显示ProgressDialog，直到AsyncTask完成，然后更新UI

Question

This is the flow of my app:

1) The user takes a picture or video

2) The media is saved to internal storage

3) The path is assigned to a custom object

4) The UI is updated to indicate that the user can continue

The UI updates only if the custom object has either an imagepath or a videopath. I've just started using AsyncTask to save to internal storage in a background thread so the app will not hang while saving large files, but I'm having some problems.

What I want to do: Display a ProgressDialog until doInBackground() finishes, then assign the path to my Object, and then continue on the main thread to update the UI.

Right now, the main thread will continue while AsyncTask is still working, and the UI will not update correctly since the path has not yet been assigned to the Object.

I've read about AsyncTask#get() , but I'm not sure how to implement it with the ProgressDialog . I tried, and the main thread didn't seem to wait for the results before continuing.

I'd really appreciate any help. Thank you!

My AsyncTask :

private class SaveMediaTask extends AsyncTask<Integer, Void, String>
{
    private ProgressDialog progressDialog;
    private int mediaType;

    public SaveMediaTask()
    {
        progressDialog = new ProgressDialog(getActivity(), ProgressDialog.THEME_HOLO_LIGHT);
        progressDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);
        this.progressDialog.setTitle("Processing");
    }

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

    protected String doInBackground(Integer... mediaType)
    {
        //save to internal storage and return the path
        return path;
    }

    protected void onPostExecute(String path)
    {
        //by the time this runs, the UI has already tried to update itself on the main thread,
        //and found that myObject does not yet have a path. Once this runs, it is too late.
        myObject.setPath(path);

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

How I call it, immediately after the user leaves the camera:

new SaveMediaTask().execute(MEDIA_TYPE_IMAGE);

//WAIT HERE AND DISPLAY PROGRESSDIALOG UNTIL TASK IS DONE

//update UI

Answer 1

Your onPostExecute should notify your activity, that it can continue. So basically:

// Start the task from your activity
new SaveMediaTask().execute(MEDIA_TYPE_IMAGE);

// Method that will be called when task is completed
public void taskComplete() {
    // update UI
}

...

protected void onPostExecute(String path) {
    myObject.setPath(path);

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

    ((YourActivity)getActivity()).taskComplete();
}

Answer 2

You can move your code marked with //update UI to the end of your onPostExecute method. onPostExecute is always called on UI thread and is a good place to update UI to reflect the AsyncTask work results.

Answer 3

the main thread didn't seem to wait for the results before continuing.

the main thread wouldn't wait. This isn't how AsyncTask work. AsyncTask runs in background along with your main thread.

then continue on the main thread to update the UI.

You don't need to continue on the main thread to update the UI once AsyncTask task is done, you could simply execute that post doInBackground task in onPostExecute() after dismissing the progressDialog , ie progressDialog.dismiss(); because onPostExecute runs in the UI thread.

Also, the good way is to start progressDialog inside onPreExecute() method and dismiss it in onPostExecute without checking if the progessDialog is still showing or not because onPostExecute() would only run if the doInBackground() method is finished doing its job.

What I want to do: Display a ProgressDialog until doInBackground() finishes, then assign the path to my Object, and then continue on the main thread to update the UI.

• Start ProgressDialog in onPreExecute ,
• Assign the path to your object in doInBackground
• Dismiss ProgressDialog in onPostExecute ,
• Continue your main thread work in onPostExecute since it runs in UI thread.

You can also update your UI thread while doInBackground is still running by invoking publishProgress() . Each call to this method will trigger the execution of onProgressUpdate() on the UI thread.

Tip: Its a good idea to dismiss progressDialog inside onCancelled() method beside dismissing it in onPostExecute() in case if you ever cancel your task inside doInBackground