Focus order of elements
Each panel has a focus ring that defines the focus order of elements. By default, a depth-first search (DFS) on the visual tree defines the focus order of elements. For example, the focus order for the tree depicted below is F, B, A, D, C, E, G, I, H.
Some events use the focus order to define which element holds the focus. For example, the target for a keyboard event is the element in focus.
使用 focusable 属性可控制元素的可聚焦性。默认情况下，VisualElements 无法聚焦，但是某些子类（例如 TextField）默认可聚焦。
Use the tabIndex property to control the focus order as follows (tabIndex default value of 0):
- 如果
tabIndex为负，则无法用 Tab 键导航到该元素。 - 如果
tabIndex为零，该元素保持默认的 Tab 顺序（通过焦点环算法确定的顺序）。 - 如果
tabIndex为正，该元素将置于tabIndex为零 (tabIndex = 0) 或tabIndex更小的其他元素之前。