关于detach和remove的不同的文章博客，在网上随便一搜，一箩筐，大都是这么说的
- 生命周期的不同：
当Fragment被detach后，Fragment的生命周期执行完onDestroyView就终止了onPause()-onStop-onDestroyView()。
Fragment被remove后，Fragment的生命周期会一直执行完onDetach，它的onPause()-onStop-onDestroyView()-onDestory()-onDetach()。
- 是否从FragmentManager中移除
当Fragment被detach后，并不会从FragmentManager中移除，而被remove的话，会从FragmentManager中移除。
这些区别可以当做结论，但不是我想要的，我只想知道，为什么会有区别！
我们知道Fragment的生命周期和状态变化依靠FragmentManager和FragmentTransaction共同管理着。
Fragment的add,detach,hide,show等操作都需要通过Fragment的事务处理对象FragmentTransaction进行操作，之后通过FragmentTransaction的commit() 或commitAllowingStateLoss()提交此次事务，而这两种提交事务的方式都是异步的，FragmentTransaction是一个抽象类，BackStackRecord是它的唯一实现类，当我们调用commit方法时，只是FragmentTransaction把自己当做一个任务添加到了它所持有的FragmentManager的一个叫mPendingActions的List集合，然后通过Handler发送了一个消息(Runnable)，等到下一次主线程准备好以后，才执行我们所提交的事务，这一块就不细讲了，不是我们这次讨论的重点
@Override
public int commitAllowingStateLoss() {
return commitInternal(true);
}
@Override
public void commitNow() {
disallowAddToBackStack();
mManager.execSingleAction(this, false);
}
int commitInternal(boolean allowStateLoss) {
//...省略其他代码
mManager.enqueueAction(this, allowStateLoss);
return mIndex;
}
一下是FragmentManager中的代码:
public void enqueueAction(OpGenerator action, boolean allowStateLoss) {
if (!allowStateLoss) {
checkStateLoss();
}
//...省略其他代码
if (mPendingActions == null) {
mPendingActions = new ArrayList<>();
}
mPendingActions.add(action);
scheduleCommit();
}
}
void scheduleCommit() {
synchronized (this) {
boolean postponeReady =
mPostponedTransactions != null && !mPostponedTransactions.isEmpty();
boolean pendingReady = mPendingActions != null && mPendingActions.size() == 1;
if (postponeReady || pendingReady) {
mHost.getHandler().removeCallbacks(mExecCommit);
mHost.getHandler().post(mExecCommit);
}
}
}
既然FragmentTransaction把自己当做一个action提交出去，我们主要关注当action被执行的时候，做了什么就行了，当线程执行到我们提交的事务时，会回调BackStackRecord类的executeOps()方法：
void executeOps() {
final int numOps = mOps.size();
for (int opNum = 0; opNum < numOps; opNum++) {
final Op op = mOps.get(opNum);
final Fragment f = op.fragment;
if (f != null) {
f.setNextTransition(mTransition, mTransitionStyle);
}
switch (op.cmd) {
case OP_ADD:
f.setNextAnim(op.enterAnim);
mManager.addFragment(f, false);
break;
case OP_REMOVE:
f.setNextAnim(op.exitAnim);
mManager.removeFragment(f);
break;
case OP_HIDE:
f.setNextAnim(op.exitAnim);
mManager.hideFragment(f);
break;
case OP_SHOW:
f.setNextAnim(op.enterAnim);
mManager.showFragment(f);
break;
case OP_DETACH:
f.setNextAnim(op.exitAnim);
mManager.detachFragment(f);
break;
case OP_ATTACH:
f.setNextAnim(op.enterAnim);
mManager.attachFragment(f);
break;
case OP_SET_PRIMARY_NAV:
mManager.setPrimaryNavigationFragment(f);
break;
case OP_UNSET_PRIMARY_NAV:
mManager.setPrimaryNavigationFragment(null);
break;
default:
throw new IllegalArgumentException("Unknown cmd: " + op.cmd);
}
if (!mReorderingAllowed && op.cmd != OP_ADD && f != null) {
mManager.moveFragmentToExpectedState(f);//更新该fragment状态，调用相应生命周期函数
}
}
if (!mReorderingAllowed) {
// Added fragments are added at the end to comply with prior behavior.
mManager.moveToState(mManager.mCurState, true);
}
}
我们着重看FragmentManager中detachFragment和removeFragment
public void detachFragment(Fragment fragment) {
if (DEBUG) Log.v(TAG, "detach: " + fragment);
if (!fragment.mDetached) {
fragment.mDetached = true;
if (fragment.mAdded) {
// We are not already in back stack, so need to remove the fragment.
if (DEBUG) Log.v(TAG, "remove from detach: " + fragment);
synchronized (mAdded) {
mAdded.remove(fragment);
}
if (fragment.mHasMenu && fragment.mMenuVisible) {
mNeedMenuInvalidate = true;
}
fragment.mAdded = false;
}
}
}
public void removeFragment(Fragment fragment) {
if (DEBUG) Log.v(TAG, "remove: " + fragment + " nesting=" + fragment.mBackStackNesting);
final boolean inactive = !fragment.isInBackStack();
if (!fragment.mDetached || inactive) {
synchronized (mAdded) {
mAdded.remove(fragment);
}
if (fragment.mHasMenu && fragment.mMenuVisible) {
mNeedMenuInvalidate = true;
}
fragment.mAdded = false;
fragment.mRemoving = true; //mRemoving被标志位true
}
}
看过两个方法，发现两着做的事情几乎一样，都是从mAdded列表中清除该fragment，并把mAdded标为false,不同的是removeFragment会把mRemoving标为true；executeOps()方法最后会通过mManager.moveFragmentToExpectedState(f)把fragment更新为最新状态，期间会调用相应的生命周期函数。
void moveFragmentToExpectedState(Fragment f) {
if (f == null) {
return;
}
int nextState = mCurState;
if (f.mRemoving) {
if (f.isInBackStack()) {
nextState = Math.min(nextState, Fragment.CREATED);
} else {
//如果当前fragment被remove了,那么它的下一个状态为为最低状态，即Fragment.INITIALIZING
nextState = Math.min(nextState, Fragment.INITIALIZING);
}
}
moveToState(f, nextState, f.getNextTransition(), f.getNextTransitionStyle(), false);
}
moveToState方法比较多，也是FragmentManager最重要的方法，Fragment状态的改变以及生命周期的回调，都是在这个方法中进行
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.ACTIVITY_CREATED) {
newState = Fragment.ACTIVITY_CREATED;
}
if (f.mState <= newState) {
//当前状态值小于等于最新状态，fragment走向活跃
// 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.
if (f.mFromLayout && !f.mInLayout) {
return;
}
if (f.getAnimatingAway() != null || f.getAnimator() != null) {
// The fragment is currently being animated... but! Now we
// want to move our state back up. Give up on waiting for the
// animation, move to whatever the final state should be once
// the animation is done, and then we can proceed from there.
f.setAnimatingAway(null);
f.setAnimator(null);
moveToState(f, f.getStateAfterAnimating(), 0, 0, true);
}
switch (f.mState) {
case Fragment.INITIALIZING:
if (newState > Fragment.INITIALIZING) {
if (DEBUG) Log.v(TAG, "moveto CREATED: " + f);
if (f.mSavedFragmentState != null) {
f.mSavedFragmentState.setClassLoader(mHost.getContext()
.getClassLoader());
f.mSavedViewState = f.mSavedFragmentState.getSparseParcelableArray(
FragmentManagerImpl.VIEW_STATE_TAG);
f.mTarget = getFragment(f.mSavedFragmentState,
FragmentManagerImpl.TARGET_STATE_TAG);
if (f.mTarget != null) {
f.mTargetRequestCode = f.mSavedFragmentState.getInt(
FragmentManagerImpl.TARGET_REQUEST_CODE_STATE_TAG, 0);
}
if (f.mSavedUserVisibleHint != null) {
f.mUserVisibleHint = f.mSavedUserVisibleHint;
f.mSavedUserVisibleHint = null;
} else {
f.mUserVisibleHint = f.mSavedFragmentState.getBoolean(
FragmentManagerImpl.USER_VISIBLE_HINT_TAG, true);
}
if (!f.mUserVisibleHint) {
f.mDeferStart = true;
if (newState > Fragment.ACTIVITY_CREATED) {
newState = Fragment.ACTIVITY_CREATED;
}
}
}
f.mHost = mHost;
f.mParentFragment = mParent;
f.mFragmentManager = mParent != null
? mParent.mChildFragmentManager : mHost.getFragmentManagerImpl();
// If we have a target fragment, push it along to at least CREATED
// so that this one can rely on it as an initialized dependency.
if (f.mTarget != null) {
if (mActive.get(f.mTarget.mIndex) != f.mTarget) {
throw new IllegalStateException("Fragment " + f
+ " declared target fragment " + f.mTarget
+ " that does not belong to this FragmentManager!");
}
if (f.mTarget.mState < Fragment.CREATED) {
moveToState(f.mTarget, Fragment.CREATED, 0, 0, true);
}
}
dispatchOnFragmentPreAttached(f, mHost.getContext(), false);
f.mCalled = false;
f.onAttach(mHost.getContext());
if (!f.mCalled) {
throw new SuperNotCalledException("Fragment " + f
+ " did not call through to super.onAttach()");
}
if (f.mParentFragment == null) {
mHost.onAttachFragment(f);
} else {
f.mParentFragment.onAttachFragment(f);
}
dispatchOnFragmentAttached(f, mHost.getContext(), false);
if (!f.mIsCreated) {
dispatchOnFragmentPreCreated(f, f.mSavedFragmentState, false);
f.performCreate(f.mSavedFragmentState);
dispatchOnFragmentCreated(f, f.mSavedFragmentState, false);
} else {
f.restoreChildFragmentState(f.mSavedFragmentState);
f.mState = Fragment.CREATED;
}
f.mRetaining = false;
}
// fall through
case Fragment.CREATED:
// This is outside the if statement below on purpose; we want this to run
// even if we do a moveToState from CREATED => *, CREATED => CREATED, and
// * => CREATED as part of the case fallthrough above.
ensureInflatedFragmentView(f);
if (newState > Fragment.CREATED) {
if (DEBUG) Log.v(TAG, "moveto ACTIVITY_CREATED: " + f);
if (!f.mFromLayout) {
ViewGroup container = null;
if (f.mContainerId != 0) {
if (f.mContainerId == View.NO_ID) {
throwException(new IllegalArgumentException(
"Cannot create fragment "
+ f
+ " for a container view with no id"));
}
container = (ViewGroup) mContainer.onFindViewById(f.mContainerId);
if (container == null && !f.mRestored) {
String resName;
try {
resName = f.getResources().getResourceName(f.mContainerId);
} catch (NotFoundException e) {
resName = "unknown";
}
throwException(new IllegalArgumentException(
"No view found for id 0x"
+ Integer.toHexString(f.mContainerId) + " ("
+ resName
+ ") for fragment " + f));
}
}
f.mContainer = container;
f.performCreateView(f.performGetLayoutInflater(
f.mSavedFragmentState), container, f.mSavedFragmentState);
if (f.mView != null) {
f.mInnerView = f.mView;
f.mView.setSaveFromParentEnabled(false);
if (container != null) {
container.addView(f.mView);
}
if (f.mHidden) {
f.mView.setVisibility(View.GONE);
}
f.onViewCreated(f.mView, f.mSavedFragmentState);
dispatchOnFragmentViewCreated(f, f.mView, f.mSavedFragmentState,
false);
// Only animate the view if it is visible. This is done after
// dispatchOnFragmentViewCreated in case visibility is changed
f.mIsNewlyAdded = (f.mView.getVisibility() == View.VISIBLE)
&& f.mContainer != null;
} else {
f.mInnerView = null;
}
}
f.performActivityCreated(f.mSavedFragmentState);
dispatchOnFragmentActivityCreated(f, f.mSavedFragmentState, false);
if (f.mView != null) {
f.restoreViewState(f.mSavedFragmentState);
}
f.mSavedFragmentState = null;
}
// fall through
case Fragment.ACTIVITY_CREATED:
if (newState > Fragment.ACTIVITY_CREATED) {
if (DEBUG) Log.v(TAG, "moveto STARTED: " + f);
f.performStart();
dispatchOnFragmentStarted(f, false);
}
// fall through
case Fragment.STARTED:
if (newState > Fragment.STARTED) {
if (DEBUG) Log.v(TAG, "moveto RESUMED: " + f);
f.performResume();
dispatchOnFragmentResumed(f, false);
f.mSavedFragmentState = null;
f.mSavedViewState = null;
}
}
} else if (f.mState > newState) {
//当前状态值大于最新状态，fragment走向消亡
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.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;
// Set here to ensure that Observers are called after
// the Fragment's view is set to null
f.mViewLifecycleOwner = null;
f.mViewLifecycleOwnerLiveData.setValue(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方法中根据fragment的状态值大小分为两条线，如果新状态比fragment的当前状态值大，那么当前fragment就属于走向活跃的路线，反之，走向消亡的路线，有一点需要注意，switch中的每一个case都是没有break的，也就是说，进入到了某一条线路后，会把剩下的生命周期走完。
我们关注的detach和remove自然是消亡路线。贴一下关键代码
case Fragment.CREATED:
if (newState < Fragment.CREATED) {
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;
}
}
}
}
当fragment进入case Fragment.CREATED：代码块后，会判断它的最新状态是否低于Fragment.CREATED，而被remove后的fragment的最新状态是INITIALIZING，是比CREATED还要低的。将会走 f.performDestroy()，f.performDetach()以及 makeInactive(f)等关键函数，f.performDestroy()，f.performDetach()分别会调用fragment的onDestroy和onDetach函数，我们看看makeInactive(f)函数：
void makeInactive(Fragment f) {
if (f.mIndex < 0) {
return;
}
if (DEBUG) Log.v(TAG, "Freeing fragment index " + f);
// Don't remove yet. That happens in burpActive(). This prevents
// concurrent modification while iterating over mActive
mActive.put(f.mIndex, null);
f.initState();
}
简单明了，把当前fragment在mActive集合中置为null，FragmentManager也就不再持有该fragment了。
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