What Are Angular Directives?
Angular Directive is a TypeScript class which is
declared as a @directive decorator.
The directives allow you to attach behavior to
DOM elements and the @directive decorator provide you an additional metadata
that determines how directives should be processed, instantiated, and used at
run-time.
What Are decorators?
The Decorators are functions that modify
JavaScript classes and it also used for attaching metadata to classes.
Directive decorator and metadata Properties -
@Directive({
selector?: string
inputs?: string[]
outputs?: string[]
host?: {...}
providers?: Provider[]
exportAs?: string
queries?: {...}
})
Selector
–
It is a CSS selector that tells Angular to create an instance of this component
wherever it finds the corresponding tag in template HTML.
For example,
it is – <app-login></app-login>
CSS selector also triggers the instantiation of a
directive.
The selector may be declared by element name,
class name, attribute name, and attribute name & value.
Suppose we have a directive with an <input type="checkbox">
selector and the HTML looks like this.
<form>
<label>Name
-</label>
<input
type="text">
<label>Are
you agree? </label>
<input
type="checkbox">
<form>
The directive will only be instantiated on the <input type="checkbox">
element.
Inputs–
The list of class property names to data-bind as component inputs
Outputs
-
The list of class property names that expose output events that others can
subscribe too
Host–
These properties use to map the class property to host element bindings for properties,
events, actions, and attributes.
The host looks like this.
@Directive({
selector: 'button',
host: {'(click)':
'onClick($event.target)'}
})
Providers
-
list of providers available to this component and its children
Queries–
To configure queries that can be injected into the component
We
have 3 types of Directives in Angular -
1. Component
2. Attribute
Directives
3. Structural
Directives
Components
- The component is a directive with their own templates and it is responsible
for how a component should be processed, instantiated and used at run-time.
Structural
Directives - The structural directive is a directive and it
is responsible for change the DOM layout by adding, removing, and manipulating
elements.
The most of the common built-in structural
directives are NgIf, NgFor, and NgSwitch.
Attribute
Directives - The Attribute directive is a directive and it
is responsible for change the behavior of a specified element or component.
What are the differences between @Component and @Directive?
The components are used, when you want to create
new elements in the DOM with their own HTML template.
The attribute directives are used, when you want
to change or update the existing elements in the DOM.
How to Create Custom Directives?
Let's start to create a simple directive.
I assuming you have installed the Angular CLI and
all the necessary configurations are running in your app. Now, go to your
project directory and execute the below CLI command for creating your custom
directive –
ng g directive myCustom
After execute the above CLI command, created two
files in the project - src/app folder
1. src/app/my-custom.directive.spec.ts
2. src/app/my-custom.directive.ts
And update files reference automatically in your
project module – “src/app/app.module.ts”
Lest see in the code-sample, how it look like-
my-custom.directive.ts –
import
{ Directive } from
'@angular/core';
@Directive({
selector: '[appMyCustom]'
})
export
class MyCustomDirective
{
constructor() { }
}
And app.module.ts –
import
{ MyCustomDirective
} from './my-custom.directive'
//AppModule class with @NgModule
decorator
@NgModule({
//Static, this is the compiler configuration
//declarations is used for configure the selectors.
declarations: [
AppComponent,
MyCustomDirective,
],
//Composability and Grouping
//imports used for composing NgModules together.
imports: [
BrowserModule
],
//Runtime or injector configuration
//providers is used for runtime injector
configuration.
providers: [],
//bootstrapped entry component
bootstrap: [AppComponent]
})
export
class AppModule
{ }