Handle event callbacks and value changes

Events in UI Toolkit are similar to HTML events. When an event occurs, UI Toolkit sends it to the target visual element and all elements within the propagation path in the visual tree.

事件处理顺序如下：

1. Execute event callbacks on elements from the root element down to the event target. This is the trickle-down phase of the dispatch process.
2. Execute event callbacks on elements from the event target up to the root. This is the bubble-up phase of the dispatch process.

在沿着传播路径发送事件时，Event.currentTarget 属性更新为当前正在处理事件的元素。在事件回调函数中：

• Event.currentTarget 是回调注册的视觉元素。
• Event.target 是原始事件发生的视觉元素。

有关更多信息，请参阅分发事件。

Register an event callback

You can register an event callback to customize the behavior of an individual instance of an existing class, such as reacting to a mouse click on a text label. To register a callback for an event, use the RegisterCallback() method to register the callback directly on the element.

传播路径上的每个元素（目标除外）对一个事件可以接收两次：

• 在涓滴阶段一次。
• 在冒泡阶段一次。

By default, a registered callback executes during the target phase and the bubble-up phase. This default behavior ensures that a parent element reacts after its child element.

However, if you want a parent element to react before its child, register your callback with the TrickleDown.TrickleDown option like this:

using UnityEngine;
using UnityEngine.UIElements;

...
VisualElement myElement = new VisualElement();

// Register a callback for the trickle-down phase.
myElement.RegisterCallback<PointerDownEvent>(MyCallback, TrickleDown.TrickleDown);
...

此代码将通知分发程序在目标阶段和涓滴阶段执行回调。

To add a custom behavior to a specific visual element, register an event callback on that element like this:

// Register a callback on a pointer down event
myElement.RegisterCallback<PointerDownEvent>(MyCallback);

The signature for the callback function looks like this:

void MyCallback(PointerDownEvent evt) { /* ... */ }

For an element whose child elements handle the event, to register a callback, use the Q() method to find the child element and register the callback on it.

The following example registers a callback on a slider’s drag container element to handle the pointer up event for the slider. In this case, you must register the callback on the drag container element instead of the slider itself because the drag container captures the pointer during pointer down events, which makes it the only receiver for the next pointer up event.

var dragContainer = slider.Q("unity-drag-container");
dragContainer.RegisterCallback<PointerUpEvent> ( evt => Debug.Log("PointerUpEvent"));

Note: You can register multiple callbacks for an event. However, you can only register the same callback function on the same event and propagation phase once.

To remove a callback from a VisualElement, call the myElement.UnregisterCallback() method.

Send custom data to an event callback

您可以将自定义数据随同回调一起发送到事件。要附加自定义数据，您必须通过扩展该调用的方式来注册回调。

The following example registers a callback for PointerDownEvent and sends custom data to the callback function:

// Send user data along to the callback
myElement.RegisterCallback<PointerDownEvent, MyType>(MyCallbackWithData, myData);

The signature for the callback function looks like this:

void MyCallbackWithData(PointerDownEvent evt, MyType data) { /* ... */ }

Manage the value of a control

UI controls use the value property to hold data for their internal state. For example:

• A Toggle holds a Boolean value that changes when the Toggle is turned on or off.
• An IntegerField holds an integer that holds the field’s value.

To get the value of a control:

• Get the value from the control directly: int val = myIntegerField.value;.

• Listen to a ChangeEvent sent by the control and process the change when it happens. You must register your callback to the event like this:

  //RegisterValueChangedCallback is a shortcut for RegisterCallback<ChangeEvent>.
  //It constrains the right type of T for any VisualElement that implements an
  //INotifyValueChange interface.
  myIntegerField.RegisterValueChangedCallback(OnIntegerFieldChange);
  

  The signature for the callback function looks like this:

  void OnIntegerFieldChange(ChangeEvent<int> evt) { /* ... */ }
  

To change the value of a control:

• Directly change the value variable: myControl.value = myNewValue;. This triggers a new ChangeEvent.
• Use myControl.SetValueWithoutNotify(myNewValue);. This doesn’t trigger a new ChangeEvent.

For more information, see Change events

其他资源

• Synthesize and send events
• Events reference
• Manipulators
• Change events