因为我实现的是一个下压按钮，最好是能够在鼠标点到按钮图像上的时候，让用户有压下按钮的感受。
Since I am implementing a push-button, I better do something so the user has the visual impression of pushing a button when the mouse is clicked over the button graphic.
因为我实现的是一个下压按钮，最好是能够在鼠标点到按钮图像上的时候，让用户有压下按钮的感受。
Since I am implementing a push-button, I better do something so the user has the visual impression of pushing a button when the mouse is clicked over the button graphic.
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