().  However* we need to know this, to know whether to retain fragments.  This will* tell us what we need to know.*/void onReallyStop() {mFragments.dispatchReallyStop();}

        最后调用dispatchReallyStop方法，该方法也会进入5个参数的moveToState方法中，相应分支代码如下：

                case Fragment.STOPPED:if (newState < Fragment.STOPPED) {if (DEBUG) Log.v(TAG, "movefrom STOPPED: " + f);f.performReallyStop();}

        继续调用performReallyStop方法，如下：

void performReallyStop() {if (mChildFragmentManager != null) {mChildFragmentManager.dispatchReallyStop();}mState = ACTIVITY_CREATED;}

        首先是分发事件，然后是改变fragment的状态，此时fragment的mState 为 ACTIVITY_CREATED。

        到这里，onStop执行完成。

此时，Fragment的mState为ACTIVITY_CREATED。