Fragment的commit分析

Android

一般使用的Fragment方式

FragmentManager manager = getFragmentManager();
FragmentTransaction transaction = manager.beginTransaction();
transaction.show(fragment);
transaction.commit();
// 或者状态丢失的提交
transaction.commitAllowingStateLoss();

有时候会遇到Crash，大概意思是在状态保存后不能提交事务操作。

IllegalStateException: Can not perform this action after onSaveInstanceState at android.support.v4.app.FragmentManagerImpl.checkStateLoss(FragmentManager.java:1493) at android.support.v4.app.FragmentManagerImpl.enqueueAction(FragmentManager.java:1511) at android.support.v4.app.BackStackRecord.commitInternal(BackStackRecord.java:638) at android.support.v4.app.BackStackRecord.commit(BackStackRecord.java:617) 

从异常的堆栈信息分析开始，调用FragmentTransaction的commit方法

FragmentTranstraction.commit() -> FragmentManager.enqueueAction() -> FragmentManager.checkStateLoss()

FragmentTranstraction的实现类 BackStackRecord的具体方法实现

    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(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;
    }

FragmentManager的实现类FragmentManagerImpl

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);
        }
    }
}
private void checkStateLoss() {
    if (mStateSaved) {
        throw new IllegalStateException(
                "Can not perform this action after onSaveInstanceState");
    }
    if (mNoTransactionsBecause != null) {
        throw new IllegalStateException(
                "Can not perform this action inside of " + mNoTransactionsBecause);
    }
}

所以关键的是找到mStateSaved这个标记的变化的代码，
FragmentManager的实现类FragmentManagerImpl的saveAllState中对其标记为true

Parcelable saveAllState() {
        // Make sure all pending operations have now been executed to get
        // our state update-to-date.
        execPendingActions();
        // android.os.Build.VERSION.SDK_INT >= 11;
        if (HONEYCOMB) {
            // As of Honeycomb, we save state after pausing.  Prior to that
            // it is before pausing.  With fragments this is an issue, since
            // there are many things you may do after pausing but before
            // stopping that change the fragment state.  For those older
            // devices, we will not at this point say that we have saved
            // the state, so we will allow them to continue doing fragment
            // transactions.  This retains the same semantics as Honeycomb,
            // though you do have the risk of losing the very most recent state
            // if the process is killed...  we'll live with that.
            mStateSaved = true;
        }
        //....
}

saveAllState方法在FragmentActivity 的onSaveInstanceState被调用

    /**
     * Save all appropriate fragment state.
     */
    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        Parcelable p = mFragments.saveAllState();
        if (p != null) {
            outState.putParcelable(FRAGMENTS_TAG, p);
        }
    }

FragmentController的saveAllState方法

public Parcelable saveAllState() {
    return mHost.mFragmentManager.saveAllState();
}

最后再调用到FragmentManager的saveAllState

为了状态的正常保存和恢复，在Activity的状态保存后不能再提交事务了，
要么选择使用commitAllowingStateLoss来避免崩溃，或者在commit之前先调用FragmentManager.noteStateNotSaved来把状态改为false

public void noteStateNotSaved() {
    mStateSaved = false;
}