Why use Handler?

Question

I came across this code in a very basic Handler tutorial. The code is working fine but I do not understand why I have to use Handler for progressDialog.dismiss()

Answer 1

Why to use Handler in Android?

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First: Let us know what is thread:

• Threads help in multi-tasking
• Threads can be taught as a mini process running under a main process
• Threads enable at-least the appearance parallel execution

Second: Let us know about the application thread:-

• When the android application is first started the runtime system will create a single main thread, this main thread will take care of execution of all the components in android

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Android UI-Toolkit is not thread safe

• As stated there are many components in the android main thread, now suppose one of the components takes a long time for execution then this makes the main thread unresponsive and it will show the application unresponsive
• Sub-threads cannot directly manipulate the application (main) thread in the android
• Handler acts as a interface and collects the messages from the sub-threads and update the main application thread one by one as the messages arrive, Thread handlers are implemented in main thread.

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Handler class:

• For the purpose of multithreading we shall use handler class which comes from the package android.os.Handler
• Each thread is handled by one instance of handler class

• From the above figure we can see that Each thread is handled by one instance of the Handler class
• threads communicate between each other with the help of messages
• This handler class helps to maintain kind of sync-coordination b/w threads by allowing them to run together achieving multithreading

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Instance of handler is made

Handler handlerObject = new Handler();

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Final piece on using handler is to use Runnable Interface:

• handler class utilizes runnable interface to implement multithreading
• We override run method to execute a thread a specified number of time

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Class NameOfClass implements Runnable
{
    Public void run()
    {
        //Body of run method
    }
}

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Putting all together

//Create handler in the thread it should be associated with 
//in this case the UI thread
final Handler handler = new Handler();
Runnable runnable = new Runnable() {
    public void run() {
        while(running){
            //Do time consuming stuff

            //The handler schedules the new runnable on the UI thread
            handler.post(new Runnable() {
                //Ex.. using progressbar to set the pogress
                            //Updating the UI is done inside the Handler
            });
        }
    }
};
new Thread(runnable).start();