As an example, you might want to send an email from your application when the user clicks on a “Feedback” button. This allows different applications to collaborate, but without having to know too much about each other. It is possible for one app to launch an activity in another application, using something called an Intent (which we will discuss later). Where Android is a little different is that a back stack can contain activities from different applications, grouped into something which is called a Task. So far, this all sounds very straight forward, right? Every phone OS these days works in this fashion. If the application then navigates to another Activity, the appropriate lifecycle events are called and then it is pushed onto the stack, becoming the topmost (active) activity. When you click on an application in the home app, Android will start a back stack, and push on the home activity. Most applications will consist of ore than one screen, and Android has a very strong back mechanism through its use of a dedicated back button. Linking Activities Together with the Back Stack Developers should put code in the onStart() and onStop() methods to save any state required. It is possible to override this, but it serves as an example. As an example, by default, Android will re-create an Activity when the screen rotates, to allow the activity to use a different layout and components. It is important that the developer puts the right code in his activity to save any state information. There are also corresponding onCreate(), onStart() and onResume() methods that are used when the activity is progressively brought to its running state.Īctivities may be started, or re-started at any time. Finally, when the application is shut down, the onDestroy() method is called. As before, it is the developer’s responsibility to release any memory it can when it is sent the onStop() method. When this happens, the activity may be stopped. Android may also request that an Activity be stopped because it wants to recover memory. Animation, Graphics) that are running when the onPause() method is called. It is the developer’s responsibility to stop any CPU hungry tasks (e.g. When Android changes activities, it will be paused. When an activity is on screen, it is running (green in the diagram).
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