1.Fragment想必是经常使用的，其存在的意义总结为：
1.1 Fragment是Activity界面的一部分，Fragment是依附于Activity之上的；
1.2 Activity是Fragment的基础，Fragment是Activity的延续和发展
1.3 一个Activity可以有多个Fragment，一个Fragment也可以被多个Activity重复使用；
1.4 一个Fragment除了Activity处于onResume状态下，他可以自己灵活的控制自己的生命周期，其他状态下，其生命周期都是由Activity所决定的；
1.5 Fragment的出现增强了UI界面布局的灵活性，可以实现动态的改变UI布局；
以上不是今天的重点，今天跟大家从源码的角度聊一聊Fragment
2.绕不开的生命周期
fragment_lifecycle.png
2.1既然Fragment 是Activity界面的一部分，就肯定绕不开Activity
当FragmentActivity在创建调用onCreate方法中
public class FragmentActivity extends ComponentActivity implements
ActivityCompat.OnRequestPermissionsResultCallback,
ActivityCompat.RequestPermissionsRequestCodeValidator {
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
//FragmentController是Fragment的主要控制器，分发事件
mFragments.dispatchCreate();
}
}
public class FragmentController {
public void dispatchCreate() {
//FragmentManager mFragmentManager 管理类继续分发事件
mHost.mFragmentManager.dispatchCreate();
}
}
public abstract class FragmentManager implements FragmentResultOwner {
void dispatchCreate() {
mStateSaved = false;
mStopped = false;
mNonConfig.setIsStateSaved(false);
//进入状态递推阶段
dispatchStateChange(Fragment.CREATED);
}
private void dispatchStateChange(int nextState) {
try {
mExecutingActions = true;
mFragmentStore.dispatchStateChange(nextState);
//移动状态处理
moveToState(nextState, false);
if (USE_STATE_MANAGER) {
Set controllers = collectAllSpecialEffectsController();
for (SpecialEffectsController controller : controllers) {
controller.forceCompleteAllOperations();
}
}
} finally {
mExecutingActions = false;
}
execPendingActions(true);
}
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 (USE_STATE_MANAGER) {
mFragmentStore.moveToExpectedState();
} else {
// Must add them in the proper order. mActive fragments may be out of order
for (Fragment f : mFragmentStore.getFragments()) {
//遍历所有的被add 的fragment全部状态移动
moveFragmentToExpectedState(f);
}
// Now iterate through all active fragments. These will include those that are removed
// and detached.
for (FragmentStateManager fragmentStateManager :
mFragmentStore.getActiveFragmentStateManagers()) {
Fragment f = fragmentStateManager.getFragment();
if (!f.mIsNewlyAdded) {
moveFragmentToExpectedState(f);
}
boolean beingRemoved = f.mRemoving && !f.isInBackStack();
if (beingRemoved) {
mFragmentStore.makeInactive(fragmentStateManager);
}
}
}
startPendingDeferredFragments();
if (mNeedMenuInvalidate && mHost != null && mCurState == Fragment.RESUMED) {
mHost.onSupportInvalidateOptionsMenu();
mNeedMenuInvalidate = false;
}
}
void moveFragmentToExpectedState(@NonNull Fragment f) {
if (!mFragmentStore.containsActiveFragment(f.mWho)) {
if (isLoggingEnabled(Log.DEBUG)) {
Log.d(TAG, "Ignoring moving " + f + " to state " + mCurState
+ "since it is not added to " + this);
}
return;
}
//真正发起状态递推的核心方法
moveToState(f);
if (f.mView != null) {
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:
FragmentAnim.AnimationOrAnimator anim = FragmentAnim.loadAnimation(
mHost.getContext(), f, true, f.getPopDirection());
if (anim != null) {
if (anim.animation != null) {
f.mView.startAnimation(anim.animation);
} else {
anim.animator.setTarget(f.mView);
anim.animator.start();
}
}
}
}
if (f.mHiddenChanged) {
completeShowHideFragment(f);
}
}
void moveToState(@NonNull Fragment f) {
moveToState(f, mCurState);
}
void moveToState(@NonNull Fragment f, int newState) {
FragmentStateManager fragmentStateManager = mFragmentStore.getFragmentStateManager(f.mWho);
if (fragmentStateManager == null) {
fragmentStateManager = new FragmentStateManager(mLifecycleCallbacksDispatcher,
mFragmentStore, f);
fragmentStateManager.setFragmentManagerState(Fragment.CREATED);
}
if (f.mFromLayout && f.mInLayout && f.mState == Fragment.VIEW_CREATED) {
newState = Math.max(newState, Fragment.VIEW_CREATED);
}
newState = Math.min(newState, fragmentStateManager.computeExpectedState());
if (f.mState <= newState) {//生命周期状态正推
if (f.mState < newState && !mExitAnimationCancellationSignals.isEmpty()) {
cancelExitAnimation(f);
}
switch (f.mState) {
case Fragment.INITIALIZING:
if (newState > Fragment.INITIALIZING) {
fragmentStateManager.attach();
}
// fall through
case Fragment.ATTACHED:
if (newState > Fragment.ATTACHED) {
fragmentStateManager.create();
}
// fall through
case Fragment.CREATED：
if (newState > Fragment.INITIALIZING) {
fragmentStateManager.ensureInflatedView();
}
if (newState > Fragment.CREATED) {
fragmentStateManager.createView();
}
// fall through
case Fragment.VIEW_CREATED:
if (newState > Fragment.VIEW_CREATED) {
fragmentStateManager.activityCreated();
}
// fall through
case Fragment.ACTIVITY_CREATED:
if (newState > Fragment.ACTIVITY_CREATED) {
fragmentStateManager.start();
}
// fall through
case Fragment.STARTED:
if (newState > Fragment.STARTED) {
fragmentStateManager.resume();
}
}
} else if (f.mState > newState) {//生命周期状态反推
switch (f.mState) {
case Fragment.RESUMED:
if (newState < Fragment.RESUMED) {
fragmentStateManager.pause();
}
// fall through
case Fragment.STARTED:
if (newState < Fragment.STARTED) {
fragmentStateManager.stop();
}
// fall through
case Fragment.ACTIVITY_CREATED:
if (newState < Fragment.ACTIVITY_CREATED) {
if (isLoggingEnabled(Log.DEBUG)) {
Log.d(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) {
fragmentStateManager.saveViewState();
}
}
}
// fall through
case Fragment.VIEW_CREATED:
if (newState < Fragment.VIEW_CREATED) {
FragmentAnim.AnimationOrAnimator anim = null;
if (f.mView != null && f.mContainer != null) {
// Stop any current animations:
f.mContainer.endViewTransition(f.mView);
f.mView.clearAnimation();
if (!f.isRemovingParent()) {
if (mCurState > Fragment.INITIALIZING && !mDestroyed
&& f.mView.getVisibility() == View.VISIBLE
&& f.mPostponedAlpha >= 0) {
anim = FragmentAnim.loadAnimation(mHost.getContext(),
f, false, f.getPopDirection());
}
f.mPostponedAlpha = 0;
ViewGroup container = f.mContainer;
View view = f.mView;
if (anim != null) {
FragmentAnim.animateRemoveFragment(f, anim,
mFragmentTransitionCallback);
}
container.removeView(view);
if (FragmentManager.isLoggingEnabled(Log.VERBOSE)) {
Log.v(FragmentManager.TAG, "Removing view " + view + " for "
+ "fragment " + f + " from container " + container);
}
if (container != f.mContainer) {
return;
}
}
}
if (mExitAnimationCancellationSignals.get(f) == null) {
fragmentStateManager.destroyFragmentView();
}
}
// fall through
case Fragment.CREATED:
if (newState < Fragment.CREATED) {
if (mExitAnimationCancellationSignals.get(f) != null) {
newState = Fragment.CREATED;
} else {
fragmentStateManager.destroy();
}
}
// fall through
case Fragment.ATTACHED:
if (newState < Fragment.ATTACHED) {
fragmentStateManager.detach();
}
}
}
if (f.mState != newState) {
if (isLoggingEnabled(Log.DEBUG)) {
Log.d(TAG, "moveToState: Fragment state for " + f + " not updated inline; "
+ "expected state " + newState + " found " + f.mState);
}
f.mState = newState;
}
}
}
Fragment 中的所有状态值
public class Fragment implements ComponentCallbacks, OnCreateContextMenuListener, LifecycleOwner,
ViewModelStoreOwner, HasDefaultViewModelProviderFactory, SavedStateRegistryOwner,
ActivityResultCaller {
static final Object USE_DEFAULT_TRANSITION = new Object();
static final int INITIALIZING = -1; // Not yet attached. 初始胡状态
static final int ATTACHED = 0; // Attached to the host. 挂载
static final int CREATED = 1; // Created. 创建
static final int VIEW_CREATED = 2; // View Created. 创建View
static final int AWAITING_EXIT_EFFECTS = 3; // Downward state, awaiting exit effects
static final int ACTIVITY_CREATED = 4; // Fully created, not started.
static final int STARTED = 5; // Created and started, not resumed.
static final int AWAITING_ENTER_EFFECTS = 6; // Upward state, awaiting enter effects
static final int RESUMED = 7; // Created started and resumed.
int mState = INITIALIZING;
}
从这里可以看出来，通过状态地推，调用 FragmentStateManager类 触发fragment的生命周期的方法被调用
class FragmentStateManager {
void attach() {
// .....代码省略.....
mFragment.performAttach();
mDispatcher.dispatchOnFragmentAttached(mFragment, false);
}
void create() {
// .....代码省略.....
mFragment.performCreate(mFragment.mSavedFragmentState);
// .....代码省略.....
}
void createView() {
// .....代码省略.....
mFragment.performCreateView(layoutInflater, container, mFragment.mSavedFragmentState);
// .....代码省略.....
mFragment.mState = Fragment.VIEW_CREATED;
}
void activityCreated() {
// .....代码省略.....
mFragment.performActivityCreated(mFragment.mSavedFragmentState);
mDispatcher.dispatchOnFragmentActivityCreated(
mFragment, mFragment.mSavedFragmentState, false);
}
void pause() {
// .....代码省略.....
mFragment.performPause();
mDispatcher.dispatchOnFragmentPaused(mFragment, false);
}
void stop() {
// .....代码省略.....
mFragment.performStop();
mDispatcher.dispatchOnFragmentStopped(mFragment, false);
}
void start() {
// .....代码省略.....
mFragment.performStart();
mDispatcher.dispatchOnFragmentStarted(mFragment, false);
}
void resume() {
// .....代码省略.....
mFragment.performResume();
mDispatcher.dispatchOnFragmentResumed(mFragment, false);
}
}
总结：通过源码的贴出，不能看出Fragment生命周期的就是一个状态的管理，通过FragmentController 发事件分发给FragmentManager 类做状态的递推，触发FragmentStateManager 调用fragment的生命周期