Android fragment源码完全解析

缘起

一直以来,笔者对fragment相关的这一堆API,内部具体干了哪些事情不是很清楚,所以经常会用起来心里有点小疙瘩,不是那么踏实。总想着花些时间去源码中看看,大概从2、3年前也偶尔会点进去看看,那时看的有点云里雾里的,不是特别明白,最近又点进去看了看,加上可以单步代码,算是对其理解有所小成,于是乎趁着记忆还是新鲜的,特此记录下。

涉及到的类

和fragment相关的类有以下几个:

什么是Fragment

Fragment是Android3.0引入的API,号称是为了解决屏幕碎片化和帮助重用代码的构造。中文翻译为碎片、片段,或者理解成子Act,相比Activity更轻量级、更灵活,使用姿势一般有以下2种:

  1. 静态写在xml中,就和我们使用一般的widget一样,如下:


    写在act的xml布局文件中
  2. 在代码里动态生成,添加、删除等等,代码如下:

通过代码添加

参考fragment开发文档

什么是FragmentTransaction

它封装了一系列对fragment的操作,并一次性执行这些操作。看眼它的API就能明白,全是各种add/remove/replace/show/hide等等操作。

什么是FragmentManager

它是和某个act相关联的,并不是全局唯一的,而是每个act都有一个自己的FragmentManager,内部有自己的状态mCurState,对应外部act的生命周期状态。它提供和act中fragment交互的API。

示例代码

private void addTestFragment() {
        FragmentManager fm = getFragmentManager();
        Fragment fragment = TestFragment.newInstance();
        FragmentTransaction ft = fm.beginTransaction();
//        ft.add(R.id.pf_base_fragment_container, fragment, "test_fragment");
        ft.replace(R.id.pf_base_fragment_container, fragment);
        ft.addToBackStack(null);
        ft.commit();
    }

之后的源码解析都按照这段示例代码来讲,源码基于Android6.0。
在开始分析之前,还得交代几个事情,如果我们仔细看了官方API会发现这里的FragmentManagerFragmentTransaction都是abstract的,其真正的实现API对外当然是没有暴露,不过我们看代码知道分别是,如下图:

FragmentManager.java
BackStackRecord.java

具体分析

通过FragmentTransaction这样的API,我们可以一次执行多个操作,就像这样:

ft.remove(R.id.fragment_container, fragment1);
ft.add(R.id.fragment_container, fragment2);
ft.hide(fragment3);
...等等还可以写很长

它是怎么做到的呢?关键是其内部的双向链表结构:

Op的定义

接下来我们看下在此结构上,FragmentTransaction的一堆操作方法的实现,代码如下:

void addOp(Op op) {
        // 典型的双向链表插入操作
        if (mHead == null) {
            mHead = mTail = op;
        } else {
            op.prev = mTail;
            mTail.next = op;
            mTail = op;
        }
        // 记录这些动画信息
        op.enterAnim = mEnterAnim;
        op.exitAnim = mExitAnim;
        op.popEnterAnim = mPopEnterAnim;
        op.popExitAnim = mPopExitAnim;
        mNumOp++;
    }

    public FragmentTransaction add(Fragment fragment, String tag) {
        doAddOp(0, fragment, tag, OP_ADD);
        return this;
    }

    public FragmentTransaction add(int containerViewId, Fragment fragment) {
        doAddOp(containerViewId, fragment, null, OP_ADD);
        return this;
    }

    public FragmentTransaction add(int containerViewId, Fragment fragment, String tag) {
        doAddOp(containerViewId, fragment, tag, OP_ADD);
        return this;
    }

    private void doAddOp(int containerViewId, Fragment fragment, String tag, int opcmd) {
        fragment.mFragmentManager = mManager;

        if (tag != null) {
            if (fragment.mTag != null && !tag.equals(fragment.mTag)) {
                throw new IllegalStateException("Can't change tag of fragment "
                        + fragment + ": was " + fragment.mTag
                        + " now " + tag);
            }
            // 记录这些信息到fragment对象上
            fragment.mTag = tag;
        }

        if (containerViewId != 0) {
            if (fragment.mFragmentId != 0 && fragment.mFragmentId != containerViewId) {
                throw new IllegalStateException("Can't change container ID of fragment "
                        + fragment + ": was " + fragment.mFragmentId
                        + " now " + containerViewId);
            }
            // 记录这些信息到fragment对象上
            fragment.mContainerId = fragment.mFragmentId = containerViewId;
        }

        Op op = new Op();
        op.cmd = opcmd;
        op.fragment = fragment;
        addOp(op);
    }

    public FragmentTransaction replace(int containerViewId, Fragment fragment) {
        return replace(containerViewId, fragment, null);
    }

    public FragmentTransaction replace(int containerViewId, Fragment fragment, String tag) {
        if (containerViewId == 0) {
            throw new IllegalArgumentException("Must use non-zero containerViewId");
        }

        doAddOp(containerViewId, fragment, tag, OP_REPLACE);
        return this;
    }

    public FragmentTransaction remove(Fragment fragment) {
        Op op = new Op();
        op.cmd = OP_REMOVE;
        op.fragment = fragment;
        addOp(op);

        return this;
    }

    public FragmentTransaction hide(Fragment fragment) {
        Op op = new Op();
        op.cmd = OP_HIDE;
        op.fragment = fragment;
        addOp(op);

        return this;
    }

    public FragmentTransaction show(Fragment fragment) {
        Op op = new Op();
        op.cmd = OP_SHOW;
        op.fragment = fragment;
        addOp(op);

        return this;
    }

    public FragmentTransaction detach(Fragment fragment) {
        Op op = new Op();
        op.cmd = OP_DETACH;
        op.fragment = fragment;
        addOp(op);

        return this;
    }

    public FragmentTransaction attach(Fragment fragment) {
        Op op = new Op();
        op.cmd = OP_ATTACH;
        op.fragment = fragment;
        addOp(op);

        return this;
    }

    public FragmentTransaction setCustomAnimations(int enter, int exit) {
        return setCustomAnimations(enter, exit, 0, 0);
    }

    public FragmentTransaction setCustomAnimations(int enter, int exit,
            int popEnter, int popExit) {
        mEnterAnim = enter;
        mExitAnim = exit;
        mPopEnterAnim = popEnter;
        mPopExitAnim = popExit;
        return this;
    }

    public FragmentTransaction setTransition(int transition) {
        mTransition = transition;
        return this;
    }

    @Override
    public FragmentTransaction addSharedElement(View sharedElement, String name) {
        String transitionName = sharedElement.getTransitionName();
        if (transitionName == null) {
            throw new IllegalArgumentException("Unique transitionNames are required for all" +
                    " sharedElements");
        }
        if (mSharedElementSourceNames == null) {
            mSharedElementSourceNames = new ArrayList<String>();
            mSharedElementTargetNames = new ArrayList<String>();
        }
        mSharedElementSourceNames.add(transitionName);
        mSharedElementTargetNames.add(name);
        return this;
    }

    public FragmentTransaction setTransitionStyle(int styleRes) {
        mTransitionStyle = styleRes;
        return this;
    }

    public FragmentTransaction addToBackStack(String name) {
        if (!mAllowAddToBackStack) {
            throw new IllegalStateException(
                    "This FragmentTransaction is not allowed to be added to the back stack.");
        }
        mAddToBackStack = true;
        mName = name;
        return this;
    }

从源码我们可以看出,各种不同操作方法内部实际上只是个不同cmd的Op对象,通过双向链表链起来了而已,至于为什么要是双向的呢,单链表不行吗?那是因为Fragment在act层面维护了一个回退栈,即如果你调用了ft.addToBackStack(null);这样的代码,那么当你按下back键的时候不是直接结束当前act,而是先从fragment栈里尝试pop出来栈顶的fragment。它不仅要向前进,还得支持向后退。
紧接着的是commit的源码,如下:

public int commit() {
        return commitInternal(false);
    }

    public int commitAllowingStateLoss() {
        return commitInternal(true);
    }

    int commitInternal(boolean allowStateLoss) {
        if (mCommitted) {
            throw new IllegalStateException("commit already called");
        }
        if (FragmentManagerImpl.DEBUG) {
            Log.v(TAG, "Commit: " + this);
            LogWriter logw = new LogWriter(Log.VERBOSE, TAG);
            PrintWriter pw = new FastPrintWriter(logw, false, 1024);
            dump("  ", null, pw, null);
            pw.flush();
        }
        mCommitted = true;
        if (mAddToBackStack) {
            mIndex = mManager.allocBackStackIndex(this);
        } else {
            mIndex = -1;
        }
        mManager.enqueueAction(this, allowStateLoss);
        return mIndex;
    }

// FragmentManagerImpl#enqueueAction方法
/**
     * Adds an action to the queue of pending actions.
     *
     * @param action the action to add
     * @param allowStateLoss whether to allow loss of state information
     * @throws IllegalStateException if the activity has been destroyed
     */
    public void enqueueAction(Runnable action, boolean allowStateLoss) {
        if (!allowStateLoss) {
            checkStateLoss();
        }
        synchronized (this) {
            if (mDestroyed || mHost == null) {
                throw new IllegalStateException("Activity has been destroyed");
            }
            if (mPendingActions == null) {
                mPendingActions = new ArrayList<Runnable>();
            }
            mPendingActions.add(action);
            if (mPendingActions.size() == 1) {
                mHost.getHandler().removeCallbacks(mExecCommit);
                mHost.getHandler().post(mExecCommit);
            }
        }
    }

我们看到commit方法并没有立即执行这个动作,只是入队了action(通过mPendingActions.add(action);),系统会在下次event loop到来时执行它。

FragmentManagerImpl的其他关键方法
Runnable mExecCommit = new Runnable() {
        @Override
        public void run() {
            execPendingActions();
        }
    };

/**
     * Only call from main thread!
     */
    public boolean execPendingActions() {
        if (mExecutingActions) {
            throw new IllegalStateException("Recursive entry to executePendingTransactions");
        }
        
        if (Looper.myLooper() != mHost.getHandler().getLooper()) {
            throw new IllegalStateException("Must be called from main thread of process");
        }

        boolean didSomething = false;

        while (true) {
            int numActions;
            
            synchronized (this) {
                if (mPendingActions == null || mPendingActions.size() == 0) {
                    break;
                }
                
                numActions = mPendingActions.size();
                if (mTmpActions == null || mTmpActions.length < numActions) {
                    mTmpActions = new Runnable[numActions];
                }
                mPendingActions.toArray(mTmpActions);
                mPendingActions.clear();
                mHost.getHandler().removeCallbacks(mExecCommit);
            }
            
            mExecutingActions = true;
            for (int i=0; i<numActions; i++) {
                mTmpActions[i].run();
                mTmpActions[i] = null;
            }
            mExecutingActions = false;
            didSomething = true;
        }

        if (mHavePendingDeferredStart) {
            boolean loadersRunning = false;
            for (int i=0; i<mActive.size(); i++) {
                Fragment f = mActive.get(i);
                if (f != null && f.mLoaderManager != null) {
                    loadersRunning |= f.mLoaderManager.hasRunningLoaders();
                }
            }
            if (!loadersRunning) {
                mHavePendingDeferredStart = false;
                startPendingDeferredFragments();
            }
        }
        return didSomething;
    }

具体要执行的action就是BackStackRecord#run方法,因为我们注意到它自己实现了Runnable接口,来看下其实现:

public void run() {
        if (FragmentManagerImpl.DEBUG) {
            Log.v(TAG, "Run: " + this);
        }

        if (mAddToBackStack) {
            if (mIndex < 0) {
                throw new IllegalStateException("addToBackStack() called after commit()");
            }
        }

        bumpBackStackNesting(1);

        SparseArray<Fragment> firstOutFragments = new SparseArray<Fragment>();
        SparseArray<Fragment> lastInFragments = new SparseArray<Fragment>();
        calculateFragments(firstOutFragments, lastInFragments);
        beginTransition(firstOutFragments, lastInFragments, false);

        Op op = mHead;
        while (op != null) {
            switch (op.cmd) {
                case OP_ADD: {
                    Fragment f = op.fragment;
                    f.mNextAnim = op.enterAnim;
                    mManager.addFragment(f, false);
                }
                break;
                case OP_REPLACE: {
                    Fragment f = op.fragment;
                    int containerId = f.mContainerId;
                    if (mManager.mAdded != null) {
                        for (int i = 0; i < mManager.mAdded.size(); i++) {
                            Fragment old = mManager.mAdded.get(i);
                            if (FragmentManagerImpl.DEBUG) {
                                Log.v(TAG,
                                        "OP_REPLACE: adding=" + f + " old=" + old);
                            }
                            // 注意这个判断,必须是同一个containerId里面的,换句话说你如果是在2个不同的container里面做fragment的replace操作,互相是不会影响的
                            if (old.mContainerId == containerId) {
                                if (old == f) {
                                    op.fragment = f = null;
                                } else {
                                    if (op.removed == null) {
                                        op.removed = new ArrayList<Fragment>();
                                    }
                                    op.removed.add(old);
                                    old.mNextAnim = op.exitAnim;
                                    if (mAddToBackStack) {
                                        old.mBackStackNesting += 1;
                                        if (FragmentManagerImpl.DEBUG) {
                                            Log.v(TAG, "Bump nesting of "
                                                    + old + " to " + old.mBackStackNesting);
                                        }
                                    }
                                    mManager.removeFragment(old, mTransition, mTransitionStyle);
                                }
                            }
                        }
                    }
// 注意这个非空判断,因为replace自己的时候f会被置为null,相当于什么也不会做的
                    if (f != null) { 
                        f.mNextAnim = op.enterAnim;
                        mManager.addFragment(f, false);
                    }
                }
                break;
                case OP_REMOVE: {
                    Fragment f = op.fragment;
                    f.mNextAnim = op.exitAnim;
                    mManager.removeFragment(f, mTransition, mTransitionStyle);
                }
                break;
                case OP_HIDE: {
                    Fragment f = op.fragment;
                    f.mNextAnim = op.exitAnim;
                    mManager.hideFragment(f, mTransition, mTransitionStyle);
                }
                break;
                case OP_SHOW: {
                    Fragment f = op.fragment;
                    f.mNextAnim = op.enterAnim;
                    mManager.showFragment(f, mTransition, mTransitionStyle);
                }
                break;
                case OP_DETACH: {
                    Fragment f = op.fragment;
                    f.mNextAnim = op.exitAnim;
                    mManager.detachFragment(f, mTransition, mTransitionStyle);
                }
                break;
                case OP_ATTACH: {
                    Fragment f = op.fragment;
                    f.mNextAnim = op.enterAnim;
                    mManager.attachFragment(f, mTransition, mTransitionStyle);
                }
                break;
                default: {
                    throw new IllegalArgumentException("Unknown cmd: " + op.cmd);
                }
            }

            op = op.next;
        }

        // 不带fragment参数的状态转移
        mManager.moveToState(mManager.mCurState, mTransition,
                mTransitionStyle, true);

        if (mAddToBackStack) {
            mManager.addBackStackState(this);
        }
    }

这个方法主要做的事情是向前遍历Op双向链表,根据cmd的不同调用FragmentManagerImpl的以下方法,特别注意下这里的OP_REPLACE cmd,看代码我们知道它其实相当于add/remove的组合,即如果有added会把所以已经added了的先remove掉,再将新的add进去,效果上是这样。

public void addFragment(Fragment fragment, boolean moveToStateNow) {
        if (mAdded == null) {
            mAdded = new ArrayList<Fragment>();
        }
        if (DEBUG) Log.v(TAG, "add: " + fragment);
        makeActive(fragment);
        if (!fragment.mDetached) {
            if (mAdded.contains(fragment)) {
                throw new IllegalStateException("Fragment already added: " + fragment);
            }
            mAdded.add(fragment);
            fragment.mAdded = true;
            fragment.mRemoving = false;
            if (fragment.mHasMenu && fragment.mMenuVisible) {
                mNeedMenuInvalidate = true;
            }
            if (moveToStateNow) {
                moveToState(fragment);
            }
        }
    }
    
    public void removeFragment(Fragment fragment, int transition, int transitionStyle) {
        if (DEBUG) Log.v(TAG, "remove: " + fragment + " nesting=" + fragment.mBackStackNesting);
        // 加到回退栈中和不加的区别
        final boolean inactive = !fragment.isInBackStack();
        if (!fragment.mDetached || inactive) {
            if (false) {
                // Would be nice to catch a bad remove here, but we need
                // time to test this to make sure we aren't crashes cases
                // where it is not a problem.
                if (!mAdded.contains(fragment)) {
                    throw new IllegalStateException("Fragment not added: " + fragment);
                }
            }
            if (mAdded != null) {
                mAdded.remove(fragment);
            }
            if (fragment.mHasMenu && fragment.mMenuVisible) {
                mNeedMenuInvalidate = true;
            }
            fragment.mAdded = false;
            fragment.mRemoving = true;
// 没有加到回退栈中则状态退的更彻底,到状态0而不是状态1,区别就是最后的几个destory相关的callback会不会被调用。
            moveToState(fragment, inactive ? Fragment.INITIALIZING : Fragment.CREATED,
                    transition, transitionStyle, false);
        }
    }
    
    public void hideFragment(Fragment fragment, int transition, int transitionStyle) {
        if (DEBUG) Log.v(TAG, "hide: " + fragment);
        if (!fragment.mHidden) {
            fragment.mHidden = true;
            if (fragment.mView != null) {
                Animator anim = loadAnimator(fragment, transition, false,
                        transitionStyle);
                if (anim != null) {
                    anim.setTarget(fragment.mView);
                    // Delay the actual hide operation until the animation finishes, otherwise
                    // the fragment will just immediately disappear
                    final Fragment finalFragment = fragment;
                    anim.addListener(new AnimatorListenerAdapter() {
                        @Override
                        public void onAnimationEnd(Animator animation) {
                            if (finalFragment.mView != null) {
                                finalFragment.mView.setVisibility(View.GONE);
                            }
                        }
                    });
                    setHWLayerAnimListenerIfAlpha(finalFragment.mView, anim);
                    anim.start();
                } else {
                    fragment.mView.setVisibility(View.GONE);
                }
            }
            if (fragment.mAdded && fragment.mHasMenu && fragment.mMenuVisible) {
                mNeedMenuInvalidate = true;
            }
            fragment.onHiddenChanged(true);
        }
    }
    
    public void showFragment(Fragment fragment, int transition, int transitionStyle) {
        if (DEBUG) Log.v(TAG, "show: " + fragment);
        if (fragment.mHidden) {
            fragment.mHidden = false;
            if (fragment.mView != null) {
                Animator anim = loadAnimator(fragment, transition, true,
                        transitionStyle);
                if (anim != null) {
                    anim.setTarget(fragment.mView);
                    setHWLayerAnimListenerIfAlpha(fragment.mView, anim);
                    anim.start();
                }
                fragment.mView.setVisibility(View.VISIBLE);
            }
            if (fragment.mAdded && fragment.mHasMenu && fragment.mMenuVisible) {
                mNeedMenuInvalidate = true;
            }
            fragment.onHiddenChanged(false);
        }
    }
    
    public void detachFragment(Fragment fragment, int transition, int transitionStyle) {
        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 (mAdded != null) {
                    if (DEBUG) Log.v(TAG, "remove from detach: " + fragment);
                    mAdded.remove(fragment);
                }
                if (fragment.mHasMenu && fragment.mMenuVisible) {
                    mNeedMenuInvalidate = true;
                }
                fragment.mAdded = false;
                moveToState(fragment, Fragment.CREATED, transition, transitionStyle, false);
            }
        }
    }

    public void attachFragment(Fragment fragment, int transition, int transitionStyle) {
        if (DEBUG) Log.v(TAG, "attach: " + fragment);
        if (fragment.mDetached) {
            fragment.mDetached = false;
            if (!fragment.mAdded) {
                if (mAdded == null) {
                    mAdded = new ArrayList<Fragment>();
                }
                if (mAdded.contains(fragment)) {
                    throw new IllegalStateException("Fragment already added: " + fragment);
                }
                if (DEBUG) Log.v(TAG, "add from attach: " + fragment);
                mAdded.add(fragment);
                fragment.mAdded = true;
                if (fragment.mHasMenu && fragment.mMenuVisible) {
                    mNeedMenuInvalidate = true;
                }
                moveToState(fragment, mCurState, transition, transitionStyle, false);
            }
        }
    }

上面的run方法在对这一系列Op处理完后,调用了下面的方法:
mManager.moveToState(mManager.mCurState, mTransition, mTransitionStyle, true);,对应的源码如下:

void moveToState(Fragment f) {
        moveToState(f, mCurState, 0, 0, false);
    }

    void moveToState(int newState, boolean always) {
        moveToState(newState, 0, 0, always);
    }
    
    // 这里调用的是这个方法
    void moveToState(int newState, int transit, int transitStyle, boolean always) {
        if (mHost == null && newState != Fragment.INITIALIZING) {
            throw new IllegalStateException("No activity");
        }

        if (!always && mCurState == newState) {
            return;
        }

        mCurState = newState;
        if (mActive != null) {
            boolean loadersRunning = false;
            for (int i=0; i<mActive.size(); i++) {
                Fragment f = mActive.get(i);
                if (f != null) {
                    // fragment状态改变的方法,其各种生命周期回调也在这里调用,是最重要的一个方法!!!
                    moveToState(f, newState, transit, transitStyle, false);
                    if (f.mLoaderManager != null) {
                        loadersRunning |= f.mLoaderManager.hasRunningLoaders();
                    }
                }
            }

            if (!loadersRunning) {
                startPendingDeferredFragments();
            }

            if (mNeedMenuInvalidate && mHost != null && mCurState == Fragment.RESUMED) {
                mHost.onInvalidateOptionsMenu();
                mNeedMenuInvalidate = false;
            }
        }
    }

这里我们需要对mManager.mCurState这个值的变化细说下,其初始值是这样的:

int mCurState = Fragment.INITIALIZING;

Fragment的所有可能状态:

static final int INVALID_STATE = -1;   // Invalid state used as a null value.
    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.

FragmentManager中有一堆public void dispatchxxx这样的方法,如下:

public void dispatchCreate() {
        mStateSaved = false;
        moveToState(Fragment.CREATED, false);
    }
    
    public void dispatchActivityCreated() {
        mStateSaved = false;
        moveToState(Fragment.ACTIVITY_CREATED, false);
    }
    
    public void dispatchStart() {
        mStateSaved = false;
        moveToState(Fragment.STARTED, false);
    }
    
    public void dispatchResume() {
        mStateSaved = false;
        moveToState(Fragment.RESUMED, false);
    }
    
    public void dispatchPause() {
        moveToState(Fragment.STARTED, false);
    }
    
    public void dispatchStop() {
        moveToState(Fragment.STOPPED, false);
    }
    
    public void dispatchDestroyView() {
        moveToState(Fragment.CREATED, false);
    }

    public void dispatchDestroy() {
        mDestroyed = true;
        execPendingActions();
        moveToState(Fragment.INITIALIZING, false);
        mHost = null;
        mContainer = null;
        mParent = null;
    }

而这些正好是和act的生命周期对应起来,也就是说这些方法是随着act进入到不同的生命周期而被调用的,即mCurState的值是被这些方法触发设置的。比如act进入到了Resume状态,那么FragmentManagerImpl.mCurState也就等于Fragment.RESUMED。

最后,我们分析下fragment状态转移最重要的一个方法,如下:

void moveToState(Fragment f, int newState, int transit, int transitionStyle,
            boolean keepActive) {
        if (DEBUG && false) Log.v(TAG, "moveToState: " + f
            + " oldState=" + f.mState + " newState=" + newState
            + " mRemoving=" + f.mRemoving + " Callers=" + Debug.getCallers(5));

        // 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) {
            // 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.
            if (f.mFromLayout && !f.mInLayout) {
                return;
            }
            if (f.mAnimatingAway != 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.mAnimatingAway = null;
                moveToState(f, f.mStateAfterAnimating, 0, 0, true);
            }
            switch (f.mState) {
                case Fragment.INITIALIZING:
                    if (DEBUG) Log.v(TAG, "moveto CREATED: " + f);
                    if (f.mSavedFragmentState != null) {
                        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);
                        }
                        f.mUserVisibleHint = f.mSavedFragmentState.getBoolean(
                                FragmentManagerImpl.USER_VISIBLE_HINT_TAG, true);
                        if (!f.mUserVisibleHint) {
                            f.mDeferStart = true;
                            if (newState > Fragment.STOPPED) {
                                newState = Fragment.STOPPED;
                            }
                        }
                    }
                    f.mHost = mHost;
                    f.mParentFragment = mParent;
                    f.mFragmentManager = mParent != null
                            ? mParent.mChildFragmentManager : mHost.getFragmentManagerImpl();
                    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);
                    }

                    if (!f.mRetaining) {
                        f.performCreate(f.mSavedFragmentState);
                    }
                    f.mRetaining = false;
                    if (f.mFromLayout) {
                        // For fragments that are part of the content view
                        // layout, we need to instantiate the view immediately
                        // and the inflater will take care of adding it.
                        f.mView = f.performCreateView(f.getLayoutInflater(
                                f.mSavedFragmentState), null, f.mSavedFragmentState);
                        if (f.mView != null) {
                            f.mView.setSaveFromParentEnabled(false);
                            if (f.mHidden) f.mView.setVisibility(View.GONE);
                            f.onViewCreated(f.mView, f.mSavedFragmentState);
                        }
                    }
                case Fragment.CREATED:
                    if (newState > Fragment.CREATED) {
                        if (DEBUG) Log.v(TAG, "moveto ACTIVITY_CREATED: " + f);
                        if (!f.mFromLayout) {
                            ViewGroup container = null;
                            if (f.mContainerId != 0) {
                                container = (ViewGroup)mContainer.onFindViewById(f.mContainerId);
                                if (container == null && !f.mRestored) {
                                    throwException(new IllegalArgumentException(
                                            "No view found for id 0x"
                                            + Integer.toHexString(f.mContainerId) + " ("
                                            + f.getResources().getResourceName(f.mContainerId)
                                            + ") for fragment " + f));
                                }
                            }
                            f.mContainer = container;
                            f.mView = f.performCreateView(f.getLayoutInflater(
                                    f.mSavedFragmentState), container, f.mSavedFragmentState);
                            if (f.mView != null) {
                                f.mView.setSaveFromParentEnabled(false);
                                if (container != null) {
                                    Animator anim = loadAnimator(f, transit, true,
                                            transitionStyle);
                                    if (anim != null) {
                                        anim.setTarget(f.mView);
                                        setHWLayerAnimListenerIfAlpha(f.mView, anim);
                                        anim.start();
                                    }
                                    container.addView(f.mView);
                                }
                                if (f.mHidden) f.mView.setVisibility(View.GONE);
                                f.onViewCreated(f.mView, f.mSavedFragmentState);
                            }
                        }

                        f.performActivityCreated(f.mSavedFragmentState);
                        if (f.mView != null) {
                            f.restoreViewState(f.mSavedFragmentState);
                        }
                        f.mSavedFragmentState = null;
                    }
                case Fragment.ACTIVITY_CREATED:
                case Fragment.STOPPED:
                    if (newState > Fragment.STOPPED) {
                        if (DEBUG) Log.v(TAG, "moveto STARTED: " + f);
                        f.performStart();
                    }
                case Fragment.STARTED:
                    if (newState > Fragment.STARTED) {
                        if (DEBUG) Log.v(TAG, "moveto RESUMED: " + f);
                        f.mResumed = true;
                        f.performResume();
                        // Get rid of this in case we saved it and never needed it.
                        f.mSavedFragmentState = null;
                        f.mSavedViewState = null;
                    }
            }
        } else if (f.mState > newState) {
            switch (f.mState) {
                case Fragment.RESUMED:
                    if (newState < Fragment.RESUMED) {
                        if (DEBUG) Log.v(TAG, "movefrom RESUMED: " + f);
                        f.performPause();
                        f.mResumed = false;
                    }
                case Fragment.STARTED:
                    if (newState < Fragment.STARTED) {
                        if (DEBUG) Log.v(TAG, "movefrom STARTED: " + f);
                        f.performStop();
                    }
                case Fragment.STOPPED:
                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();
                        if (f.mView != null && f.mContainer != null) {
                            Animator anim = null;
                            if (mCurState > Fragment.INITIALIZING && !mDestroyed) {
                                anim = loadAnimator(f, transit, false,
                                        transitionStyle);
                            }
                            if (anim != null) {
                                final ViewGroup container = f.mContainer;
                                final View view = f.mView;
                                final Fragment fragment = f;
                                container.startViewTransition(view);
                                f.mAnimatingAway = anim;
                                f.mStateAfterAnimating = newState;
                                anim.addListener(new AnimatorListenerAdapter() {
                                    @Override
                                    public void onAnimationEnd(Animator anim) {
                                        container.endViewTransition(view);
                                        if (fragment.mAnimatingAway != null) {
                                            fragment.mAnimatingAway = null;
                                            moveToState(fragment, fragment.mStateAfterAnimating,
                                                    0, 0, false);
                                        }
                                    }
                                });
                                anim.setTarget(f.mView);
                                setHWLayerAnimListenerIfAlpha(f.mView, anim);
                                anim.start();

                            }
                            f.mContainer.removeView(f.mView);
                        }
                        f.mContainer = null;
                        f.mView = null;
                    }
                case Fragment.CREATED:
                    if (newState < Fragment.CREATED) {
                        if (mDestroyed) {
                            if (f.mAnimatingAway != null) {
                                // 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.
                                Animator anim = f.mAnimatingAway;
                                f.mAnimatingAway = null;
                                anim.cancel();
                            }
                        }
                        if (f.mAnimatingAway != 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.mStateAfterAnimating = newState;
                            newState = Fragment.CREATED;
                        } else {
                            if (DEBUG) Log.v(TAG, "movefrom CREATED: " + f);
                            if (!f.mRetaining) {
                                f.performDestroy();
                            }

                            f.mCalled = false;
                            f.onDetach();
                            if (!f.mCalled) {
                                throw new SuperNotCalledException("Fragment " + f
                                        + " did not call through to super.onDetach()");
                            }
                            if (!keepActive) {
                                if (!f.mRetaining) {
                                    makeInactive(f);
                                } else {
                                    f.mHost = null;
                                    f.mParentFragment = null;
                                    f.mFragmentManager = null;
                                    f.mChildFragmentManager = null;
                                }
                            }
                        }
                    }
            }
        }
        
        f.mState = newState;
    }

这个方法最终会将FragmentManager的状态赋值给fragment,另外这个方法会根据不同的state调用各种onAttach, Fragment.performXXX,进而调到用户自己override的fragment的各种生命周期方法,比如onCreate、onCreateView等等。

总结

  1. show/hideFragment只是改变fragment根View的visibility,最多带上个动画效果,另外只有本身是hidden的fragment,调用show才起作用,否则没用的,fragment.onHiddenChanged会被触发;其次不会有生命周期callback触发,当然了这些操作的前提是已经被add了的fragment;

  2. addFragment的时候,不管加不加入回退栈都一样,经历的生命周期如下:onAttach、onCreate、onCreateView、onActivityCreate、onStart、onResume;

  3. removeFragment的时候,经历的生命周期如下:onPause、onStop、onDestroyView,如果不加回退栈还会继续走
    onDestroy、onDetach;remove的时候不仅从mAdded中移除fragment,也从mActive中移除了;


    FragmentManagerImpl中关键字段申明
  4. attach/detachFragment的前提都是已经add了的fragment,其生命周期回调不受回退栈影响。attach的时候onCreateView、onActivityCreate、onStart、onResume会被调用;detach的时候onPause、onStop、onDestroyView会被调用,onDestroy、onDetach不会被调用;对应的fragment只是从mAdded中移除了;

  5. remove、detachFragment的时候,当FragmentManagerImpl.makeInactive()被调用的话,fragment就变成了一个空壳,里面绝大部分字段都会被置空,注意只是系统内部自己管理的字段,假如你在自己的fragment子类中引入了新的字段,当你重用这些类的对象时要自己处理这种情况(即系统不会reset你自己造的字段),代码如下:

void makeInactive(Fragment f) {
        if (f.mIndex < 0) {
            return;
        }
        
        if (DEBUG) Log.v(TAG, "Freeing fragment index " + f);
        mActive.set(f.mIndex, null);
        if (mAvailIndices == null) {
            mAvailIndices = new ArrayList<Integer>();
        }
        mAvailIndices.add(f.mIndex);
        mHost.inactivateFragment(f.mWho);
        f.initState();
    }

// Fragment.initState方法
/**
     * Called by the fragment manager once this fragment has been removed,
     * so that we don't have any left-over state if the application decides
     * to re-use the instance.  This only clears state that the framework
     * internally manages, not things the application sets.
     */
    void initState() {
        mIndex = -1;
        mWho = null;
        mAdded = false;
        mRemoving = false;
        mResumed = false;
        mFromLayout = false;
        mInLayout = false;
        mRestored = false;
        mBackStackNesting = 0;
        mFragmentManager = null;
        mChildFragmentManager = null;
        mHost = null;
        mFragmentId = 0;
        mContainerId = 0;
        mTag = null;
        mHidden = false;
        mDetached = false;
        mRetaining = false;
        mLoaderManager = null;
        mLoadersStarted = false;
        mCheckedForLoaderManager = false;
    }

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