Events
In this article, we will discuss events in the context of UI5 Web Components.
Components use CustomEvent
to inform developers of important state changes in the components. For example, the change
event is fired whenever the value of a ui5-input
is changed.
The @event
Decorator​
To define your own custom event, you need to use the @event
decorator.
The event
decorator is a class decorator that takes one required argument as a string to define the event name and an optional argument as an object literal to describe details of the custom element.
The details object allows developers to describe more information about the event.
import UI5Element from "@ui5/webcomponents-base/dist/UI5Element.js";
import customElement from "@ui5/webcomponents-base/dist/decorators/customElement.js";
import event from "@ui5/webcomponents-base/dist/decorators/event.js";
@customElement("my-demo-component")
@event("change", {
detail: {
valid: { type: Boolean },
},
})
class MyDemoComponent extends UI5Element {}
Note: This decorator is used only to describe the events of the component and is not meant to create emitters.
Usage​
As mentioned earlier, the @event
decorator doesn't create event emitters. To notify developers of component changes, we have to fire events ourselves. This can be done using the fireEvent
and the newer fireDecoratorEvent
methods that comes from the UI5Element
class. The difference between the methods is explained below.
import UI5Element from "@ui5/webcomponents-base/dist/UI5Element.js";
import customElement from "@ui5/webcomponents-base/dist/decorators/customElement.js";
import property from "@ui5/webcomponents-base/dist/decorators/property.js";
import event from "@ui5/webcomponents-base/dist/decorators/event.js";
@customElement("my-demo-component")
@event("change")
class MyDemoComponent extends UI5Element {
@property()
value = "";
onNativeInputChange(e) {
this.value = e.target.value;
this.fireDecoratorEvent("change"); // or this.fireEvent("change");
}
}
Note: By default, the fireDecoratorEvent
(and fireEvent
) method returns a boolean value that helps you understand whether the event was canceled (i.e., if the preventDefault
method was called).
Event Detail​
The @event
decorator is generic and accepts a TypeScript type that describes its detail. This type is crucial for preventing incorrect detail data when the event is fired using fireDecoratorEvent
and fireEvent
methods (both generic) and for ensuring type safety when listening for the event, so you know what kind of detail data to expect.
Note: It's required to export all types that describe specific event details for all public events.
Here's an example implementation:
import UI5Element from "@ui5/webcomponents-base/dist/UI5Element.js";
import customElement from "@ui5/webcomponents-base/dist/decorators/customElement.js";
import property from "@ui5/webcomponents-base/dist/decorators/property.js";
import event from "@ui5/webcomponents-base/dist/decorators/event.js";
// Define the event detail type
export type MyDemoComponentChangeEventDetail = {
valid: boolean;
};
@customElement("my-demo-component")
@event<MyDemoComponentChangeEventDetail>("change", {
detail: {
valid: { type: Boolean },
},
})
class MyDemoComponent extends UI5Element {
@property()
value = "";
onNativeInputChange(e: Event) {
this.fireDecoratorEvent<MyDemoComponentChangeEventDetail>("change", {
valid: true,
});
}
}
export { MyDemoComponent };
Event Configuration​
Bubbling and Preventing​
Whether the events should be cancelable or able to bubble is configurable.
by setting cancelable
and bubbles
in the @event
decorator.
-
cancelable: true
means the event can be prevented by calling the nativepreventDefault()
method in the event handler- by default it'sfalse
. -
bubbles: true
means the event will bubble - by default it'sfalse
.
Since v2.4.0
this can be configured in the @event
decorator:
import UI5Element from "@ui5/webcomponents-base/dist/UI5Element.js";
import customElement from "@ui5/webcomponents-base/dist/decorators/customElement.js";
import event from "@ui5/webcomponents-base/dist/decorators/event.js";
@customElement("my-demo-component")
@event("change", {
bubbles: true // false by default
cancelable: true // false by default
})
class MyDemoComponent extends UI5Element {
onSomeAction() {
this.fireDecoratorEvent("change")
}
}
The fireDecoratorEvent
method​
The method is available since version v2.4.0
and it fires a custom event and gets the configuration for the event from the @event
decorator. In case you rely on the decorator settings, you must use the fireDecoratorEvent
method.
Keep in mind that cancelable
and bubbles
are false
by default and you must explicitly enable them in the @event
decorator if required.
- Fire event with default configuration
@event("change")
// Fires the event as NOT preventable and NOT bubbling
this.fireDecoratorEvent("change");
- Fire event with non-default configuration
@event("change", {
bubbles: true // false by default
cancelable: true // false by default
})
// Fires the event as preventable and bubbling
this.fireDecoratorEvent("change");
Note: since v2.4.0
it's recommended to describe the event in the @event
decorator and use the fireDecoratorEvent
method.
The fireEvent
method​
The method is available since the very beginning of the project and like fireDecoratorEvent
fires a custom event, but does not consider the settings in the @event
decorator. So, if you set cancelable
and bubbles
in the @event
decorator, but fire the component events via fireEvent
, the configured values won't be considered.
Another difference is the default values of the event settings. When using fireEvent
by default it assumes the event is bubbling (bubbles: true) and not preventable (cancelable: false).
- Fire event with default configuration
// Fires the event as NOT preventable and bubbling
this.fireEvent("change");
- Fire event with non-default configuration
The method allows configuring the cancelable
and bubbles
fields via function arguments - the third and fourth parameters respectively.
// Fires the event as preventable and non-bubbling
this.fireEvent("change", {}, true, false);
noConflict mode​
By default, UI5 Web Components fire all custom events twice: once with their name (e.g., change
) and once more with a ui5-
prefix (e.g., ui5-change
). For example, when the ui5-switch
is toggled, it fires a change
event and a ui5-change
event.
This noConflict
setting allows us to prevent clashes between native and custom events.
The noConflict
setting (@ui5/webcomponents-base/config/NoConflict.js
) controls this behavior:
false
(default): Events fire both with and without theui5-
prefix.true
: Events fire only with theui5-
prefix, avoiding name collisions with third-party libraries.- Object: Specific events listed in the object fire only with the
ui5-
prefix, while all other events fire both ways. Example:In this case, only{
"events": ["selection-change", "header-click"]
}selection-change
andheader-click
fire with theui5-
prefix, leaving these names available for other uses.
Note: With this setting, when attaching an event listener to a UI5 web component used inside a template, the event name must be specified with the ui5- prefix.