The classic example is a GUI where your program must respond when the user does something like press a key or move the mouse.
经典的例子便是GUI，程序必须响应用户的某些行为，比如按键或是鼠标移动。
Think about what happens when users press their mouse on a button, realize it's a mistake and move their mouse off of the button before releasing it.
试想一下这样的情况：用户在某个按钮上按下鼠标，发现错了，立即松开鼠标而不是释放鼠标。
When you are finished, press the mouse button or enter to leave the splitter bar in their new location.
完成后，按鼠标按钮或回车键以离开移动到新位置的分割条。
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