Lecture Mobile Application Development (119310)

Android Framework Basics

Agenda

• Framework Components Part VIII - AsyncTask
  • The Main- / UI-Thread
  • Async programming using AsyncTask

UI-Thread

• Android modifies the user interface and handles input events from one single user interface thread. This thread is also called the main thread.
• Android collects all events in a queue and dispatches them to be executed in the main thread

UI-Thread

• Long running tasks can block the UI and events (e.g. touches, keystrokes) can not be processed
• If the UI is blocked for more than 5 seconds the “Application is Not Responding” (ANR) dialog will be shown

UI-Thread

• A better way is to execute long running tasks in a separate thread
• But beware: The UI must not be manipulated outside the Main-Thread!

UI-Thread

• Important: Do not block the UI with a long running task
• Do not access the Android UI toolkit from outside the Main-Thread

UI-Thread

• Solution: Use Android Methods to execute UI updates in Main Thread, e.g.:
  • Activity.runOnUIThread(Runnable)
  • View.post(Runnable)
  • View.postDelayed(Runnable, long)

UI-Thread

• Example for loading image in background using thread

public void onClick(View v) {
    new Thread(new Runnable() {
        public void run() {
            final Bitmap bitmap = loadImageFromNetwork("http://example.com/image.png");
            mImageView.post(new Runnable() {
                public void run() {
                    mImageView.setImageBitmap(bitmap);
                }
            });
        }
    }).start();
}

UI-Thread

• Using the runOnUIThread() and post() methods can become hard to maintain in a more complex application
• Thus Android provides the AsyncTask class to do background processing and update the UI with progress information
• AsyncTask takes care of the creation of the worker threads using a thread pool
• It simplifies thread handling and the synchronization with the UI-thread

Multithreading using AsyncTask

Multithreading using AsyncTask

• Simple AsyncTask example

 private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {
     protected Long doInBackground(URL... urls) {
         int count = urls.length;
         long totalSize = 0;
         for (int i = 0; i < count; i++) {
             totalSize += Downloader.downloadFile(urls[i]);
             publishProgress((int) ((i+1 / (float) count) * 100));
         }
         return totalSize;
     }
     protected void onProgressUpdate(Integer... progress) {
         setProgressPercent(progress[0]);
     }
     protected void onPostExecute(Long result) {
         showDialog("Downloaded " + result + " bytes");
     }
 }

Multithreading using AsyncTask

• Starting the AsycTask

new DownloadFilesTask().execute(url1, url2, url3);

Android-Assignment - AsyncTask

• Conduct Android AsyncTask assignment
• Write an app that shows how to update a progress bar using AsyncTask

Summary

• Working with multithreading is very important to achieve responsive applications
• Unfortunately it makes programming more complex and error prone
• To simplify asynchronous programming Android provides the AsyncTask class
• This can be used when a long running process needs to be executed within an activity and requires to frequently update the UI
• For other long running tasks, Services are a good alternative

Recab Questions

• When would you use a Service and when an AsyncTask?
• Why is it not allowed to manipulate the UI outside of the UI-Thread?
• How is AsyncTask executed when it is started multiple times after another?
• What happens when you do long running tasks on the UI-Thread?
• Which of the AsyncTask methods are executed on the Main-Thread and which on another thread?