Creating an own thread and providing Lopper and MessageQueue is not the right way to deal with the problem. So, Android has provided HandlerThread subclass of Thread to streamline the process. Internally it does the same things that we have done but in a robust way. So, always use HandlerThread. One of the ways to create the HandlerThread is to subclass it and most of the time you will be using this method.
Note: We have instantiated the Handler when the onLooperPrepared is called. So, that Handler can be associated with that Looper. Note: HandlerThread needs to call myHandlerThread. I would suggest practicing the above codes, so you can grasp their little details. The link to this Example. Ensure that prepare method was called inside a thread and only then call loop. Each thread can have only one Looper.
So calling prepare method twice inside same thread will also lead to an exception. The last methods which are interesting to us in Looper class are quit and quitSafely. These methods are used to stop event loop. The first one is not safe as it will terminate processing the queue and some events might be left unprocessed.
The second waits until all the messages are processed and then terminates, therefore it is called safe. Basically it works the following way:. So, we see that Handler does two things: 1 posts messages to be placed in a queue, 2 handles messages when they were processed by Looper.
As we already know there a basically two major types of messages: what and Runnable. In order to send already created message e. From names it is clear that sendMessage sends message to be processed as soon as possible, sendMessageDelayed sends it to be processed after some timeout and sendMessageAtTime sends it to be processed at some particular moment of time.
Pretty obvious. Also there is a way to directly post Runnable without first creating explicit message for that. For this there are post , postDelayed and postAtTime methods. They have same meaning as sendMessageXXX methods, the difference is just that we send Runnable instead of already created message. I strongly recommend you to look at source code or documentation to learn more about public API.
In order to handle messages when they were processed by Looper we can do one of two things:. When messages will be ready they will be sent to these callbacks by Looper. Last but not least Handler is able not only to add messages to event queue but also to remove them. For such cases there are removeMessages methods, which can trigger removing unprocessed messages which met some criteria from MessageQueue.
Source code for Handler one can see here. HandlerThread is just a thread which has event loop. There is not much to talk here, you can look at source code here it is less than lines including javadoc.
If we need event loop inside some thread the basic structure will be the following:. And as a rule of thumb Handler is good to use in the following cases:. Debounce is a way to skip some frequent calls to have some predefined min delay between these calls and always receiving the latest one. It might be easier to describe this by example, so imagine that you have a search query text input. In this article, you will learn how to use Handler in Android.
Uses of Handler To schedule messages and runnables to be executed at some point in the future. To enqueue an action to be performed on a different than your own. Two options for communicating with the Handler are messages and Runnable objects. What is a message? Message contains a description and an arbitrary data object that can be sent to a handler.
While the construction of a message is public, the best way to get one of these is to call Message. How to send message objects? You referesh a webview using the reload method of the webview. We pass the delay time in milliseconds. Android is an animation rich framework. The ability to use animations has been made easier given the presence of a Hnadler.
We will pass the view to animate, the animation resource id, the delay time and the Context object. There we pass our Runnable annonymous class as well as the delay time. As you can see that method takes a runnable object. The post method causes the Runnable r to be added to the message queue. This example will explore various practical usage scenarios that you are likely to get into while using the android.
Handler class. Create a java project in android studio. You can also create a kotlin one and use the code converter to convert from java to kotlin.
This involves adding a bunch of textviews to your main activity layout. Also add a button. You can organize them using a vertical linear layout:. We then wrote our code. In this tutorial we want to see how to post updates from a background thread to the user interface thread using a Handler.
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