Question

我有一个主活动类，我从中调用两个异步任务来加载外部（来自url）数据。我想显示进度条或简单的Toast消息，但我无法检索应用程序上下文。

这是异步类代码：

public class URLDataReader extends AsyncTask<URL, Void, String> {

   @Override
   protected void onPreExecute() {
      // here where I need the context
   }

    String result = null;

    @Override
    protected String doInBackground(URL... params) {

        try {
            BufferedReader reader = new BufferedReader(
                    new InputStreamReader(
                            params[0].openStream()
                    )
            );

            result = reader.readLine();

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

        return result;
    }
}

这是家庭活动类代码：

public class Home extends ActionBarActivity{

    Context context = getApplicationContext();

    public class Post{

        String id;
        String title;
        String description;
        String release;

        public String getTitle() {
            return title;
        }

        public String getDescription() {
            return description;
        }

        public String getRelease() {
            return release;
        }

        public void setTitle(String title) {
            this.title = title;
        }

        public void setDescription(String description) {
            this.description = description;
        }

        public void setRelease(String release) {
            this.release = release;
        }

        public void setId(String id) {
            this.id = id;
        }

        public String getId() {

            return id;
        }

    }

    @Override
    protected void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home);

        String result = null;
        ArrayList<Post> focusOn = new ArrayList<Post>();

        try {
        URL address = new URL("http://www.youth-stories.com/api/all.php");
        URLDataReader reader = new URLDataReader();
        result = reader.execute(address).get();

          //Toast.makeText(getApplicationContext(),result,Toast.LENGTH_LONG).show();
    }catch (IOException e){
        e.printStackTrace();
    } catch(InterruptedException e){
        e.printStackTrace();
    } catch (ExecutionException e){
        e.printStackTrace();
    }

        try {
            JSONObject obj = new JSONObject(result);
            String success = (String) obj.getString("success");
            JSONArray records = obj.getJSONArray("records");

            for(int i = 0; i < records.length(); i++) {
                Post tmp = new Post();
                tmp.setId(records.getJSONObject(i).getString("id"));
                tmp.setTitle(records.getJSONObject(i).getString("title"));
                tmp.setDescription(records.getJSONObject(i).getString("contents"));
                tmp.setRelease(records.getJSONObject(i).getString("data_post"));
                focusOn.add(tmp);
            }

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


        //wrapper
        LinearLayout container = (LinearLayout)findViewById(R.id.wrapper);

        for(int i = 0; i < focusOn.size(); i++){
            //item
            LinearLayout item = new LinearLayout(getApplicationContext());
            container.addView(item);
            item.setOrientation(LinearLayout.HORIZONTAL);

            //image

            Bitmap imageHandler = null;
            URL address = null;

           try {
               address = new URL("http://www.youth-stories.com/public/admin/CH_FocusOn/images/"+focusOn.get(i).getId()+"_thumb2.jpg");
               URLImageReader reader = new URLImageReader();
               imageHandler = reader.execute(address).get();

           }catch(MalformedURLException e){
               e.printStackTrace();
           }catch (ExecutionException e){
               e.printStackTrace();
           }catch(InterruptedException e){
               e.printStackTrace();
           }

            //Toast.makeText(getApplicationContext(),address.toString(),Toast.LENGTH_LONG).show();
            ImageView asset = new ImageView(getApplicationContext());
            //asset.setImageResource(R.drawable.ic_launcher);
            asset.setImageBitmap(imageHandler);
            item.addView(asset);

            LinearLayout.LayoutParams imgSettings = new LinearLayout.LayoutParams(300,300);
            asset.setLayoutParams(imgSettings);

            //inside
            LinearLayout contents = new LinearLayout(getApplicationContext());
            contents.setOrientation(LinearLayout.VERTICAL);
            item.addView(contents);
            //title
            TextView title = new TextView(getApplicationContext());
            title.setText(focusOn.get(i).getTitle());
            title.setTextAppearance(this, R.style.title);
            contents.addView(title);
            //description
            TextView description = new TextView(getApplicationContext());
            description.setText(focusOn.get(i).getDescription());
            description.setTextAppearance(this, R.style.description);
            contents.addView(description);
            //date
            TextView date = new TextView(getApplicationContext());
            date.setText(focusOn.get(i).getRelease());
            date.setTextAppearance(this, R.style.description);
            contents.addView(date);

        }
    }

}

现在正确传递了上下文，我可以编写一个简单的Toast但是当我尝试显示进度对话框应用程序崩溃时。这是修改后的代码：

public class URLImageReader extends AsyncTask<URL, Void, Bitmap> {

    Context context = null;
    ProgressDialog dialog = null;

    public URLImageReader(Context context){
        this.context = context;
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        dialog = new ProgressDialog(context);
        dialog.setMessage("Loading, please wait.");
        dialog.show();

    }

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


    @Override
    protected Bitmap doInBackground(URL... params) {

        Bitmap image = null;

        try {
            URL url= params[0];
            image = BitmapFactory.decodeStream(url.openConnection().getInputStream());


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

        return image;
    }
}

Answer 1

我想显示进度条或简单的Toast消息，但我想无法检索应用程序上下文

使用URLDataReader类构造函数获取Context以显示Toast和ProgressBar：

1。在URLDataReader类中创建一个以Context为参数的构造函数：

private Context mContext;
public URLDataReader(Context mContext){
  this.mContext=mContext;
}

2. 来自Home活动创建URLDataReader类的对象：

 URLDataReader reader = new URLDataReader(Home.this);

现在在mContext上使用onPreExecute来显示进度条。

注意：

下面

imageHandler = reader.execute(address).get();

调用get()方法停止执行当前线程，直到doInBackground方法执行未完成。

imageHandler = reader.execute(address);

如果要显示进度条，只需调用执行方法并使用get()处理onPostExecute方法响应
，则无需调用doInBackground方法

Answer 2

首先，在context内分配onCreate()，如：

`context = this;`

并为constructor context定义asynctask class，然后将其从您调用的地方传递

`URLDataReader reader = new URLDataReader(context);`

并在URLDataReader.java

中

Context contextAsynctask;  
public URLDataReader(Context context)
        {  
          this.contextAsynctask = context;      
        }

忘记在context班级

中写下您声明Asynctask的部分

Answer 3

在URLDataReader的构造函数中发送上下文，以便在Home类的onCreate方法中

    URL address = new URL("http://www.youth-stories.com/api/all.php");
    URLDataReader reader = new URLDataReader(context);
    result = reader.execute(address).get();

然后在URLDataReader类

中

void URLDataReader (Context _context) {
this.context = _context; // declare a global variable called context in your URLDataReader  class
}

在异步类中检索上下文

3 个答案: