An instance of this class holds a tree of `VisualElementAsset`s, created from a UXML file. Each node in the file corresponds to a `VisualElementAsset`. You can clone a `VisualTreeAsset` to yield a tree of `VisualElement`s.
| contentHash | A hash value computed from the template content. |
| stylesheets | The stylesheets used by this VisualTreeAsset. |
| templateDependencies | The UXML templates used by this VisualTreeAsset. |
| VisualTreeAsset | 构造函数。 |
| CloneTree | 从资源构建 VisualElements 树。 |
| Instantiate | 从资源构建 VisualElements 树。 |