事件的传播阶段。
When an element receives an event, the event is propagated along a path built from the elements between the panel root and the target element. In the TrickleDown phase, the event is sent from the root element to the target parent element. In the AtTarget phase, the event is sent to the target element. In the BubbleUp phase, the event is sent from the target parent element to the root element. In the last phase, the DefaultAction phase, the event is resent to the target element.
| None | 未传播事件。 |
| TrickleDown | The event is being sent from the root element to the target parent element. |
| AtTarget | 正在将事件发送到事件目标。 |
| BubbleUp | 正在将事件从事件目标父元素向上发送到根元素。 |
| DefaultAction | 正在将事件发送给目标元素以执行事件的默认操作。事件处理程序在此阶段不获取事件。而是对目标调用 ExecuteDefaultAction。 |