上下文無法解析爲類型

Question

前幾天我開始關注Android開發。我試圖從REST API中獲取數據，並且使用AsyncTask類來完成HTTP事務。問題是，我無法爲我的主要活動獲取上下文，以便查找我的ListView並將其內容分配給onPostExecute（）。

這裏是MainActivity.java：

public class MainActivity extends Activity { 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     new DownloadMediaList(this).execute(); 
    } 
} 

這裏是DownloadMediaList.java：

public class DownloadMediaList extends AsyncTask<Void, Void, ArrayList<Media>> { 

    ListView listView = null; 
    Context mainContext = null; 

    public DownloadMediaList(Context main){ 
     this.mainContext = main; 
    } 

    @Override 
    protected void onPreExecute(){ 
     listView = (ListView) mainContext.this.findViewById(R.id.media_list); 
    } 

    // Operations that we do on a different thread. 
    @Override 
    protected ArrayList<Media> doInBackground(Void... params){ 
     // Set an ArrayList to store the medias. 
     ArrayList<Media> mediaList = new ArrayList<Media>(); 

     // Call the REST API and get the request info and media list in a JSONObject. 
     RESTFunctions restRequest = new RESTFunctions(); 
     JSONObject jsonMedia = restRequest.getMediaList(); 

     // Try catch to catch JSON exceptions. 
     try { 
      // Store the media list into a JSONArray. 
      JSONArray mediaArray = jsonMedia.getJSONArray("media"); 

      // Create an instance of media to store every single media later. 
      Media media = new Media(); 

      // Loop through the JSONArray and add each media to the ArrayList. 
      for (int i=0; i<mediaArray.length();i++){ 
       media = new Media(); 
       JSONObject singleMedia = mediaArray.getJSONObject(i); 
       media.setTitle(singleMedia.getString("titre")); 
       media.setYear(singleMedia.getString("annee")); 
       media.setLength(singleMedia.getString("duree")); 
       mediaList.add(media); 
      } 
     } catch (JSONException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     } 

     // Return the ArrayList. 
     return mediaList; 
    } 

    // Operations we do on the User Interface. Synced with the User Interface. 
    @Override 
    protected void onPostExecute(ArrayList<Media> mediaList){ 
     // Set TextViews in ListViews here 
    } 
} 

這些類兩個單獨的文件。

此行特別是給我找麻煩：

listView = (ListView) mainContext.this.findViewById(R.id.media_list); 

我在做什麼錯？它告訴我Context無法解析爲一個類型，即使我已經導入了android.content.Context。我嘗試了實例化上下文，但我無法做到這一點。

Answer 1

在您的活動類

public interface TheInterface { 
public void theMethod(ArrayList<Media> result); 

} 
    } 

然後

DownloadMediaList task = new DownloadMediaList (MainAactivity.this,new TheInterface() { 
      @Override 
      public void theMethod(ArrayList<Media> result) { 
       // result available here do whatever with list media 
      } 
     }); 

然後在的AsyncTask

Context mainContext = null; 
TheInterface mlistener; 
public DownloadMediaList(Context main,TheInterface listener){ 
    this.mainContext = main; 
    mlistener = listener; 
} 

然後在onPostExecute

@Override 
protected void onPostExecute(ArrayList<Media> mediaList){ 
    if (mlistener != null) 
    { 
     mlistener.theMethod(mediaList); 
    } 
} 

編輯：

作爲替代。

您可以在asycntask中定義接口並讓您的活動類實現接口。

Answer 2

Context s沒有意見，也沒有findViewById方法 - 嘗試使mainContext爲Activity代替。

Answer 3

更換

listView = (ListView) mainContext.this.findViewById(R.id.media_list); 

與

listView = (ListView) mainContext.findViewById(R.id.media_list); 

和錯誤應該消失。

Answer 4

您可以將AsyncTask作爲內部類添加到您的活動中。

public class MainActivity extends Activity { 
    private Context context; 
@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    context=this; 
    new DownloadMediaList(this).execute(); 
} 

private class DownloadMediaList extends AsyncTask<Void, Void, ArrayList<Media>> { 

ListView listView = null; 




@Override 
protected void onPreExecute(){ 
    listView = (ListView) context.findViewById(R.id.media_list); 
} 

// Operations that we do on a different thread. 
@Override 
protected ArrayList<Media> doInBackground(Void... params){ 
    // Set an ArrayList to store the medias. 
    ArrayList<Media> mediaList = new ArrayList<Media>(); 

    // Call the REST API and get the request info and media list in a JSONObject. 
    RESTFunctions restRequest = new RESTFunctions(); 
    JSONObject jsonMedia = restRequest.getMediaList(); 

    // Try catch to catch JSON exceptions. 
    try { 
     // Store the media list into a JSONArray. 
     JSONArray mediaArray = jsonMedia.getJSONArray("media"); 

     // Create an instance of media to store every single media later. 
     Media media = new Media(); 

     // Loop through the JSONArray and add each media to the ArrayList. 
     for (int i=0; i<mediaArray.length();i++){ 
      media = new Media(); 
      JSONObject singleMedia = mediaArray.getJSONObject(i); 
      media.setTitle(singleMedia.getString("titre")); 
      media.setYear(singleMedia.getString("annee")); 
      media.setLength(singleMedia.getString("duree")); 
      mediaList.add(media); 
     } 
    } catch (JSONException e) { 
     // TODO Auto-generated catch block 
     e.printStackTrace(); 
    } 

    // Return the ArrayList. 
    return mediaList; 
} 

// Operations we do on the User Interface. Synced with the User Interface. 
@Override 
protected void onPostExecute(ArrayList<Media> mediaList){ 
    // Set TextViews in ListViews here 
} 
    }}