Posts Tagged ‘handler’

Android Architecture: Message-based MVC

Wednesday, November 17th, 2010

How do you separate application state, user interaction logic and data presentation in your Android apps?

Platform designers did not enforce any high-level application architecture framework upon us but left us enough options to implement our own solutions based on application requirements and scale. Most simple applications will get away with just storing data in their widgets (such as in text fields, spinners etc.) and their state manipulation right in event handlers such as OnClickListener‘s. However, if you are going to write a complex application or plan to sophisticate your simple app further, you should really think how to layer it well so that the architecture supports adding new features and satisfies the expected performance, flexibility, responsiveness and other requirements, and your code does not become a mess.

In this article, I will show you one practical approach to dividing application code into three layers according the MVC paradigm and connecting the view to the controller using the Android messaging framework. I used it in my own code and although it might not be 100% academically correct or applicable for every possible app, I love the benefits it gives to me as my app grows more and more complex.


Android Guts: Intro to Loopers and Handlers

Thursday, June 3rd, 2010

One of the reasons I love Android API is because it contains so many useful little things. Many of them are not even specific to the platform and I really miss them in Java SE. Today I’d like to give a short introduction to two nice classes – Looper and Handler. They are used in Android UI internally, and available to us developers so we can do some cool things with their help.

So what can you do with Loopers and Handlers? Basically, they implement a common concurrency pattern that I call the Pipeline Thread. Here’s how it works:

  • The Pipeline Thread holds a queue of tasks which are just some units of work that can be executed or processed.
  • Other threads can safely push new tasks into the Pipeline Thread’s queue at any time.
  • The Pipeline Thread processes the queued tasks one after another. If there are no tasks queued, it blocks until a task appears in the queue.
  • Sometimes tasks can called messages and other names.

This architecture has some valuable traits and is widely used in frameworks and applications on different platforms.

In this post, we are going to build a simple app that emulates a queue of background downloads while showing its status in the UI. It will be based on a Pipeline Thread that we will build using Looper and Handler. As usual, the complete source is available at the bottom of the article.