滑块的移动调用名为fct的回调来改变显示的颜色条的长度，那个回调会调用ProgressBar的value函数来设置新的长度。

Moving the slider changes the length of the displayed color bar by invoking the callback named FCT, which calls the value function on the ProgressBars to set the new length.

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清单6显示的是小部件样例的事件处理函数和回调函数。

Listing 6 shows the event handlers and the callback function of the sample widget.

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如果注册了一个路径，它就使用定义的回调函数来生成要显示的内容。

If a path is registered, it USES the defined function as a callback to produce the content presented.

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