在需要（重新）生成 VisualElement 可视内容时调用。
当处理时，可以在 VisualElement 的内容区域中生成自定义几何体。 仅当检测到 VisualElement 需要重新生成其可视内容时，才调用此委托。刷新面板时，并非每帧都调用它。生成的内容将被缓存并保持不变，直到 VisualElement 上影响可视内容的属性发生更改或调用了 VisualElement.MarkDirtyRepaint。 在此委托的处理程序中执行代码时，限制更改 VisualElement 的任何属性。正确的处理程序应将 VisualElement 视为“只读”，并在不引起副作用的情况下生成几何体。在此事件期间对 VisualElement 所做的更改可能会丢失或至少是滞后出现。
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