What is the process for consumers to provide constructor parameters in Angular 2?

Question

What is the process for consumers to provide constructor parameters in Angular 2?

Is it possible to modify the field of a component instance? Let's consider an example in test.component.ts:

@Component({
    selector: 'test',
})
export class TestComponent {    
    @Input() temp;
    temp2;
    constructor(arg) {
        this.temp = arg;
        this.temp2 = arg * 2;
    }
}

I am looking to set the values of temp and temp2 using the constructor. One common approach is to use input property like this:

<test [temp]='1'></test>

But this method modifies the properties after construction, causing temp2 not to update accordingly. What can be done to provide constructor arguments for a component from the consumer's perspective so that "temp" and "temp2" are set during construction?

Thank you!

javascript angularjs typescript angular

Answer 1

Answer №1

Component inputs can only be accessed from the ngOnInit method due to the component lifecycle:

@Component({
    selector: 'test',
})
export class TestComponent {    
    @Input() temp;

    ngOnInit() {
        console.log(this.temp);
    }
}

In addition, parameters in the component constructor must be provided through dependency injection.

Therefore, using the constructor for the temp property is not possible because of the component lifecycle. If you want to make it available through dependency injection, use the @Inject decorator to specify what needs to be injected.

For more information on this topic, refer to the following question:

Difference between Constructor and ngOnInit

Answer 2

Component inputs can only be accessed from the ngOnInit method due to the component lifecycle:

@Component({
    selector: 'test',
})
export class TestComponent {    
    @Input() temp;

    ngOnInit() {
        console.log(this.temp);
    }
}

In addition, parameters in the component constructor must be provided through dependency injection.

Therefore, using the constructor for the temp property is not possible because of the component lifecycle. If you want to make it available through dependency injection, use the @Inject decorator to specify what needs to be injected.

For more information on this topic, refer to the following question:

Difference between Constructor and ngOnInit

Answer 3

Answer №2

service.ts

import {Injectable} from 'angular/core';

@Injectable()
export class customService{
  message:string="Welcome to Angular";
}

app.ts

import {customService} from './customService';
...
...
bootstrap[App,[customService]]

import {customService} from './customService';
@Component({
    selector: 'example',
})
export class ExampleComponent {    
    data;
    constructor(customService:customService) {
        this.data = customService.message;
        console.log(this.data) //Welcome to Angular
    }
}

Answer 4

service.ts

import {Injectable} from 'angular/core';

@Injectable()
export class customService{
  message:string="Welcome to Angular";
}

app.ts

import {customService} from './customService';
...
...
bootstrap[App,[customService]]

import {customService} from './customService';
@Component({
    selector: 'example',
})
export class ExampleComponent {    
    data;
    constructor(customService:customService) {
        this.data = customService.message;
        console.log(this.data) //Welcome to Angular
    }
}

Answer 5

Answer №3

Based on the response by Thierry Templier, it seems like your issue is being addressed, however,

You mentioned in a comment:

I have updated the question for clarity. When using an input property, I can modify temp but temp2 does not update accordingly.

I hope this helps you achieve what you are looking for.

    import {Input, Component} from 'angular2/core'

    @Component({
      selector: 'my-test',
      template: `
      <h1> arg value:    {{ arg }} </h1>
      <h1> temp value:   {{ temp }} </h1>
      <h1> temp1 value:  {{ temp1 }} </h1>
    `
    })

    export class test {
      @Input() arg  : number;
      temp : number;
      temp1: number;

      constructor(){

      }

      ngOnInit(){
        this.temp  = this.arg;
        this.temp1 = this.arg * 2;
      }

    }

    @Component({
      selector: 'my-app',
      directives: [test],
      template: `
      <h2>Hello {{name}}</h2>
      <my-test [arg]="1"></my-test>
    `
    })
    export class App {
      constructor() {
       this.name = 'Angular2';
      } 
    }

Check out the Plunker example

Answer 6

Based on the response by Thierry Templier, it seems like your issue is being addressed, however,

You mentioned in a comment:

I have updated the question for clarity. When using an input property, I can modify temp but temp2 does not update accordingly.

I hope this helps you achieve what you are looking for.

    import {Input, Component} from 'angular2/core'

    @Component({
      selector: 'my-test',
      template: `
      <h1> arg value:    {{ arg }} </h1>
      <h1> temp value:   {{ temp }} </h1>
      <h1> temp1 value:  {{ temp1 }} </h1>
    `
    })

    export class test {
      @Input() arg  : number;
      temp : number;
      temp1: number;

      constructor(){

      }

      ngOnInit(){
        this.temp  = this.arg;
        this.temp1 = this.arg * 2;
      }

    }

    @Component({
      selector: 'my-app',
      directives: [test],
      template: `
      <h2>Hello {{name}}</h2>
      <my-test [arg]="1"></my-test>
    `
    })
    export class App {
      constructor() {
       this.name = 'Angular2';
      } 
    }

Check out the Plunker example

What is the process for consumers to provide constructor parameters in Angular 2?

Answer №1

Answer №2

Answer №3

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