理论上，程序的切换不难，但是程序占用的的资源共享很难——包括屏幕和输入设备
Let’s nail down some terminology that’s consistent with some events I’ll discuss later:
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When an application is run from the Start screen, it is said to be launched.
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When an application is terminated as a result of the Back button, it is closed.
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When the program is running and the user presses the Start button, the program is said to be deactivated, even though it really is quite dead. This is the tombstoned state.
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When a program comes out of tombstoning as the user navigates back to it, it is said to be activated, even though it’s really starting up from scratch.