Tasks and Back Stack

Even though the activities may be from different applications, Android maintains this seamless user experience by keeping both activities in the same task.

A task is a collection of activities that users interact with when performing a certain job. The activities are arranged in a stack (the "back stack"), in the order in which each activity is opened.

The device Home screen is the starting place for most tasks. When the user touches an icon in the application launcher (or a shortcut on the Home screen), that application's task comes to the foreground. If no task exists for the application (the application has not been used recently), then a new task is created and the "main" activity for that application opens as the root activity in the stack.

When the current activity starts another, the new activity is pushed on the top of the stack and takes focus. The previous activity remains in the stack, but is stopped. When an activity stops, the system retains the current state of its user interface. When the user presses the BACK key, the current activity is popped from the top of the stack (the activity is destroyed) and the previous activity resumes (the previous state of its UI is restored).

Activities in the stack are never rearranged, only pushed and popped from the stack—pushed onto the stack when started by the current activity and popped off when the user leaves it using the BACK key. As such, the back stack operates as a "last in, first out" object structure.

If the user continues to press BACK, then each activity in the stack is popped off to reveal the previous one, until the user returns to the Home screen (or to whichever activity was running when the task began). When all activities are removed from the stack, the task no longer exists.

A task is a cohesive unit that can move to the "background" when users begin a new task or go to the Home screen, via the HOME key.

Because the activities in the back stack are never rearranged, if your application allows users to start a particular activity from more than one activity, a new instance of that activity is created and popped onto the stack (rather than bringing any previous instance of the activity to the top). As such, one activity in your application might be instantiated multiple times (even from different 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 key while in Activity B, Activity A resumes with its state restored.
•When the user leaves a task by pressing the HOME key, 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 key, 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.

you can use these manifest attributes and intent flags to define how activities are associated with tasks and how the behave in the back stack.

Launch modes allow you to define how a new instance of an activity is associated with the current task.

As such, if Activity A starts Activity B, Activity B can define in its manifest how it should associate with the current task (if at all) and Activity A can also request how Activity B should associate with current task. If both activities define how Activity B should associate with a task, then Activity A's request (as defined in the intent) is honored over Activity B's request (as defined in its manifest).

Some the launch modes available in the manifest are not available as flags for an intent and, likewise, some launch modes available as flags for an intent cannot be defined in the manifest.

The affinity indicates which task an activity prefers to belong to. By default, all the activities from the same application have an affinity for each other. So, by default, all activities in the same application prefer to be in the same task.

If the user leaves a task for a long time, the system clears the task of all activities except the root activity. When the user returns to the task again, only the root activity is restored. The system behaves this way, because, after an extended amount of time, users likely have abandoned what they were doing before and are returning to the task to begin something new.