栈顶的地址就代表最后一次被调用的函数（也就是当前的活动函数）。
The address at the top of the stack represents the function that was last called (that is, the active function).
在碰到退出函数时，当前的活动函数就会返回，并释放栈顶元素。
When an exit address is encountered, the current active function has returned and the top element on the stack is discarded.
任务是由栈中的活动组成的，而不是清单文件中声明的某个类或元素。
A task is a stack of activities, not a class or an element in the manifest file.
任务就是以栈的形式组织起来起来的相互关联的一组活动。
栈中最底部的是任务的起始活动——一般是用户在滑出的程序列表中选择启动的活动。
The root activity in the stack is the one that began the task — typically, it's an activity the user selected in the application launcher.
当用户按下BACK键后，正在运行的活动被弹出栈，旧一个恢复成为正在运行的活动。
When the user presses the BACK key, the current activity is popped from the stack, and the previous one resumes as the running activity.
旧一个活动还在栈中。
如果已经有了一个亲和度和新活动相同的任务，活动就被压入那个任务的栈中。
If there's already an existing task with the same affinity as the new activity, the activity is launched into that task.
如果已经有了一个亲和度和新活动相同的任务，活动就被压入那个任务的栈中。
If there's already an existing task with the same affinity as the new activity, the activity is launched into that task.
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