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Efficient Android Threading by Anders Goransson

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Chapter 10. Tying a Background task to the UI Thread with AsyncTask

The most important role for asynchronous tasks on Android, as we’ve discussed, is to relieve the UI thread from long-running operations. This calls for defining an execution environment, creating the task to do the long operation, and finally determining how the UI thread and the background threads communicate. All of these properties are encapsulated in an AsyncTask to make asynchronous execution as easy as it gets.

This chapter gets into the details of AsyncTask class and shows how smoothly it can handle background task execution, but also raises concerns about the pitfalls you need to watch for.

Fundamentals

As the name indicates, an AsyncTask is an asynchronous task that is executed on a background thread. The only method you need to override in the class is doInBackground(). Hence, a minimal implementation of an AsyncTask looks like this:

public class MinimalTask extends AsyncTask {
    @Override
    protected Object doInBackground(Object... objects) {
        // Implement task to execute on background thread.
    }
}

The task is executed by calling the execute method, which triggers a callback to doInBackground on a background thread.

new MinimalTask().execute(Object... objects);

When an AsyncTask finishes executing, it cannot be executed again—i.e., execute is a one-shot operation and can be called only once per AsyncTask instance, the same behavior as a Thread.

In addition to background execution, AsyncTask offers a data passing mechanism ...

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