Multiple AsyncTasks load the same WebView

Question

I'm trying to run many AsyncTasks to do loadData() on the same WebView. For example, I have 3 threads with 3 contents: "One", "Two", "Three" and a WebView with content "abc". (like the code below)

After 3 tasks finished, I want the WebView has content: "abcOneTwoThree". The idea of this code is that three threads will append its content to WebView at anytime, so the result could be "abcTwoOneThree" or "abcTwoThreeOne", etc ...

I read many concurrency articles but still don't understand how to implement this. This is my code. It just prints "abcThree".

public class UsingSameWebViewActivity extends Activity implements OnClickListener {
    private Button button1;
    private WebView webView;
    private String result;

    @Override
    public void onCreate(final Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        button1 = (Button)findViewById(R.id.button1);
        button1.setOnClickListener(this);
        webView = (WebView)findViewById(R.id.webView1);
        result = "abc";
    }

    @Override
    public void onClick(final View v) {
        final String[] contents = {"One", "Two", "Three"};
        for(int i = 0; i < 3; ++i) {
            final FooTask task = new FooTask(webView, result);
            task.execute(contents[i]);
        }
    }

    private class FooTask extends AsyncTask<String, Void, String> {
        private final WebView resultView;
        private String result;

        // This is what I try to make it work right.
        private synchronized String getResult() {
            return result;
        }

        public FooTask(final WebView resultView, final String result) {
            this.resultView = resultView;
            this.result = result;
        }

        @Override
        protected String doInBackground(final String... params) {
        // Do a long time work randomly then returns a string.
            return params[0];
        }

        @Override
        protected void onPostExecute(final String content) {
            result = getResult() + content;
            resultView.loadData(result, "text/html", "utf-8");
        }
    }
}

score 1 · Answer 1 · answered Aug 01 '11 at 10:24

1

There is nothing much to do here.. just add this line task.execute(contents[i]); in postExecute() of AsynTask and make contents[i] as a class variable.. task.execute(contents[i]); call this twice since you want it to do "one" and "two"

answered Aug 01 '11 at 10:24

ngesh

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No, I want each task appends its content to result variable then load it on WebView immediately. i.e. Task A appends 'One' to result, Task B appends 'Two' to result, ... the order is not important. If I misunderstood your answer, please add some code. – emeraldhieu Aug 01 '11 at 10:45
Thats what i'm trying to say.. You can call asyncTask inside postExecute multiple times.. and it probably will serve your purpose.. – ngesh Aug 01 '11 at 10:58

score 1 · Accepted Answer · answered Aug 01 '11 at 11:51

1

Remove the private String result; line from the Async Task class.

answered Aug 01 '11 at 11:51

Arnab Chakraborty

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It works but what if FooTask is not inner class. I need to pass 'result' to FooTask as in my code? – emeraldhieu Aug 01 '11 at 12:58
Then make `result` a `static` member of the `async task`. Your problem essentially is that you are appending your content with an empty string, make it static and it should work. – Arnab Chakraborty Aug 01 '11 at 13:01
... but using static is quite weird. :| don't know if it is a good practice. – emeraldhieu Aug 01 '11 at 13:33

score 1 · Answer 3 · answered Aug 01 '11 at 20:50

In onPostExecute() you want to "return" your result somewhere. This is to say FooTask needs a place to put its result. A likely candidate would be some sort of method FooTask can call on its caller to place the result. If you do this note that the method must be synchronized, otherwise some of the returns may be lost.

Alternately you could give FooTask a Handler that it can send a Message to with the result. In this case, you will not need to synchronize anything since all the Messages will be sent to the main/UI thread which will process them serially.

Multiple AsyncTasks load the same WebView

3 Answers3