Скачать или смотреть Activity Life Cycle in Android Application

As a user navigates through, out of, and back to your app, the Activity instances in your app transition through different states in their lifecycle. The Activity class provides a number of callbacks that allow the activity to know that a state has changed: that the system is creating, stopping, or resuming an activity, or destroying the process in which the activity resides.

Within the lifecycle callback methods, you can declare how your activity behaves when the user leaves and re-enters the activity. For example, if you're building a streaming video player, you might pause the video and terminate the network connection when the user switches to another app. When the user returns, you can reconnect to the network and allow the user to resume the video from the same spot. In other words, each callback allows you to perform specific work that's appropriate to a given change of state. Doing the right work at the right time and handling transitions properly make your app more robust and performant. For example, good implementation of the lifecycle callbacks can help ensure that your app avoids:

Crashing if the user receives a phone call or switches to another app while using your app.
Consuming valuable system resources when the user is not actively using it.
Losing the user's progress if they leave your app and return to it at a later time.
Crashing or losing the user's progress when the screen rotates between landscape and portrait orientation.
This document explains the activity lifecycle in detail. The document begins by describing the lifecycle paradigm. Next, it explains each of the callbacks: what happens internally while they execute, and what you should implement during them. It then briefly introduces the relationship between activity state and a process’s vulnerability to being killed by the system. Last, it discusses several topics related to transitions between activity states.