提交onSaveInstanceState之后无法执行此操作

Question

当ft.commit（）出现异常时，我不知道为什么。

java.lang.IllegalStateException: Can not perform this action after onSaveInstanceState
           at android.support.v4.app.FragmentManagerImpl.checkStateLoss(FragmentManager.java:1448)
           at android.support.v4.app.FragmentManagerImpl.enqueueAction(FragmentManager.java:1466)
           at android.support.v4.app.BackStackRecord.commitInternal(BackStackRecord.java:634)
           at android.support.v4.app.BackStackRecord.commit(BackStackRecord.java:613)
           at MainActivity.attachFragment(MainActivity.java:242)
           at MainActivity.attachFragment(MainActivity.java:225)
           at MainActivity.showHome(MainActivity.java:171)
           at MainActivity.onComplete(MainActivity.java:278)
           at MDownloadManager.onDownloadComplete(MDownloadManager.java:83)
           at DownloadRequestQueue$CallBackDelivery$2.run(DownloadRequestQueue.java:61)
           at android.os.Handler.handleCallback(Handler.java:733)
           at android.os.Handler.dispatchMessage(Handler.java:95)
           at android.os.Looper.loop(Looper.java:149)
           at android.app.ActivityThread.main(ActivityThread.java:5257)
           at java.lang.reflect.Method.invokeNative(Method.java)
           at java.lang.reflect.Method.invoke(Method.java:515)
           at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
           at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:609)
           at dalvik.system.NativeStart.main(NativeStart.java)

这是崩溃即将来临的我的方法。

FragmentManager fm = getSupportFragmentManager();
        FragmentTransaction ft = fm.beginTransaction();

        if(addToBackStack) {
            ft.addToBackStack(null);
            ft.add(R.id.frame_container, fragment, tag);
        } else {
            ft.replace(R.id.frame_container, fragment, tag);
        }

        ft.commit();

你知道什么地方出了问题吗？我不在项目中使用onSaveInstanceState。

Answer 1

IllegalStateException解决方案的完整解决方案

覆盖onSaveInstanceSate是一种黑客，不一定对所有场景都有效。同样，使用commitAllowingStateLoss（）是危险的，并且可能导致UI异常。

我们需要了解，当我们在Activity状态丢失后尝试提交片段时，会遇到IllegalStateException-Activity不在前台（要了解有关Activity状态的更多信息，请阅读）。因此，为避免（解决）此异常，我们只是将片段事务延迟到恢复状态之前

声明两个私有布尔变量

public class MainActivity extends AppCompatActivity {

    //Boolean variable to mark if the transaction is safe
    private boolean isTransactionSafe;

    //Boolean variable to mark if there is any transaction pending
    private boolean isTransactionPending;

现在，在onPostResume（）和onPause中，我们设置和取消设置布尔变量isTransactionSafe。想法是仅在活动处于前台时才将事务标记为安全，这样就不会造成状态损失。

/*
onPostResume is called only when the activity's state is completely restored. In this we will
set our boolean variable to true. Indicating that transaction is safe now
 */
public void onPostResume(){
    super.onPostResume();
    isTransactionSafe=true;
}
/*
onPause is called just before the activity moves to background and also before onSaveInstanceState. In this
we will mark the transaction as unsafe
 */

public void onPause(){
    super.onPause();
    isTransactionSafe=false;

}

private void commitFragment(){
    if(isTransactionSafe) {
        MyFragment myFragment = new MyFragment();
        FragmentManager fragmentManager = getFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.add(R.id.frame, myFragment);
        fragmentTransaction.commit();
    }
}

到目前为止，我们所做的一切都可以从IllegalStateException中保存下来，但是如果在活动移至后台之后完成交易，我们的交易将会丢失，就像commitAllowStateloss（）一样。为了解决这个问题，我们提供了isTransactionPending布尔变量

public void onPostResume(){
   super.onPostResume();
   isTransactionSafe=true;
/* Here after the activity is restored we check if there is any transaction pending from
the last restoration
*/
   if (isTransactionPending) {
      commitFragment();
   }
}


private void commitFragment(){

 if(isTransactionSafe) {
     MyFragment myFragment = new MyFragment();
     FragmentManager fragmentManager = getFragmentManager();
     FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
     fragmentTransaction.add(R.id.frame, myFragment);
     fragmentTransaction.commit();
     isTransactionPending=false;
 }else {
     /*
     If any transaction is not done because the activity is in background. We set the
     isTransactionPending variable to true so that we can pick this up when we come back to
foreground
     */
     isTransactionPending=true;
 }
}