使用者通过目镜观察，并使用手机上的按键移动屏幕上的两条直线，直到它们重叠到一起。
The user looks into the eyepiece and uses the buttons on the phone to move two lines until they appear as one.
应避免对每次按键或鼠标移动都发出异步调用。
Avoid making an asynchronous call for every keystroke or mouse move.
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。
The classic example is a GUI where your program must respond when the user does something like press a key or move the mouse.
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