用户单击并且保持鼠标左键于按下状态时，按钮是加亮的。
A button is highlighted when the user clicks and holds the left mouse button. The highlighting is removed when the user releases the mouse button.
怎么样完成击一次BUTTON显示按下状态，再点击一次BUTTON再起来？
How kind of hit a BUTTON complete display pressed, and then click a BUTTON rise again?
当我按下enter时，它会显示一些状态信息，直到管道到达PLAYING状态。
When I press Enter, it prints several status messages until the pipeline reaches the PLAYING state.
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