Android app force close when dismissing progress dialog in asynctask

Question

I am using a subclass inside my main activity to do an asynctask. I plan to move it into a separate file afterwards I just prefer doing it this way to confirm it is working. I am trying to do a HTTPRequest with a progress dialog because of the potentially long waiting times. I am doing nothing in the doInBackground() because I've read you cannot access the UI layer from there. Ultimately what is happening is I show my progress dialog on onPreExecute() and dismiss it in onPostExecute() . I am calling each task separately above which could be 99% of my problem but I need to be able to pass my activity to the tasks for them to function properly.

I've broken a lot of the steps down into little pieces. I am getting a null pointer exception which leads me to believe my problem comes from not using execute() but I just can't seem to figure out how that works.

new CodeRetrievalItem().onPreExecute(MyActivity.this);

new CodeRetrievalItem().onPostExecute(MyActivity.this);

class CodeRetrievalItem extends AsyncTask<Void, Void, Void>{

ProgressDialog dialog;
protected void onPreExecute(Activity actpass){
        dialog = new  ProgressDialog(actpass);
    dialog.setMessage("Loading");
    dialog.setIndeterminate(false); 
    dialog.setCancelable(false);
    dialog.show();

}

@Override
protected Void doInBackground(Void... arg0) {
    // TODO Auto-generated method stub
    return null;
}

protected void onPostExecute(Activity actpass){

            // Execute HTTP Request
    try{
        dialog.dismiss();
    }
    catch(Exception e){
        Toast.makeText(actpass, "IOE response " + e.toString(), Toast.LENGTH_LONG).show();
    }

}

}

Answer 1

You are using AsyncTask completely wrong. There are several major mistakes in your code.

AsyncTask protected methods

Do not call them yourself. In the documentation for AsyncTask , you can find:

Do not call onPreExecute(), onPostExecute(Result), doInBackground(Params...), onProgressUpdate(Progress...) manually.

You have to start your AsyncTask by calling execute(Params...) method. This will cause that:

1. onPreExecute() is called prior to the start of background execution and is called on the main thread
2. doInBackground(Params...) is called in background
3. After background execution is finished onPostExecute(Result) is called on the main thread.

Unless you call execute(Params...) nothing of this will happen.

Generic parameters

Moreover there is a problem with your generic parameters. AsyncTask has three generic paramters AsyncTask<Params, Progress, Result> (in your case AsyncTask<Void, Void, Void> , you have to match these parameters in the methods' parameters.

Therefore create:

1. onPreExecute() instead of onPreExecute(Activity actpass)
2. onPostExecute(Void) instead of onPostExecute(Activity actpass)

Those methods you defined aren't bound to the async execution in any way, they are just some extra methods in the class.

If you need to access the activity inside the AsyncTask pass it in different way (eg in a constructor). Beware that passing an Activity to an AsyncTask might lead to possible memory leaks (it's good to utilize WeakReference<T> here) and accessing dialogs in onPostExecute(Result) might cause crash if not properly handled.

Answer 2

change your code like this

new CodeRetrievalItem(MyActivity.this).execute();



class CodeRetrievalItem extends AsyncTask<Void, Void, Void>{
Activity context;
public CodeRetrievalItem(Activity actpass)
{
context=actpass;
}
ProgressDialog dialog;
protected void onPreExecute(Activity actpass){
        dialog = new  ProgressDialog(context);
    dialog.setMessage("Loading");
    dialog.setIndeterminate(false); 
    dialog.setCancelable(false);
    dialog.show();

}

@Override
protected Void doInBackground(Void... arg0) {
    // TODO Auto-generated method stub
    return null;
}

protected void onPostExecute(Activity actpass){

            // Execute HTTP Request
    try{
        dialog.dismiss();
    }
    catch(Exception e){
        Toast.makeText(context, "IOE response " + e.toString(), Toast.LENGTH_LONG).show();
    }