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Introduction to Handler in Kotlin Android

Introduction to Handler in Kotlin Android

A Handler is a class provided by the Android framework that allows you to communicate between different threads in an Android application. It is commonly used to schedule tasks to be executed on a certain thread, or to queue messages or runnables to be processed later.

Step 1: Create a Handler object​

To use the Handler class in your Kotlin Android application, you need to create an instance of it. You can do this by either creating a new instance directly or by using the Handler() constructor.

val handler = Handler()

Step 2: Post a Runnable to the Handler​

One of the key functionalities of a Handler is its ability to post runnables to be executed on a specific thread. You can use the post() method of the Handler object to achieve this.

handler.post {
    // Code to be executed on the thread associated with the Handler
}

For example, let's say you want to update the UI from a background thread. You can use a Handler to post a runnable that performs the UI update.

val handler = Handler()

Thread {
    // Perform some background task

    handler.post {
        // Update the UI
    }
}.start()

Step 3: Delayed Execution with Handler​

The Handler class also provides a way to schedule a runnable to be executed after a certain delay. You can use the postDelayed() method to achieve this.

handler.postDelayed({
    // Code to be executed after the delay
}, delayInMillis)

For example, if you want to show a toast message after a delay of 1 second, you can use the following code:

val handler = Handler()

handler.postDelayed({
    Toast.makeText(context, "Delayed toast message", Toast.LENGTH_SHORT).show()
}, 1000)

Step 4: Remove Pending Runnables​

Sometimes, you may need to remove any pending runnables from the Handler's message queue. You can use the removeCallbacks() method to achieve this.

handler.removeCallbacks(runnable)

For example, if you have scheduled a runnable to be executed after a delay but want to cancel it before it runs, you can use the following code:

val handler = Handler()

val runnable = Runnable {
    // Code to be executed
}

handler.postDelayed(runnable, 1000)

// Cancel the scheduled runnable
handler.removeCallbacks(runnable)

Step 5: Communication between Threads​

One of the main use cases of a Handler is to communicate between different threads. You can send messages from one thread to another using a Handler.

val handler = Handler()

Thread {
    // Perform some background task

    val message = Message.obtain()
    message.what = MESSAGE_ID
    message.obj = data

    handler.sendMessage(message)
}.start()

In the receiving thread, you can override the handleMessage() method of a Handler to handle the incoming message.

val handler = object : Handler(Looper.getMainLooper()) {
    override fun handleMessage(message: Message) {
        when (message.what) {
            MESSAGE_ID -> {
                val data = message.obj as Data
                // Process the data
            }
        }
    }
}

Handler Examples​