Android Activity，back stack管理

To summarize the default behavior for activities and tasks:

• When Activity A starts Activity B, Activity A is stopped, but the system retains its state (such as scroll position and text entered into forms). If the user presses the Back button while in Activity B, Activity A resumes with its state restored.
• When the user leaves a task by pressing the Home button, the current activity is stopped and its task goes into the background. The system retains the state of every activity in the task. If the user later resumes the task by selecting the launcher icon that began the task, the task comes to the foreground and resumes the activity at the top of the stack.
• If the user presses the Back button, the current activity is popped from the stack and destroyed. The previous activity in the stack is resumed. When an activity is destroyed, the system does not retain the activity's state.
• Activities can be instantiated multiple times, even from other tasks.