Question

I am coding an app that give me some problems. I have an activity A with a button. When the user presses the button, it retrieves data from the Internet using an AsyncTask class, and when the data is retrieved, the AsyncTask class calls a method declared in activity A that starts a new activity B.

But this has a problem I don’t know how to fix. If the user presses the button twice (or more than once), the data is retrieved two times and activity B is launched twice. So, once I am in activity B, when I press the back button that instance of Activity B is destroyed but I’m still in the activity B (because it was launched twice).

Some code from the AsyncTask class which download the data:

@Override
protected String doInBackground(String... urls) {
    try {
        String data = downloadUrl(urls[0]);
        return downloadUrl(urls[0]);
    } catch (IOException e) {
        return m_errorCODE;
    }
}

@Override
protected void onPostExecute(String result) {
    ((MainActivity) mParentActivity).displayInfo(result);
}

Method displayInfo() from MainActivity which launches the activity B:

public void displayInfo(String result) {
        int duration = Toast.LENGTH_LONG;
        Toast toast = null;
        setSupportProgressBarIndeterminateVisibility(false);
        if (result == "404 ERROR") {
            toast = Toast.makeText(getBaseContext(), R.string.error_file_not_found, duration);
            toast.show();
        } else if (result == "CONNECTION_ERROR") {
            toast = Toast.makeText(getBaseContext(), R.string.error_connection, duration);
            toast.show();
        } else {

            Intent intent = new Intent(this, DisplayInfo.class);
            m_data = result;
            intent.putExtra(DOWNLOADED_DATA, m_data);
            intent.putExtra(FORMAT_OK, Boolean.toString(m_formatOK));
            intent.putExtra(URL_FILE, urlFile);
            startActivity(intent);
        }
    }

So I have three problems:

1. How can I download the data only once?
2. How can I open activity B only once?
3. How can I know in my Activity A is still alive when it executes onPostExecute()?

To solve #2 I tried using intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP) before starting activity B. It opens only one activity B, but if I press back, activity A starts again another instance of activity B.

Do I need to paste more code? Thanks in advance.

Answer 1

For #1 and #2 you can use an instance variable in your activity that gets set after a button is clicked. In the onClick() method of the button you can check this variable to see if you should start the AsyncTask or if it was already started.

For #3 you could maybe test if mParentActivity is not null? I am not sure about that though.