Android 在主 Activity 中获取 AsyncTask 的结果并在 onCreate 方法之外将 View 添加到 XML 布局

Question

我正在尝试从 Youtube 获取要显示在我的 Activity 上的视频列表。我使用 onCreate 和 getResult 这两个方法创建了主要 Activity MediaActivity.java，这是从 onPostExecute AsyncTask 类中的方法。这是 MediaActivity 代码:

    public class MediaActivity extends Activity {
    YoutubeConnect youtubeRequest = new YoutubeConnect(); 
    ViewGroup media;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        youtubeRequest.execute();
        setContentView(R.layout.media);
        media = (ViewGroup) this.findViewById(R.layout.media);

    }

    public void getResult(String result) throws JSONException {


        JSONObject jsonObj = new JSONObject(result);

        JSONObject jsonObj2 = jsonObj.getJSONObject("feed");
        JSONArray the_json_array = jsonObj2.getJSONArray("entry");

        int size = the_json_array.length();

        for (int i = 0; i < size; i++) {

            //get all infos
               [...]
            LinearLayout main = new LinearLayout(this);
            main.setOrientation(0);
            main.setLayoutParams(new LayoutParams(LayoutParams.WRAP_CONTENT,LayoutParams.WRAP_CONTENT));

            //thumb
            WebView thumb = new WebView(this);
            thumb.loadUrl(video_thumb);
            main.addView(thumb);

            LinearLayout right = new LinearLayout(this);
            right.setOrientation(1);
            right.setLayoutParams(new LayoutParams(LayoutParams.WRAP_CONTENT,LayoutParams.WRAP_CONTENT));

            //title
            TextView title = new TextView(this);
            title.setText(video_title);
            right.addView(title);

            //description
            TextView description = new TextView(this);
            description.setText(video_description.substring(100)+"...");
            right.addView(description);

            //views
            TextView views = new TextView(this);
            views.setText("Views: "+video_views);
            right.addView(views);

            main.addView(right);                

            this.media.addView(main);
        }


     }      

}

这是我的自定义 AsyncTask 类代码:

 public class YoutubeConnect extends AsyncTask<String, String, String> {

    public YoutubeConnect() {
    }

    @Override
    protected String doInBackground(String... params) {
         try {
             String APIurl = "https://gdata.youtube.com/feeds/api/videos?author=author&alt=json";
                DefaultHttpClient httpClient = new DefaultHttpClient();
                HttpGet request = new HttpGet(APIurl);
                HttpResponse response = httpClient.execute(request);
                BufferedReader reader = new BufferedReader(new InputStreamReader(response.getEntity().getContent(), "UTF-8"));
                StringBuilder builder = new StringBuilder();
                for (String line = null; (line = reader.readLine()) != null;) {
                    builder.append(line).append("\n");
                }
                return builder.toString();

            } catch (Exception e) {
                return null;
            }

    }

    @Override
    protected void onPostExecute(String result) {
        super.onPostExecute(result);
        MediaActivity mainthread = new MediaActivity();
        try {
            mainthread.getResult(result);
        } catch (JSONException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }




 }

这是日志:

 05-17 11:01:26.383: E/AndroidRuntime(2687): FATAL EXCEPTION: main
05-17 11:01:26.383: E/AndroidRuntime(2687): java.lang.NullPointerException
05-17 11:01:26.383: E/AndroidRuntime(2687):     at android.content.ContextWrapper.getResources(ContextWrapper.java:81)
05-17 11:01:26.383: E/AndroidRuntime(2687):     at android.view.View.<init>(View.java:2696)
05-17 11:01:26.383: E/AndroidRuntime(2687):     at android.view.ViewGroup.<init>(ViewGroup.java:374)
05-17 11:01:26.383: E/AndroidRuntime(2687):     at android.widget.LinearLayout.<init>(LinearLayout.java:166)
05-17 11:01:26.383: E/AndroidRuntime(2687):     at wachipi.palio.MediaActivity.getResult(MediaActivity.java:47)
05-17 11:01:26.383: E/AndroidRuntime(2687):     at wachipi.palio.YoutubeConnect.onPostExecute(YoutubeConnect.java:41)
05-17 11:01:26.383: E/AndroidRuntime(2687):     at wachipi.palio.YoutubeConnect.onPostExecute(YoutubeConnect.java:1)
05-17 11:01:26.383: E/AndroidRuntime(2687):     at android.os.AsyncTask.finish(AsyncTask.java:602)
05-17 11:01:26.383: E/AndroidRuntime(2687):     at android.os.AsyncTask.access$600(AsyncTask.java:156)
05-17 11:01:26.383: E/AndroidRuntime(2687):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:615)
05-17 11:01:26.383: E/AndroidRuntime(2687):     at android.os.Handler.dispatchMessage(Handler.java:99)
05-17 11:01:26.383: E/AndroidRuntime(2687):     at android.os.Looper.loop(Looper.java:137)
05-17 11:01:26.383: E/AndroidRuntime(2687):     at android.app.ActivityThread.main(ActivityThread.java:4424)
05-17 11:01:26.383: E/AndroidRuntime(2687):     at java.lang.reflect.Method.invokeNative(Native Method)
05-17 11:01:26.383: E/AndroidRuntime(2687):     at java.lang.reflect.Method.invoke(Method.java:511)
05-17 11:01:26.383: E/AndroidRuntime(2687):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
05-17 11:01:26.383: E/AndroidRuntime(2687):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
05-17 11:01:26.383: E/AndroidRuntime(2687):     at dalvik.system.NativeStart.main(Native Method)

在各种尝试之前，setContenView 和 ViewGroup 行位于 getResult 方法中，但它给了我其他错误。我也尝试过使用调试器，但它给我错误 "source not found"，我认为我的安装中缺少某些东西。我真的不明白为什么它会抛出这个错误并且找不到任何相关信息。

Answer 1

该异常可能来自您实例化一个新的 MediaActivity 而不是使用您启动线程的 Activity 实例:

MediaActivity mainthread = new MediaActivity();//don't do this

而是在 YoutubeConnect 任务中传递您的 MediaActivity 的引用，并使用该引用调用 getResult:

private MediaActivity activityRef;

public YoutubeConnect(MediaActivity activityRef) {
     this.activityRef = activityRef;
}

在 onPosteExecute 方法中:

@Override
protected void onPostExecute(String result) {
    try {
        activityRef.getResult(result); // you should have some ckecks to see if the activity is still available.
    } catch (JSONException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

然后创建一个新的 YoutubeConnect 任务:

    YoutubeConnect youtubeRequest;
    ViewGroup media;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.media);
        youtubeRequest = new YoutubeConnect(this); 
        youtubeRequest.execute();
        media = (ViewGroup) this.findViewById(R.layout.media);
    }

另外，还有这个:

media = (ViewGroup) this.findViewById(R.layout.media);

您正在通过布局文件名而不是 id 在当前布局中搜索 View。应该是这样的:

media = (ViewGroup) this.findViewById(R.id.view_id_from_the_layout);