什么时候在Android中创建Activity子类的对象？

Question

We write different activity classes in Android and declare those activities in AndroidManifest.xml

However, what I am not getting is when is the object of that Activity class created or how that activity class is loaded into memory ?

The doubt might seem naive but am confused.

Answer 1

An activity is a single, focused thing that the user can do. Almost all activities interact with the user, so the Activity class takes care of creating a window for you in which you can place your UI with setContentView(View). The activity contains the user interface of your application. There are various states of activity like Running, Paused, Stopped and Killed. The Activity base class contains several events that govern the life cycle of an activity.

Now, when Activities's onDestroy method is called, your Activities aren't necessarily garbage collected. When the system gets low on memory the process that your Application lives in can be killed, meaning your Application will disappear; Application's onTerminate method may or may not be called. At that time all the Activities, Services, etc, are killed too. The Application instance is always instantiated first, an Activity must have an associated Application, just like how you define it in the AndroidManifest.xml.

As always, the best resource for understanding all this is the official documentation .

Answer 2

An Activity is instantiated automatically by your Application when it receives an Intent that corresponds with one of the Activities you describe in your manifest. You don't need to worry about any constructor methods, or keeping a reference to your activity after its instantiated. That is done behind the scenes. Android handles the lifecycle, so you use the lifecycle callbacks to handle creation and cleanup of your own objects.

When someone clicks the icon for your app in the launcher, what actually happens is the launcher sends an Intent to your application to launch the activity associated with that Intent. If your application isn't open yet, Android will launch it so it can receive the Intent.