java - onPostExecute 中来自 AsyncTask 的 setText

Question

我有以下 AsyncTask 从 xml 文件中获取一些数据:

public class GetTripDetails extends AsyncTask<String, Integer, String[]> {

    private Context context;
    private String[] details = new String[4];

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
    }

    public GetTripDetails (Context parsedContext){
        context = parsedContext;
    }

    @Override
    protected String[] doInBackground(String... params) {
        SharedPreferences sharedpreferences;
        String activeTrip;

        try {
            sharedpreferences = context.getSharedPreferences("prefs", Context.MODE_PRIVATE);
            activeTrip = sharedpreferences.getString("activeTrip", null); //Liest den Namen des aktiven Trips

            File file = new File(Environment.getExternalStorageDirectory() + "/" + "android/data/com.travip/" + activeTrip + ".trp");
            FileInputStream fis = new FileInputStream(file);

            XmlPullParserFactory xmlFactoryObject = XmlPullParserFactory.newInstance();
            XmlPullParser myparser = xmlFactoryObject.newPullParser();
            myparser.setInput(fis, null);

            int event = myparser.getEventType();
            while (event != XmlPullParser.END_DOCUMENT)  {
                String tag_name = myparser.getName();
                switch (event){
                    case XmlPullParser.START_TAG:
                        break;
                    case XmlPullParser.END_TAG:
                        if(tag_name.equals("trip")){
                            details[0] = myparser.getAttributeValue(null, "name");
                            details[1] = myparser.getAttributeValue(null, "startdate");
                            details[2] = myparser.getAttributeValue(null, "enddate");
                            details[3] = myparser.getAttributeValue(null, "description");
                        }
                        break;
                }
                event = myparser.next();
            }

        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        } catch (IllegalStateException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (XmlPullParserException e) {
            e.printStackTrace();
        }

        return details;
    }

    @Override
    protected void onProgressUpdate(Integer... values) {
        super.onProgressUpdate(values);
    }

    @Override
    protected void onPostExecute(String[] result) {
        Toast.makeText(context, result[0], Toast.LENGTH_LONG).show();

        tv_title.setText(result[0]);
        tv_subtitle.setText(result[1] + " - " + result[2]);

        super.onPostExecute(result);
    }

}

尽管 tv_title 和 tv_subtitle 是公共(public)的并且 AsyncTask 在同一个类中，但它会产生一个空指针异常。我试图在 onStart() 中执行它(-> 在 TextViews 应该已经初始化之后)，但它也没有用。谢谢你的建议

更新:
日志:

08-08 12:09:01.446 22390-22390/com.travip E/AndroidRuntime: FATAL EXCEPTION: main
                                                        Process: com.travip, PID: 22390
                                                        java.lang.RuntimeException: Unable to start activity ComponentInfo{com.travip/com.travip.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
                                                            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2440)
                                                            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2500)
                                                            at android.app.ActivityThread.access$900(ActivityThread.java:163)
                                                            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1362)
                                                            at android.os.Handler.dispatchMessage(Handler.java:102)
                                                            at android.os.Looper.loop(Looper.java:148)
                                                            at android.app.ActivityThread.main(ActivityThread.java:5585)
                                                            at java.lang.reflect.Method.invoke(Native Method)
                                                            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:730)
                                                            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:620)
                                                         Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
                                                            at com.travip.MainActivity$GetTripDetails.onPreExecute(MainActivity.java:192)
                                                            at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:604)
                                                            at android.os.AsyncTask.execute(AsyncTask.java:551)
                                                            at com.travip.MainActivity.onCreate(MainActivity.java:88)
                                                            at android.app.Activity.performCreate(Activity.java:6279)
                                                            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108)
                                                            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2393)
                                                            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2500) 
                                                            at android.app.ActivityThread.access$900(ActivityThread.java:163) 
                                                            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1362) 
                                                            at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                            at android.os.Looper.loop(Looper.java:148) 
                                                            at android.app.ActivityThread.main(ActivityThread.java:5585) 
                                                            at java.lang.reflect.Method.invoke(Native Method) 
                                                            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:730) 
                                                            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:620) 

完整代码:

public class MainActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener, Overview.OnFragmentInteractionListener, Activities.OnFragmentInteractionListener, Marked.OnFragmentInteractionListener, Map.OnFragmentInteractionListener, Nearby.OnFragmentInteractionListener {
NavigationView navigationView;
String activeMenu;

public ImageView expand_collapse;
public TextView tv_title;
public TextView tv_subtitle;
public View headerLayout;

DrawerLayout drawer;

@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);

    Mapbox.getInstance(getApplicationContext(), getString(R.string.access_token));

    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawer.setDrawerListener(toggle);
    toggle.syncState();

    navigationView = (NavigationView) findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);
    navigationView.setCheckedItem(R.id.nav_overview); //Setzt den aktiven Menüeintrag
    activeMenu = "main";

    headerLayout = navigationView.getHeaderView(0);
    headerLayout.setOnClickListener(new View.OnClickListener() { //Ändert das Menü, wenn auf den Header gecklickt wird
        @Override
        public void onClick(View v) {
            toggleMenu();
        }
    });

    expand_collapse = (ImageView) headerLayout.findViewById(R.id.expand_collapse);
    tv_title = (TextView) headerLayout.findViewById(R.id.tv_name);
    tv_subtitle = (TextView) headerLayout.findViewById(R.id.subtitle);

    new GetTripDetails(getApplicationContext()).execute();
}
//Here is the asynctask (see above)

更新 2:
logcat(初始化 TextView 后):

08-08 12:20:59.008 934-934/com.travip E/AndroidRuntime: FATAL EXCEPTION: main
                                                    Process: com.travip, PID: 934
                                                    java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
                                                        at com.travip.MainActivity$GetTripDetails.onPostExecute(MainActivity.java:255)
                                                        at com.travip.MainActivity$GetTripDetails.onPostExecute(MainActivity.java:183)
                                                        at android.os.AsyncTask.finish(AsyncTask.java:651)
                                                        at android.os.AsyncTask.access$500(AsyncTask.java:180)
                                                        at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:668)
                                                        at android.os.Handler.dispatchMessage(Handler.java:102)
                                                        at android.os.Looper.loop(Looper.java:148)
                                                        at android.app.ActivityThread.main(ActivityThread.java:5585)
                                                        at java.lang.reflect.Method.invoke(Native Method)
                                                        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:730)
                                                        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:620)

更新 3:
nav_header_main.xml(抽屉导航的headerLayout(TextView所在的位置)):

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="@drawable/side_nav_bar"
android:gravity="bottom"
android:theme="@style/ThemeOverlay.AppCompat.Dark"
android:id="@+id/nav_header_main" >

<RelativeLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/nav_header_top"
    android:background="?attr/selectableItemBackground">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:paddingTop="@dimen/nav_header_vertical_spacing"
        android:id="@+id/title"
        android:text="Spanien"
        android:textAppearance="@style/TextAppearance.AppCompat.Body1" />
    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/title"
        android:id="@+id/subtitle"
        android:text="21.3.17 - 1.4.17"
        android:textAppearance="@style/TextAppearance.AppCompat.Small" />

    <ImageView
        android:id="@+id/expand_collapse"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentEnd="true"
        android:layout_alignParentRight="true"
        android:layout_alignParentBottom="true"
        android:src="@drawable/ic_expand"
        android:background="?attr/actionBarItemBackground"/>

</RelativeLayout>

activity_main.xml:

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:mapbox="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:openDrawer="start">

<include
    layout="@layout/fragment_overview"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/fragment_container"/>

<android.support.design.widget.NavigationView
    android:id="@+id/nav_view"
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:layout_gravity="start"
    android:fitsSystemWindows="true"
    app:headerLayout="@layout/nav_header_main"
    app:menu="@menu/activity_main_drawer"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"/>

 </android.support.v4.widget.DrawerLayout>

Answer 1

为避免上下文内存泄漏，我建议您将 GetTripDetails 类设为静态，并将构造函数中的上下文参数替换为将任务结果返回到 Activity 的适当回调接口(interface)。它也肯定会解决您的问题，因为在您的情况下，没有设置您的任务的引用。

static class GetTripDetails extends AsyncTask<String, Void, String[]> {

    private WeakReference<Callback> callbackRef;

    public GetTripDetails(final Callback callback){
        callbackRef = new WeakReference<>(callback);
    }

    @Override
    protected String[] doInBackground(String... params) {
        // Do your stuff
        return details;
    }

    @Override
    protected void onPostExecute(String[] result) {
        final Callback callback = callbackRef.get();
        if (callback == null) {
            return;
        }
        callback.onResult(result);
    }

    interface Callback {
        void onResult(String[] result);
    }

}

在你的 Activity 中:

private final Callback callback = new Callback() {
    void onResult(String[] result) {
        // set values in your textview..
    } 
}

private GetTripDetails task;

@Override 
protected void onCreate(Bundle savedInstanceState) {
    //...
    task = new GetTripDetails(callback);
    task.execute(); 
}