android - 在 Postexec 上提交 fragment

Question

我是 Android 开发新手，提交 fragment 时遇到问题。异步任务完成后，我尝试从 OnPost 方法提交 fragment ，但应用程序销毁了。如果我通常从 MapsActivity.class 提交 fragment ，一切正常。

 @Override
    protected void onPostExecute(String s) {


        try {
            JSONObject jsonObject = new JSONObject(s);
            JSONArray jsonArray = jsonObject.getJSONArray("routes").getJSONObject(0).getJSONArray("legs").getJSONObject(0).getJSONArray("steps");
            int count = jsonArray.length();


            Log.i("Distance", jsonArray.toString());




            String[] polyline_array = new String[count];
            JSONObject jsonObject2;
            JSONObject jsonObject3;
            for (int i =0;i<count;i++){

                jsonObject2 = jsonArray.getJSONObject(i);

                String polygone = jsonObject2.getJSONObject("polyline").getString("points");


                polyline_array[i] = polygone;

            }

            distancetotal = 0;

            for (int i =0;i<count;i++){
                jsonObject3 = jsonArray.getJSONObject(i);
                String distance = jsonObject3.getJSONObject("distance").getString("value");






                    this.distancetotal += Double.valueOf(distance);




            }





            float umweg =  ((totaldistance/distancetotal)-1)*100;
             umwegint = (int) umweg;




            MapsActivity mapsActivity = new MapsActivity();
            mapsActivity.a = String.valueOf(distancetotal);
            mapsActivity.c = String.valueOf(umwegint);
            mapsActivity.updateUi();

            int count2 = polyline_array.length;
            for (int i = 0; i< count2;i++){

                PolylineOptions options2 = new PolylineOptions();
                options2.color(Color.GREEN);
                options2.width(10);
                options2.addAll(PolyUtil.decode(polyline_array[i])).geodesic(true);



                mMap.addPolyline(options2);






            }





        } catch (JSONException e) {
            e.printStackTrace();
        }


    }

MapsActivity.class

 public class MapsActivity extends FragmentActivity implements  OnMapReadyCallback, View.OnClickListener, RoutingListener{

    public void updateUi(){


            Fragment newfragment = new ResultFragment();
            fragmentTransaction = getSupportFragmentManager().beginTransaction();
            fragmentTransaction.replace(R.id.fragment4, newfragment);
            fragmentTransaction.commit();

        }

}

java.lang.IllegalStateException: Activity has been destroyed
                  at 

android.support.v4.app.FragmentManagerImpl.enqueueAction(FragmentManager.java:2087)
                      at android.support.v4.app.BackStackRecord.commitInternal(BackStackRecord.java:678)
                      at android.support.v4.app.BackStackRecord.commit(BackStackRecord.java:632)
                      at taxiapp.com.taxitracker.MapsActivity.updateUi(MapsActivity.java:607)
                      at taxiapp.com.taxitracker.GetDirectionsData.onPostExecute(GetDirectionsData

Answer 1

我猜想在 MapsActivity 中，您通过调用 GetDirectionsData(从 AsyncTask 扩展)开始一个异步任务，当任务完成时，您将结果返回给MapsActivity` 更新 UI。

根本原因:这些代码行导致您的应用崩溃

MapsActivity mapsActivity = new MapsActivity();
mapsActivity.a = String.valueOf(distancetotal);
mapsActivity.c = String.valueOf(umwegint);
mapsActivity.updateUi();

原因:在Android中你不允许创建一个activity实例，系统会为你创建并调用onCreate方法来通知你activity已经创建。当您创建一个新的 MapsActivity 实例并调用 updateUi 方法时，系统无法在任何地方找到该 Activity (因为您创建的 Activity 不是系统创建的)并且他们认为该 Activity 可能会被摧毁。

解决方案:在GetDirectionsData类中添加这几行代码

public class GetDirectionsData extends AsyncTask<String, Void, String> {

    // Add this line
    private WeakReference<MapsActivity> mapsActivityWeakReference;

    // Add this constructor
    public GetDirectionsData(MapsActivity mapsActivity) {
        mapsActivityWeakReference = new WeakReference<>(mapsActivity);
    }

    @Override
    protected void onPostExecute(String s) {
        super.onPostExecute(s);

        // Your code
        float umweg =  ((totaldistance/distancetotal)-1)*100;
        umwegint = (int) umweg;

        // Remove these lines of code
        /*MapsActivity mapsActivity = new MapsActivity();
        mapsActivity.a = String.valueOf(distancetotal);
        mapsActivity.c = String.valueOf(umwegint);
        mapsActivity.updateUi();*/

        // Add these lines of code instead
        MapsActivity mapsActivity = mapsActivityWeakReference.get();
        if (mapsActivity != null) {
            mapsActivity.a = String.valueOf(distancetotal);
            mapsActivity.c = String.valueOf(umwegint);
            mapsActivity.updateUi();
        }

        int count2 = polyline_array.length;
        // Your code
    }
}

在 MapsActivity 类中

DirectionsData directionsData = new GetDirectionsData(this);
directionsData.execute();