Mobile Computing Dr. Mohsin Ali Memon

Threads • Basic approach - Make a task list with Executors.newFixedThreadPool - Add tasks to list with taskList.execute(someRunnable) • Three variations on the theme - Separate classes that implement Runnable - Main Activity implements Runnable - Inner classes that implement Runnable - Race conditions and synchronization • Related topics - Helpful Thread-related methods

Motivation for Concurrent Programming • Pros - Advantages even on single-processor systems • Efficiency - Downloading image files • Convenience - A clock icon • Responsiveness - If you block the main thread, event handlers will not respond - Some devices have multiple cpus or cores • Find out via Runtime.getRuntime().availableProcessors() • Cons - Significantly harder to debug and maintain than single- threaded apps

Steps for Concurrent Programming

Steps for Concurrent Programming cont’d

Approach One: Separate Classes that Implement Runnable

Separate Runnable Class: Template Code This code should not update the UI. If you need to update UI, use AsyncTask class.

Thread Mechanism One: Example

Thread Mechanism One: Example con’d

Helper Class

Thread Mechanism One: Example Result

Pros and Cons of Approach

Approach Two: Main App Implements Runnable

Main App Implements Runnable: Template Code

Thread Mechanism Two: Example

Thread Mechanism Two: Results

Pros and Cons of Approach

Approach Three: Inner Class that Implements Runnable

Inner Class implements Runnable: Template code

Thread Mechanism Three: Example

Thread Mechanism Three: Example cont’d

Thread Mechanism Three: Example Result

Pros and cons of approach

Summary

Race Condition

Race Conditions: Example

Race Conditions: Example cont’d

Race Conditions: Result

Race Conditions: Solution

Race Conditions: Solution

Helpful –Thread related Methods

Methods in ExecutorService Class

Threads Cannot Update UI

Updating UI All at Once

Displaying a network image

BitmapUtils: Getting a Bitmap

Making a View for an Image

BitmapUtils:Getting a view for image

Example: Unthreaded Image Viewer

Activity

Results

Updating UI with View.post

Example: Multithreaded Image Viewer

Main Activity

Main Activity Button Handler

Main Activity: Runnable Inner Class (For Background)

Main Activity: Runnable Inner Class (For UI Thread)

Results

Downloading Images via AsyncTask