编程语言
首页 > 编程语言> > 从源码角度理解Can not perform this action after onSaveInstanceState异常

从源码角度理解Can not perform this action after onSaveInstanceState异常

作者:互联网

在开发中经常遇到Fatal Exception: java.lang.IllegalStateException:Can not perform this action after onSaveInstanceState异常,那这个异常出现原因是什么呢,怎么解决呢?

问题描述
出现Fatal Exception: java.lang.IllegalStateException:Can not perform this action after onSaveInstanceState异常有两种情况:

  1. FragmentTransaction的commit()时出现:
    具体堆栈信息如下:

  2. Activity/FragmentActivity的onBackPressed时出现:
    具体堆栈信息如下:

问题原因和解决方法
出现Fatal Exception: java.lang.IllegalStateException:Can not perform this action after onSaveInstanceState异常细分为两种情况,但产生原因是一样的,都是因为在存储状态之后调用了commit()/onBackPressed()方法,在commit()/onBackPrssed()中会调用checkStateLoss()方法,具体如下:

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

其中如果this.mStateSaved为true,就会抛出这个异常。而对于mStateSaved用了保存Fragment状态,是在Activity#onSaveInstanceState时通过调用FragmentManager#saveAllState方法,来进行Fragment的状态保存,同时设置mStateSaved为true,用来标识状态已被保存过。下面具体分析两种情况:
1.FragmentTransaction的commit()时出现:
FragmentTransaction的commit()会调用BackStackRecord.java的commit()方法,可以看到commit()和commitAllowingStateLoss()的具体实现,如下:
BackStackRecord.java

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

    @Override
    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);
            pw.close();
        }
        mCommitted = true;
        if (mAddToBackStack) {
            mIndex = mManager.allocBackStackIndex(this);
        } else {
            mIndex = -1;
        }
        mManager.enqueueAction(this, allowStateLoss);
        return mIndex;
    }

可以看出commit()和commitAllowingStateLoss()都是调用commitInternal()方法,只是传参不一样,而在commitInternal()方法中,根据传参会调用FragmentManager的enqueueAction方法,具体如下:

public void enqueueAction(OpGenerator 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);
            scheduleCommit();
        }
    }

如果allowStateLoss为true的话会调用checkStateLoss()方法进行检测。所以从源码可知commit()和commitAllowingStateLoss()的区别在于,commit()方法会检测fragment的状态,而commitAllowingStateLoss()不会对状态进行检测,状态会丢失。
由此得到结论和解决方法:
(1)在activity的生命周期方法中提交事务要小心,越早越好,比如onCreate或是在接收用户的输入时来提交。尽量避免在onActivityResult()方法中提交。
(2)避免在异步的回调方法中执行commit。因为他们感知不到当前activity生命周期的状态。
(3)如果ui状态的改变对用户来说是可以接受的话使用commitAllowingStateLoss()代替commit()。
2.Activity/FragmentActivity的onBackPressed时出现:
按返回键时会调用super.onBackPressed()方法,如下:
FragmentActivity.java中

@Override
    public void onBackPressed() {
        if (!mFragments.getSupportFragmentManager().popBackStackImmediate()) {
            super.onBackPressed();
        }
    }

会调用FragmentManagerImpl的popBackStackImmediate()方法,如下:

@Override
    public boolean popBackStackImmediate() {
        checkStateLoss();
        return popBackStackImmediate(null, -1, 0);
    }

会调用checkStateLoss()方法,同上,会检测状态。
那这个问题怎么解决呢?
(1)添加try…catch,感觉这种方法并没有解决根本问题,不推荐。
(2)重写onSaveInstanceState方法,不调用super,但onSaveInstanceState方法是用来存储状态使用的,不调用super,防止系统保存fragment的状态,可能会引发一引起其他的问题;再有就是,对于support包下,在其onStop时也会把mStateSaved置为true,仍然能够遇到state loss exception。不推荐。
(3)设置标志位,状态保存过后,不处理KEY事件。具体如下:

public class FragmentStateLossActivity extends Activity {
    private static final String TAG = "Fragment state loss";
    private boolean mStateSaved;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_fragment_state_loss);
        mStateSaved = false;
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        // Not call super won't help us, still get crash
        super.onSaveInstanceState(outState);
        imStateSaved = true;
    }

    @Override
    protected void onResume() {
        super.onResume();
        mStateSaved = false;
    }

    @Override
    protected void onStop() {
        super.onStop();
        mStateSaved = true;
    }

    @Override
    protected void onStart() {
        super.onStart();
        mStateSaved = false;
    }

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        if (!mStateSaved) {
            return super.onKeyDown(keyCode, event);
        } else {
            // State already saved, so ignore the event
            return true;
        }
    }

    @Override
    public void onBackPressed() {
        if (!mStateSaved) {
            super.onBackPressed();
        }
    }
}

但这种方法虽然能从根本上解决crash,但相对比较麻烦。
目前还没有发现更好的方法。

参考
https://juejin.im/entry/58636864128fe10069efc4b5
https://huxian99.github.io/2016/08/28/cj3qymo360000owxk9zp17alo/

标签:perform,方法,mStateSaved,源码,Override,commit,onSaveInstanceState,super
来源: https://www.cnblogs.com/sishuiliuyun/p/14294353.html