在flutter中只用GestureDetector添加点击事件，发现在空白区域点击无效，事件不响应。解决办法：
GestureDetector(
behavior: HitTestBehavior.opaque,
)
说明:
/// How to behave during hit tests.
enum HitTestBehavior {
/// Targets that defer to their children receive events within their bounds
/// only if one of their children is hit by the hit test.
deferToChild,
/// Opaque targets can be hit by hit tests, causing them to both receive
/// events within their bounds and prevent targets visually behind them from
/// also receiving events.
opaque,
/// Translucent targets both receive events within their bounds and permit
/// targets visually behind them to also receive events.
translucent,
}
翻译一下就是：
当behavior选择deferToChild时，只有当前容器中的child被点击时才会响应点击事件；
当behavior选择opaque时，点击整个区域都会响应点击事件，但是点击事件不可穿透向下传递，注释翻译：阻止视觉上位于其后方的目标接收事件，所以我需要的这种效果直接将behavior设置为HitTestBehavior.opaque就可以了；
当behavior选择translucent时，同样是点击整个区域都会响应点击事件，和opaque的区别是点击事件是否可以向下传递，注释翻译：半透明目标既可以在其范围内接受事件，也可以允许视觉上位于其后方的目标接收事件。
