如何将上下文传递给 AsyncTask？

Question

How to pass context in Async Task class which is coded in different java file from Main Activity but it called from main activity?

Below is my code:

 @Override

protected void onPostExecute(List<Movie_ModelClass> result) {
        super.onPostExecute(result);

        if (result != null) {
            Movie_Adapter movieAdapter = new Movie_Adapter(new MainActivity().getApplicationContext() , R.layout.custom_row, result);
            MainActivity ovj_main = new MainActivity();
            ovj_main.lv_main.setAdapter(movieAdapter);
        } else {
            Toast.makeText(new MainActivity().getApplicationContext() ,"No Data Found", Toast.LENGTH_LONG);

        }
        if (progressDialog.isShowing()) {
            progressDialog.dismiss();
    }

Answer 1

You could just pass a Context instance as a constructor parameter (and keep a WeakReference to it to avoid memory leaks).

For example:

public class ExampleAsyncTask extends AsyncTask {
    private WeakReference<Context> contextRef;

    public ExampleAsyncTask(Context context) {
        contextRef = new WeakReference<>(context);
    }

    @Override
    protected Object doInBackground(Object[] params) {
        // ...
    }

    @Override
    protected void onPostExecute(Object result) {
        Context context = contextRef.get();
        if (context != null) {
            // do whatever you'd like with context
        }
    }
}

And the execution:

new ExampleAsyncTask(aContextInstance).execute();

Answer 2

You can just pass the context in the constructor of your AsyncTask.

MyAsyncTask.java

public class MyAsyncTask extends AsyncTask<Void, Integer, List> {

    private final Context mContext;

    public MyAsyncTask(final Context context) {
         mContext = context;
    }
}

and then just use the mContext variable in your onPostExecute() method.

When you call your AsyncTask from your MainActivity, you pass the context to the constructor of MyAsyncTask.

MainActivity.java

final MyAsyncTask task = new MyAsyncTask(getApplicationContext());
task.execute();

Answer 3

I encountered same issue when trying to compress an image using an Async class. I had a constructor in place so I just added context as below

   public BackgroundImageResize(Context context, Bitmap bm) {
    if (bm != null){
        mBitmap = bm;
    }
    this.context =context;
}

Then i called the class like below,

public void uploadDevicePhoto(Uri imageUri){

BackgroundImageResize resize = new BackgroundImageResize(this,null);
resize.execute(imageUri);

}