# ModelSchemaCheckAttributeEvent

typedef

Event fired when the [`checkAttribute`](module_engine_model_schema-ModelSchema.md#function-checkAttribute) method is called. It allows plugging in additional behavior, for example implementing rules which cannot be defined using the declarative [`ModelSchemaItemDefinition`](module_engine_model_schema-ModelSchemaItemDefinition.md) interface.

**Note:** The [`addAttributeCheck`](module_engine_model_schema-ModelSchema.md#function-addAttributeCheck) method is a more handy way to register callbacks. Internally, it registers a listener to this event but comes with a simpler API and it is the recommended choice in most of the cases.

The [`checkAttribute`](module_engine_model_schema-ModelSchema.md#function-checkAttribute) method fires an event because it is [decorated](module_utils_observablemixin-Observable.md#function-decorate) with it. Thanks to that you can use this event in various ways, but the most important use case is overriding the standard behavior of the `checkAttribute()` method. Let's see a typical listener template:

```typescript
schema.on( 'checkAttribute', ( evt, args ) => {
	const context = args[ 0 ];
	const attributeName = args[ 1 ];
}, { priority: 'high' } );
```

The listener is added with a `high` priority to be executed before the default method is really called. The `args` callback parameter contains arguments passed to `checkAttribute( context, attributeName )`. However, the `context` parameter is already normalized to a [`ModelSchemaContext`](module_engine_model_schema-ModelSchemaContext.md) instance, so you do not have to worry about the various ways how `context` may be passed to `checkAttribute()`.

So, in order to implement a rule "disallow `bold` in a text which is in a `heading1`, you can add such a listener:

```typescript
schema.on( 'checkAttribute', ( evt, args ) => {
	const context = args[ 0 ];
	const attributeName = args[ 1 ];

	if ( context.endsWith( 'heading1 $text' ) && attributeName == 'bold' ) {
		// Prevent next listeners from being called.
		evt.stop();
		// Set the checkAttribute()'s return value.
		evt.return = false;
	}
}, { priority: 'high' } );
```

Allowing attributes in specific contexts will be a far less common use case, because it is normally handled by the `allowAttributes` rule from [`ModelSchemaItemDefinition`](module_engine_model_schema-ModelSchemaItemDefinition.md). But if you have a complex scenario where `bold` should be allowed only in element `foo` which must be in element `bar`, then this would be the way:

```typescript
schema.on( 'checkAttribute', ( evt, args ) => {
	const context = args[ 0 ];
	const attributeName = args[ 1 ];

	if ( context.endsWith( 'bar foo $text' ) && attributeName == 'bold' ) {
		// Prevent next listeners from being called.
		evt.stop();
		// Set the checkAttribute()'s return value.
		evt.return = true;
	}
}, { priority: 'high' } );
```

[See source](https://github.com/ckeditor/ckeditor5/blob/master/packages/ckeditor5-engine/src/model/schema.ts#L1406)

<a id="properties">

## Properties

<a id="member-args">

### `args: [ [ context: ModelSchemaContext, attributeName: string ] ]`

<a id="member-name">

### `name: 'checkAttribute'`

#### Value

`object`

## Fired by

* [ModelSchema#checkAttribute](module_engine_model_schema-ModelSchema.md#event-checkAttribute)

---

Full index of the CKEditor 5 API reference: [llms.txt](llms.txt)