flutter可以滚动控件之 RefreshIndicator 刷新指示器

When the child's Scrollable descendant overscrolls, an animated circular progress indicator is faded into view. When the scroll ends, if the indicator has been dragged far enough for it to become completely opaque, the onRefresh callback is called. The callback is expected to update the scrollable's contents and then complete the Future it returns. The refresh indicator disappears after the callback's Future has completed.

当孩子的可滚动后代滚动时，动画循环进度指示器将淡入视图。当滚动结束时，如果指示器被拖得足够远以至于完全不透明，则调用onRefresh回调。回调应该更新滚动条的内容，然后完成它返回的Future。刷新指示器在Futurer的回调完成后消失。