FragmentManager.show(fragment) / .hide(fragment) 原理

http://www.binkery.com/archives/492.html

Fragment的创建

在Fragment的回调中 onCreateView 我们会初始化一个view 并 reture.
如下，我们reture的 myView 会被保存到Fragment中

    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View  myView = new MineView(getActivity());
        return myView;
    }

来看fragment的成员变量，那么可以猜想mView就是 onCreateView 所返回的，那么这个mView是怎么赋值的呢，我们继续追源码

    // The parent container of the fragment after dynamically added to UI.
    ViewGroup mContainer;
    
    // The View generated for this fragment.
    View mView;
    
    // The real inner view that will save/restore state.
    View mInnerView;

我们进入的源码中看Fragment中的onCreateView方法是 performCreateView调用了

View performCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        if (mChildFragmentManager != null) {
            mChildFragmentManager.noteStateNotSaved();
        }
        return onCreateView(inflater, container, savedInstanceState);
    }

但是在Fragment中 并没有地方去调用 performCreateView ，跟fragment有关的 无非就是 FragmentManager ，所以我们去 FragmentManager中找, 果然发现 mView 就是我们猜想的那样

f.mContainer = container;
                            f.mView = f.performCreateView(f.getLayoutInflater(
                                    f.mSavedFragmentState), container, f.mSavedFragmentState);

Fragment的隐藏和显示

通常我们显示或隐藏Fragment 是通过如下的方式

 ((FragmentActivity) getContext()).getSupportFragmentManager().beginTransaction().show(mFragmentAt).commitAllowingStateLoss();
 ((FragmentActivity) getContext()).getSupportFragmentManager().beginTransaction().hide(mFragmentB).commitAllowingStateLoss();

我们追进 commitAllowingStateLoss 看做了什么

    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(TAG);
            PrintWriter pw = new PrintWriter(logw);
            dump("  ", null, pw, null);
        }
        mCommitted = true;
        if (mAddToBackStack) {
            mIndex = mManager.allocBackStackIndex(this);
        } else {
            mIndex = -1;
        }
        mManager.enqueueAction(this, allowStateLoss);
        return mIndex;
    }

继续

   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();
            }
            mPendingActions.add(action);
            if (mPendingActions.size() == 1) {
                mHost.getHandler().removeCallbacks(mExecCommit);
                mHost.getHandler().post(mExecCommit);
            }
        }
    }

有用的操作就是 post(mExecCommit) , 继续

Runnable mExecCommit = new Runnable() {
        @Override
        public void run() {
            execPendingActions();
        }
    };

execPendingActions中有用的操作是 startPendingDeferredFragments(); 继续追 performPendingDeferredStart

    public void performPendingDeferredStart(Fragment f) {
        if (f.mDeferStart) {
            if (mExecutingActions) {
                // Wait until we're done executing our pending transactions
                mHavePendingDeferredStart = true;
                return;
            }
            f.mDeferStart = false;
            moveToState(f, mCurState, 0, 0, false);
        }
    }

终于到关键方法了，moveToState是处理Fragment状态的

继续读代码我们可以看到

if (f.mHidden) f.mView.setVisibility(View.GONE);
f.onViewCreated(f.mView, f.mSavedFragmentState);

由此可看hide()的基本原理就是 将Fragment的 mView 不可见

小注 ： 如果我们在hide(mFragmentB)之后 ，手动将 mFragmentB的view.setVisible(View.Visible); 的话，会导致hide失效 。