AndroidX Fragment探究-生命周期

概述

Fragment的生命周期依托FragmentActivity管理调度，在FragmentActivity的各个生命周期方法回调中，会将对应生命状态派发给依附于FragmentActivity的所有Fragment。

图1.Fragment 生命周期

源码探究

文中源码基于’androidx.fragment:fragment:1.1.0’

Fragment的生命周期派发

图2.Activity 生命周期对Fragment 生命周期的影响

上图为FragmentActivity和Fragment的生命周期对照，随着FragmentActivity的生命周期变化，也会调整Fragment的生命周期状态。

FragmentActivity首先会设置FragmentManagerImpl中的生命状态，再通过FragmentManagerImpl来更新其中的Fragment的状态。

状态常量值

在Fragment类中定义了5个状态常量值，用于标记当前状态：

• INITIALIZING（0）：初始状态、默认状态，和图2中onDestroy状态
• CREATED（1）：对应图2中Created、onDestroyView状态
• ACTIVITY_CREATED（2）：对应图2中onActivityCreated至Started、Stopped状态
• STARTED（3）：对应图2中Started、Paused状态
• RESUMED（4）：对应图2中Resumed状态

FragmentManagerImpl利用mCurState成员变量来标记当前状态，Fragment利用mState成员变量来标记当前状态。

更新FragmentManagerImpl的生命周期状态

这里以FragmentActivity的生命周期回调开始，先看派发给FragmentManagerImpl各个状态的时机。

onCreate

[FragmentActivity.java]

protected void onCreate(@Nullable Bundle savedInstanceState) {
    // 进行上下文绑定，FragmentManagerImpl会持有HostCallbacks引用
    mFragments.attachHost(null /*parent*/);
    // 省略savedInstanceState部分 ···
    // 省略mPendingFragmentActivityResults部分 ···
    super.onCreate(savedInstanceState);
    // 省略LifecycleRegistry部分 ···
    // 通过FragmentController来调用FragmentManagerImpl
    mFragments.dispatchCreate();
}

FragmentActivity中调用FragmentManagerImpl的相关方法都是通过FragmentController来间接调用。

继续看dispatchCreate方法：
[FragmentController.java]

public void dispatchCreate() {
    // 通过FragmentHostCallback来调用FragmentManagerImpl
    mHost.mFragmentManager.dispatchCreate();
}

这里最终调用了FragmentManagerImpl的dispatchCreate方法：
[FragmentManagerImpl.java]

public void dispatchCreate() {
    // mStateSaved用于标记是否保存Fragment State（通过saveAllState方法保存）
    mStateSaved = false;
    // mStopped用于标记当前是否Stopped
    mStopped = false;
    // 传入状态值CREATED
    dispatchStateChange(Fragment.CREATED);
}

该方法中调用dispatchStateChange方法并传入CREATED来更新状态。

[FragmentManagerImpl.java]

private void dispatchStateChange(int nextState) {
    try {
        mExecutingActions = true;
        // 变更当前状态为传入的目标状态
        moveToState(nextState, false);
    } finally {
        mExecutingActions = false;
    }
    // 处理待执行任务队列
    execPendingActions();
}

真正更新状态是在moveToState方法中：
[FragmentManagerImpl.java]

void moveToState(int newState, boolean always) {
    if (mHost == null && newState != Fragment.INITIALIZING) {
        throw new IllegalStateException("No activity");
    }

    // 检查状态是否相等，always此时传入的为false
    if (!always && newState == mCurState) {
        return;
    }

    // 更新状态
    mCurState = newState;

    // 省略触发Fragment更新状态部分
    // ···
}

在该方法中将传入的指定的状态赋值给了mCurState成员，从而达到FragmentManagerImpl的状态更新。

onStart

[FragmentActivity.java]

protected void onStart() {
    super.onStart();

    mStopped = false;

    // mCreated默认为false
    if (!mCreated) {
        mCreated = true;
        // 派发ACTIVITY_CREATED状态
        mFragments.dispatchActivityCreated();
    }

    mFragments.noteStateNotSaved();
    mFragments.execPendingActions();

    // NOTE: HC onStart goes here.

    mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START);
    // 派发STARTED状态
    mFragments.dispatchStart();
}

在onStart方法中，判断若是首次调用，则会调用dispatchActivityCreated方法。最后调用dispatchStart方法。这两个方法同dispatchCreate方法一样，通过FragmentController->FragmentHostCallback->FragmentManagerImpl调用对应方法。

[FragmentManagerImpl.java]

public void dispatchActivityCreated() {
    mStateSaved = false;
    mStopped = false;
    // 派发ACTIVITY_CREATED状态
    dispatchStateChange(Fragment.ACTIVITY_CREATED);
}

public void dispatchStart() {
    mStateSaved = false;
    mStopped = false;
    // 派发STARTED状态
    dispatchStateChange(Fragment.STARTED);
}

前后更新状态为ACTIVITY_CREATED和STARTED。

onPostResume

在Activity的performResume方法中会调用onPostResume方法：
[Activity.java]

final void performResume(boolean followedByPause, String reason) {
    // ···
    // 将会触发Activity的onResume回调
    mInstrumentation.callActivityOnResume(this);
    // ···
    onPostResume();
    // ···
}

而FragmentActivity重写了onPostResume方法：
[FragmentActivity.java]

protected void onPostResume() {
    super.onPostResume();
    onResumeFragments();
}

protected void onResumeFragments() {
    mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
    // 派发RESUMED状态
    mFragments.dispatchResume();
}

dispatchResume方法最终会调用FragmentManagerImpl的dispatchResume方法，将状态更新为RESUMED。

onPause

[FragmentActivity.java]

protected void onPause() {
    super.onPause();
    mResumed = false;
    // 派发STARTED状态
    mFragments.dispatchPause();
    mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE);
}

[FragmentManagerImpl.java]

public void dispatchPause() {
    dispatchStateChange(Fragment.STARTED);
}

在onPause中，将调用dispatchPause方法，又会将FragmentManagerImpl的状态置为STARTED。

onStop

[FragmentActivity.java]

protected void onStop() {
    super.onStop();

    mStopped = true;
    markFragmentsCreated();
    // 派发ACTIVITY_CREATED状态
    mFragments.dispatchStop();
    mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_STOP);
}

[FragmentManagerImpl.java]

public void dispatchStop() {
    // 标记Stopped为true
    mStopped = true;
    dispatchStateChange(Fragment.ACTIVITY_CREATED);
}

在onStop中将会更新FragmentManagerImpl的状态为ACTIVITY_CREATED。

onDestroy

[FragmentActivity.java]

protected void onDestroy() {
    super.onDestroy();
    mFragments.dispatchDestroy();
    mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY);
}

[FragmentManagerImpl.java]

public void dispatchDestroy() {
    mDestroyed = true;
    execPendingActions();
    // 更新状态为INITIALIZING
    dispatchStateChange(Fragment.INITIALIZING);
    // 解除引用
    mHost = null;
    mContainer = null;
    mParent = null;
    if (mOnBackPressedDispatcher != null) {
        // mOnBackPressedDispatcher can hold a reference to the host
        // so we need to null it out to prevent memory leaks
        mOnBackPressedCallback.remove();
        mOnBackPressedDispatcher = null;
    }
}

在onDestroy中将会更新FragmentManagerImpl的状态为INITIALIZING。

小结

在FragmentActivity的onCreate回调中将会更新FragmentManagerImpl的状态为CREATED；在onStart回调中，将会更新状态为STARTED；在onPostResume中，更新状态为RESUMED；在onPause中，更新状态为STARTED；在onStop中，更新状态为ACTIVITY_CREATED；在onDestroy中，更新状态为INITIALIZING。

更新Fragment的生命周期状态

FragmentManagerImpl在moveToState方法中更新自身状态后，便会遍历mAdded（保存添加的Fragment的集合），依次取出Fragment，调用moveFragmentToExpectedState方法并传入Fragment。在moveFragmentToExpectedState方法中又会调用另一个moveToState重载方法，在该方法中实现Fragment的生命状态更新。

[FragmentManagerImpl.java]

// 参数f为将要处理的Fragment，newState为将要设置的目标状态
void moveToState(Fragment f, int newState, int transit, int transitionStyle,
                 boolean keepActive) {
    // Fragments that are not currently added will sit in the onCreate() state.
    // 判断是否对fragment执行了detach操作
    if ((!f.mAdded || f.mDetached) && newState > Fragment.CREATED) {
        newState = Fragment.CREATED;
    }
    // 判断是否对fragment执行了remove操作
    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.
    // 判断是否延迟启动（可通过setUserVisibleHint设置）
    if (f.mDeferStart && f.mState < Fragment.STARTED && newState > Fragment.ACTIVITY_CREATED) {
        newState = Fragment.ACTIVITY_CREATED;
    }
    // Don't allow the Fragment to go above its max lifecycle state
    // Ensure that Fragments are capped at CREATED instead of ACTIVITY_CREATED.
    // 判断是否超过fragment设置的最大生命周期状态（默认为RESUMED，这里利用Lifecycle组件中的枚举值来比较）
    if (f.mMaxState == Lifecycle.State.CREATED) {
        newState = Math.min(newState, Fragment.CREATED);
    } else {
        newState = Math.min(newState, f.mMaxState.ordinal());
    }
    
    // ···
}

moveToState方法中第一部分是对目标生命周期状态进行校验和修正，确保状态处于合理的范围内，接下来便会根据目标状态和fragment当前状态（默认INITIALIZING）进行调度。

[FragmentManagerImpl.java]

// 参数f为将要处理的Fragment，newState为将要设置的目标状态
void moveToState(Fragment f, int newState, int transit, int transitionStyle,
                 boolean keepActive) {
    // ···
    
    // fragment当前生命周期状态小等于目标状态，那么会进行从create(假设)到resume(假设)的生长过程
    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.
        if (f.mFromLayout && !f.mInLayout) {
            // 跳过通过布局xml定义创建的fragment
            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);
            // 当前fragment正在进行离开动画，先更新状态至之前保存的动画结束后的目标状态
            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) {
                        // 省略mSavedFragmentState部分 ···
                    }

                    f.mHost = mHost;
                    f.mParentFragment = mParent;
                    f.mFragmentManager = mParent != null
                            ? mParent.mChildFragmentManager : mHost.mFragmentManager;

                    // 省略Target Fragment部分 ···

                    dispatchOnFragmentPreAttached(f, mHost.getContext(), false);
                    // 触发Fragment的onAttach回调方法
                    f.performAttach();
                    if (f.mParentFragment == null) {
                        // 触发FragmentActivity的onAttachFragment回调方法
                        mHost.onAttachFragment(f);
                    } else {
                        f.mParentFragment.onAttachFragment(f);
                    }
                    dispatchOnFragmentAttached(f, mHost.getContext(), false);

                    // mIsCreated默认为false
                    if (!f.mIsCreated) {
                        dispatchOnFragmentPreCreated(f, f.mSavedFragmentState, false);
                        // 设置mState为CREATED，触发Fragment的onCreate回调方法，并设置mIsCreated为true
                        f.performCreate(f.mSavedFragmentState);
                        dispatchOnFragmentCreated(f, f.mSavedFragmentState, false);
                    } else {
                        f.restoreChildFragmentState(f.mSavedFragmentState);
                        f.mState = Fragment.CREATED;
                    }
                }
                // fall through
                // 没有break，继续往下执行
            case Fragment.CREATED:
                // We want to unconditionally run this anytime we do a moveToState that
                // moves the Fragment above INITIALIZING, including cases such as when
                // we move from CREATED => CREATED as part of the case fall through above.
                if (newState > Fragment.INITIALIZING) {
                    // 布局xml定义的Fragment相关
                    ensureInflatedFragmentView(f);
                }

                if (newState > Fragment.CREATED) {
                    if (DEBUG) Log.v(TAG, "moveto ACTIVITY_CREATED: " + f);
                    // 判断该fragment不是通过布局xml中定义
                    if (!f.mFromLayout) {
                        ViewGroup container = null;
                        if (f.mContainerId != 0) {
                            // 若添加该fragment时有指定布局容器ID，则mContainerId不为0
                            if (f.mContainerId == View.NO_ID) {
                                throwException(new IllegalArgumentException(
                                        "Cannot create fragment "
                                                + f
                                                + " for a container view with no id"));
                            }
                            // 通过FragmentActivity.this.findViewById(id)获取布局容器
                            container = (ViewGroup) mContainer.onFindViewById(f.mContainerId);
                            if (container == null && !f.mRestored) {
                                String resName;
                                try {
                                    resName = f.getResources().getResourceName(f.mContainerId);
                                } catch (Resources.NotFoundException e) {
                                    resName = "unknown";
                                }
                                throwException(new IllegalArgumentException(
                                        "No view found for id 0x"
                                                + Integer.toHexString(f.mContainerId) + " ("
                                                + resName
                                                + ") for fragment " + f));
                            }
                        }
                        f.mContainer = container;
                        // 触发Fragment的onCreateView回调方法，生成view并保存在mView成员上
                        f.performCreateView(f.performGetLayoutInflater(
                                f.mSavedFragmentState), container, f.mSavedFragmentState);
                        if (f.mView != null) {
                            f.mInnerView = f.mView;
                            f.mView.setSaveFromParentEnabled(false);
                            if (container != null) {
                                // 将fragment自定义的view添加入布局容器中
                                container.addView(f.mView);
                            }
                            if (f.mHidden) {
                                f.mView.setVisibility(View.GONE);
                            }
                            // 触发Fragment的onViewCreated回调方法
                            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;
                        }
                    }

                    // mState设置为ACTIVITY_CREATED，触发Fragment的onActivityCreated回调方法
                    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);
                    // mState设置为STARTED，触发Fragment的onStart回调方法
                    f.performStart();
                    dispatchOnFragmentStarted(f, false);
                }
                // fall through
            case Fragment.STARTED:
                if (newState > Fragment.STARTED) {
                    if (DEBUG) Log.v(TAG, "moveto RESUMED: " + f);
                    // mState设置为RESUMED，触发Fragment的onResume回调方法
                    f.performResume();
                    dispatchOnFragmentResumed(f, false);
                    f.mSavedFragmentState = null;
                    f.mSavedViewState = null;
                }
        }
    } else if (f.mState > newState) {
        // ···
    }
    // ···
}

可以看出，该方法第二部分中会判断目标状态和fragment当前状态的高低，若目标状态较高，则fragment向上生长，逐步更新状态值并执行对应阶段的处理逻辑。

[FragmentManagerImpl.java]

// 参数f为将要处理的Fragment，newState为将要设置的目标状态
void moveToState(Fragment f, int newState, int transit, int transitionStyle,
                 boolean keepActive) {
    // ···
    if (f.mState <= newState) {
        // ···
    } else if (f.mState > newState) {
        // 若fragment当前生命周期状态高于目标状态，那么会进行从resumed(假设)到destroy(假设)的衰减过程
        switch (f.mState) {
            case Fragment.RESUMED:
                if (newState < Fragment.RESUMED) {
                    if (DEBUG) Log.v(TAG, "movefrom RESUMED: " + f);
                    // mState置为STARTED，触发Fragment的onPause回调
                    f.performPause();
                    dispatchOnFragmentPaused(f, false);
                }
                // fall through
            case Fragment.STARTED:
                if (newState < Fragment.STARTED) {
                    if (DEBUG) Log.v(TAG, "movefrom STARTED: " + f);
                    // mState置为ACTIVITY_CREATED，触发Fragment的onStop回调
                    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);
                        }
                    }
                    // mState置为CREATED，触发Fragment的onDestroyView回调
                    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 parent is being removed, no need to handle child animations.
                        if (f.getParentFragment() == null || !f.getParentFragment().mRemoving) {
                            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);
                            }
                            // 从布局容器中移除fragment创建的view
                            f.mContainer.removeView(f.mView);
                        }
                    }
                    // 解除引用
                    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.
                        // 省略动画相关部分 ···
                    }
                    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);
                        boolean beingRemoved = f.mRemoving && !f.isInBackStack();
                        if (beingRemoved || mNonConfig.shouldDestroy(f)) {
                            boolean shouldClear;
                            if (mHost instanceof ViewModelStoreOwner) {
                                shouldClear = mNonConfig.isCleared();
                            } else if (mHost.getContext() instanceof Activity) {
                                Activity activity = (Activity) mHost.getContext();
                                shouldClear = !activity.isChangingConfigurations();
                            } else {
                                shouldClear = true;
                            }
                            if (beingRemoved || shouldClear) {
                                mNonConfig.clearNonConfigState(f);
                            }
                            // mState置为INITIALIZING，触发Fragment的onDestroy回调
                            f.performDestroy();
                            dispatchOnFragmentDestroyed(f, false);
                        } else {
                            f.mState = Fragment.INITIALIZING;
                        }

                        // 触发Fragment的onDetach回调
                        f.performDetach();
                        dispatchOnFragmentDetached(f, false);
                        if (!keepActive) {
                            if (beingRemoved || mNonConfig.shouldDestroy(f)) {
                                makeInactive(f);
                            } else {
                                f.mHost = null;
                                f.mParentFragment = null;
                                f.mFragmentManager = null;
                                if (f.mTargetWho != null) {
                                    Fragment target = mActive.get(f.mTargetWho);
                                    if (target != null && target.getRetainInstance()) {
                                        // Only keep references to other retained Fragments
                                        // to avoid developers accessing Fragments that
                                        // are never coming back
                                        f.mTarget = target;
                                    }
                                }
                            }
                        }
                    }
                }
        }
    }

    if (f.mState != newState) {
        Log.w(TAG, "moveToState: Fragment state for " + f + " not updated inline; "
                + "expected state " + newState + " found " + f.mState);
        // 将目标状态赋值给fragment的mState
        f.mState = newState;
    }
}

当目标状态低于fragment当前生命周期状态时，则会进行和生长过程相反的过程。

总结

FragmentActivity在各个生命周期回调方法中会更新FragmentManagerImpl的生命周期状态，而FragmentManagerImpl接着就会遍历更新其所管理的Fragment集合，依次取出Fragment更新它的生命周期状态。更新Fragment的过程中会逐步调整它的状态至相邻生命周期状态，并进行对应阶段的处理和执行对应生命周期回调方法，最终更新到和FragmentManagerImpl的状态一致。