Find visual elements with UQuery

You can use UQuery to find elements from a visual tree. UQuery was inspired by JQuery and Linq, and is designed to limit dynamic memory allocation. This allows for optimal performance on mobile platforms.

Query methods

You can use UQuery through the following extension methods:

• Q
• Query

Internally, the Q and Query methods use UQueryBuilder to construct a query. These extension methods reduce the verbosity of creating a UQueryBuilder.

To use UQuery to find elements, you must load and instantiate the UXML first, and then use Query or Q to construct selection rules on a root visual element.

Query returns a list of elements that match the selection rules. You can filter the return results of Query with the public methods of UQueryBuilder, such as First, Last, AtIndex, Children, and Where.

Q is the shorthand for Query<T>.First(). It returns the first element that matches the selection rules.

Query elements

You can query elements by their name, their USS class, or their element type (C# type). You can also query with a predicate or make complex hierarchical queries.

The following sections use this example UXML to demonstrate how to find elements:

<UXML xmlns="UnityEngine.UIElements">
    <VisualElement name="container1">
      <Button name="OK" text="OK" />
      <Button name="Cancel" text="Cancel" />
    </VisualElement>
     <VisualElement name="container2">
      <Button name="OK" class="yellow" text="OK" />
      <Button name="Cancel" text="Cancel" />
    </VisualElement>
    <VisualElement name="container3">
      <Button name="OK" class="yellow" text="OK" />
      <Button name="Cancel" class="yellow" text="Cancel" />
    </VisualElement>
</UXML>

Query by name

To find elements by their name, use Query(name: "element-name") or Q(name: "element-name"). You can omit the name as it’s the first argument. For example:

The following example finds a list of elements named “Ok”:

List<VisualElement> result = root.Query("OK").ToList();

The following example uses Query to find the first element named “Ok”:

VisualElement result = root.Query("OK").First(); //or VisualElement result = root.Q("OK");            

The following example uses Q to find the first element named “Ok”:

VisualElement result = root.Q("OK");            

The following example finds the second element named “Ok”:

VisualElement result3 = root.Query("OK").AtIndex(1);

The following example finds the last element named “Ok”:

VisualElement result4 = root.Query("OK").Last();

Query by USS class

To find elements by a USS class, use Query(className: "class-name") or Q(className: "class-name").

The following example finds all the elements that have the class “yellow” and assigns them to a list:

List<VisualElement> result = root.Query(className: "yellow").ToList();

The following example finds the first element that has the class “yellow”:

VisualElement result = root.Q(className: "yellow");

Query by element type

To find elements by their element type(C# type), use Query<Type> or Q<Type>.

The following example finds the first button and adds a tooltip for it:

VisualElement result = root.Q<Button>();
result.tooltip = "This is a tooltip!";

The following example finds the third button:

VisualElement result = root.Query<Button>().AtIndex(2);

Note: You can only query by the actual type of the element, not base classes.

Query with a predicate

Other than to query elements by name, class, and type, you can also use the Where method to select all elements that satisfy a predicate. The predicate must be a function callback that takes a single VisualElement argument.

The following example finds all the elements with the “yellow” USS class that have no tooltips:

List<VisualElement> result = root.Query(className: "yellow").Where(elem => elem.tooltip == "").ToList();

Complex hierarchical queries

You can combine name, class, and type to make complex hierarchical queries.

The following example finds the first button named “OK” that has a class of “yellow”:

VisualElement result = root.Query<Button>(className: "yellow", name: "OK").First();

The following example finds the child cancel button of the “container2”:

VisualElement result = root.Query<VisualElement>("container2").Children<Button>("Cancel").First();

Operate on results

You can use the ForEach method to operate directly on the query results.

The following example adds a tooltip for any elements that have no tooltips:

root.Query().Where(elem => elem.tooltip == "").ForEach(elem => elem.tooltip="This is a tooltip!");

最佳实践

Consider the following when you use UQuery:

• UQuery traverses through the hierarchy to find elements by name, class or type. Cache results from UQuery at initialization.
• If you need to retrieve multiple elements, use the QueryState struct (returned by the element.Query() method) and enumerate it to avoid creating lists. You can also construct a query once and execute it on different elements.
• UI Toolkit doesn’t destroy visual elements that are no longer needed, it uses C# garbage collector to collect them. Be mindful to not accidentally retain references to visual elements in a class that outlives the UIDocuments or Window where the elements came from.
• Capture VisualElement variables inside closures.
• When you create or release lots of elements, enable incremental garbage collection to avoid garbage collector spikes.

其他资源

• USS selectors
• Introduction to visual elements and visual tree
• Load UXML and USS from C# scripts
• Instantiate UXML with C#