fragment 出棧過程

當咱們點擊了返回鍵，首先收到消息的仍是activity,若是回退棧有fragment，則有fragment manager處理， 不然調用finish()：

//in class Activity
/**
     * Called when the activity has detected the user's press of the back
     * key.  The default implementation simply finishes the current activity,
     * but you can override this to do whatever you want.
     */
    public void onBackPressed() {
        if (!mFragments.popBackStackImmediate()) {
            finish();
        }
    }

在FragmentManager中（由FragmentManagerImp實現），首先將事務隊列裏還沒有執行的事務執行完畢， 而後處理回退操做。實際使用的時候，時常能夠看到這樣的場景：點擊一個按鈕（會跳轉到另外一個fragment或activity）而後立刻點擊回退，能夠感覺到屏幕的閃爍， 在性能稍差的設備上尤爲明顯， 其實這些現象均可以在這裏找到答案。

@Override
    public boolean popBackStackImmediate() {
        checkStateLoss();
        executePendingTransactions();
        return popBackStackState(mActivity.mHandler, null, -1, 0);
    }

這裏操做的是事務記錄，雖然感受像是操做的fragment。彈出事務時， 能夠是彈出一個，也能夠根據條件彈出多個（能夠認爲控制mBackStack的容量），而後對棧頂的事務執行恢復顯示：即調用改事務的 popFromBackStack(boolean doStateMove)方法。

boolean popBackStackState(Handler handler, String name, int id, int flags) {
        if (mBackStack == null) {
            return false;
        }
        if (name == null && id < 0 && (flags&POP_BACK_STACK_INCLUSIVE) == 0) {
            int last = mBackStack.size()-1;
            if (last < 0) {
                return false;
            }
            final BackStackRecord bss = mBackStack.remove(last);
            bss.popFromBackStack(true);
            reportBackStackChanged();
        } else {
            int index = -1;
            if (name != null || id >= 0) {
                // If a name or ID is specified, look for that place in
                // the stack.
                index = mBackStack.size()-1;
                while (index >= 0) {
                    BackStackRecord bss = mBackStack.get(index);
                    if (name != null && name.equals(bss.getName())) {
                        break;
                    }
                    if (id >= 0 && id == bss.mIndex) {
                        break;
                    }
                    index--;
                }
                if (index < 0) {
                    return false;
                }
                if ((flags&POP_BACK_STACK_INCLUSIVE) != 0) {
                    index--;
                    // Consume all following entries that match.
                    while (index >= 0) {
                        BackStackRecord bss = mBackStack.get(index);
                        if ((name != null && name.equals(bss.getName()))
                                || (id >= 0 && id == bss.mIndex)) {
                            index--;
                            continue;
                        }
                        break;
                    }
                }
            }
            if (index == mBackStack.size()-1) {
                return false;
            }
            final ArrayList<BackStackRecord> states
                    = new ArrayList<BackStackRecord>();
            for (int i=mBackStack.size()-1; i>index; i--) {
                states.add(mBackStack.remove(i));
            }
            final int LAST = states.size()-1;
            for (int i=0; i<=LAST; i++) {
                if (DEBUG) Log.v(TAG, "Popping back stack state: " + states.get(i));
                states.get(i).popFromBackStack(i == LAST);
            }
            reportBackStackChanged();
        }
        return true;
    }

最後是BackStackRecord的 popFromBackStack方法， 和BackStackRecord的 run（） 方法是相對應的， 最後仍是要經過FragmentManager的 moveToState來執行實際的處理（激活fragment的生命週期方法）

public void popFromBackStack(boolean doStateMove) {
        if (FragmentManagerImpl.DEBUG) {
            Log.v(TAG, "popFromBackStack: " + this);
            LogWriter logw = new LogWriter(TAG);
            PrintWriter pw = new PrintWriter(logw);
            dump("  ", null, pw, null);
        }

        bumpBackStackNesting(-1);

        Op op = mTail;
        while (op != null) {
            switch (op.cmd) {
                case OP_ADD: {
                    Fragment f = op.fragment;
                    f.mNextAnim = op.popExitAnim;
                    mManager.removeFragment(f,
                            FragmentManagerImpl.reverseTransit(mTransition),
                            mTransitionStyle);
                } break;
                case OP_REPLACE: {
                    Fragment f = op.fragment;
                    if (f != null) {
                        f.mNextAnim = op.popExitAnim;
                        mManager.removeFragment(f,
                                FragmentManagerImpl.reverseTransit(mTransition),
                                mTransitionStyle);
                    }
                    if (op.removed != null) {
                        for (int i=0; i<op.removed.size(); i++) {
                            Fragment old = op.removed.get(i);
                            old.mNextAnim = op.popEnterAnim;
                            mManager.addFragment(old, false);
                        }
                    }
                } break;
                case OP_REMOVE: {
                    Fragment f = op.fragment;
                    f.mNextAnim = op.popEnterAnim;
                    mManager.addFragment(f, false);
                } break;
                case OP_HIDE: {
                    Fragment f = op.fragment;
                    f.mNextAnim = op.popEnterAnim;
                    mManager.showFragment(f,
                            FragmentManagerImpl.reverseTransit(mTransition), mTransitionStyle);
                } break;
                case OP_SHOW: {
                    Fragment f = op.fragment;
                    f.mNextAnim = op.popExitAnim;
                    mManager.hideFragment(f,
                            FragmentManagerImpl.reverseTransit(mTransition), mTransitionStyle);
                } break;
                case OP_DETACH: {
                    Fragment f = op.fragment;
                    f.mNextAnim = op.popEnterAnim;
                    mManager.attachFragment(f,
                            FragmentManagerImpl.reverseTransit(mTransition), mTransitionStyle);
                } break;
                case OP_ATTACH: {
                    Fragment f = op.fragment;
                    f.mNextAnim = op.popEnterAnim;
                    mManager.detachFragment(f,
                            FragmentManagerImpl.reverseTransit(mTransition), mTransitionStyle);
                } break;
                default: {
                    throw new IllegalArgumentException("Unknown cmd: " + op.cmd);
                }
            }

            op = op.prev;
        }

        if (doStateMove) {
            mManager.moveToState(mManager.mCurState,
                    FragmentManagerImpl.reverseTransit(mTransition), mTransitionStyle, true);
        }

        if (mIndex >= 0) {
            mManager.freeBackStackIndex(mIndex);
            mIndex = -1;
        }
    }