Unlocking the Power of Angular’s @ViewChild and @ViewChildren Decorators
When building complex Angular applications, accessing and modifying child elements within a component view can be a daunting task. Fortunately, Angular provides two powerful decorators, @ViewChild and @ViewChildren, to help you tackle this challenge.
What are View Queries?
A view query is a requested reference to a child element within a component view, containing metadata about the element. The scope of these decorators is limited to the component view and its embedded child views. They are particularly useful when you need to access and modify elements within the view in unconventional ways.
The AfterViewInit Lifecycle Hook
The AfterViewInit lifecycle hook is called when the component view and its child views are completely initialized. This is the perfect place to access view queries, as they are already resolved and set. Trying to access them before ngAfterViewInit responds may result in undefined values.
The @ViewChild Decorator
The @ViewChild decorator takes three properties: a selector, a read property, and a static property. The read and static properties are optional. The selector property specifies what child element within the component view is to be queried. There are five kinds of selectors supported:
- Classes with
@Componentor@Directivedecorators - A template reference variable as a string
- A provider defined in the child component tree of the current component
- A provider defined through a string token (although this is currently not working due to an Ivy regression)
- A TemplateRef
Using the read Property
The read property allows you to select various tokens from the elements you query. These tokens could be provider tokens used for dependency injection or, in some cases, the type of view query. This property is optional.
Using the static Property
The static property takes a boolean value and is optional. By default, it is false. If it is true, the view query is resolved before the complete component view and data-bound properties are fully initialized. If set to false, the view query is resolved after the component view and data-bound properties are completely initialized.
The @ViewChildren Decorator
The @ViewChildren decorator works similarly to the @ViewChild decorator but instead of configuring a single view query, it gets a query list. From the component view DOM, it retrieves a QueryList of child elements. This list is updated when any changes are made to the child elements.
Putting it all Together
By mastering the @ViewChild and @ViewChildren decorators, you can unlock new possibilities for customizing component views in Angular. With these powerful tools, you can access and modify child elements with ease, taking your Angular applications to the next level.
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