Input signals and model signals in Angular

Introduction

Input signals let a component receive data from outside. Model signals help track and update data inside the component easily.

When you want to pass data from a parent component to a child component.

When you need to react to changes in input data automatically.

When you want to keep the component's internal data reactive and update the UI smoothly.

When building forms or interactive UI elements that change based on user input.

When you want to separate data flow clearly between components.

Syntax

Angular

import { Component, Input, signal } from '@angular/core';

@Component({
  selector: 'app-child',
  standalone: true,
  template: `
    <p>Value: {{ value() }}</p>
  `
})
export class ChildComponent {
  @Input() value = signal('');
}

Input decorator marks a property as an input to receive data from a parent.

signal() creates a reactive value that updates the UI when changed.

Examples

This example shows a child component receiving a name input as a signal with a default value.

Angular

import { Component, Input, signal } from '@angular/core';

@Component({
  selector: 'app-child',
  standalone: true,
  template: `<p>{{ name() }}</p>`
})
export class ChildComponent {
  @Input() name = signal('Guest');
}

Here, a numeric signal count is passed as input to show a reactive counter value.

Angular

import { Component, Input, signal } from '@angular/core';

@Component({
  selector: 'app-counter',
  standalone: true,
  template: `
    <p>Count: {{ count() }}</p>
  `
})
export class CounterComponent {
  @Input() count = signal(0);
}

Sample Program

This example shows a parent component passing a signal parentMessage to a child component's input message. Clicking the button changes the message reactively in the child.

Angular

import { Component, Input, signal } from '@angular/core';

@Component({
  selector: 'app-child',
  standalone: true,
  template: `
    <p>Message: {{ message() }}</p>
  `
})
export class ChildComponent {
  @Input() message = signal('Hello');
}

@Component({
  selector: 'app-root',
  standalone: true,
  imports: [ChildComponent],
  template: `
    <app-child [message]="parentMessage"></app-child>
    <button (click)="changeMessage()">Change Message</button>
  `
})
export class AppComponent {
  parentMessage = signal('Welcome!');

  changeMessage() {
    this.parentMessage.set('Hello from Parent!');
  }
}

Important Notes

Signals are functions you call to get the current value, like message().

Use .set(newValue) to update a signal's value and refresh the UI.

Input signals keep data reactive between parent and child components without extra code.

Summary

Input signals let components receive reactive data from parents.

Model signals inside components track and update data easily.

Using signals makes UI updates automatic and simple.

Practice

1. What is the main purpose of Input signals in Angular components?

easy

A. To receive reactive data from parent components

B. To send events to parent components

C. To style the component dynamically

D. To handle user input events

Solution

1. Step 1: Understand Input signals role

   Input signals allow a component to get reactive data from its parent, keeping the data flow reactive and automatic.

2. Step 2: Differentiate from other options

   Sending events to parents is done by outputs, styling is unrelated, and user input events are handled differently.

3. Final Answer:

   To receive reactive data from parent components -> Option A

4. Quick Check:

   Input signals = receive reactive data [OK]

Hint: Input signals bring data in from parents [OK]

Common Mistakes:

• Confusing input signals with output events
• Thinking input signals handle styling
• Assuming input signals manage user events

2. Which of the following is the correct way to declare an input signal in an Angular standalone component?

easy

A. @Input() inputSignal = signal();

B. const inputSignal = signal();

C. const inputSignal = @Input(signal());

D. signal @Input() inputSignal;

Solution

1. Step 1: Recall Angular input signal syntax

   Input signals are declared with the @Input() decorator followed by a signal initialization.

2. Step 2: Check each option

   @Input() inputSignal = signal(); correctly uses @Input() decorator before the signal. const inputSignal = signal(); misses the decorator, C and D have invalid syntax.

3. Final Answer:

   @Input() inputSignal = signal(); -> Option A

4. Quick Check:

   Input signals need @Input() decorator [OK]

Hint: Use @Input() before signal declaration [OK]

Common Mistakes:

• Forgetting the @Input() decorator
• Placing @Input() incorrectly
• Using invalid syntax with signals

3. Given this Angular component code snippet:

export class MyComponent {
  @Input() count = signal(0);

  increment() {
    this.count.update(c => c + 1);
  }
}

What will be the value of count() after calling increment() twice if the initial value is 0?

medium

A. 0

B. 1

C. undefined

D. 2

Solution

1. Step 1: Understand initial signal value

   The signal count starts at 0.

2. Step 2: Apply increment method twice

   Each call to increment() updates the signal by adding 1, so after two calls: 0 + 1 + 1 = 2.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   0 + 2 increments = 2 [OK]

Hint: Each update adds 1 to the signal value [OK]

Common Mistakes:

• Thinking signals don't update automatically
• Confusing method calls with direct assignment
• Assuming initial value changes unexpectedly

4. Identify the error in this Angular component code using input signals:

export class SampleComponent {
  @Input() data = signal();

  ngOnInit() {
    this.data.set('Hello');
  }
}

medium

A. Input signals must be readonly

B. Missing initial value for signal()

C. ngOnInit() is not allowed in standalone components

D. Cannot call set() on input signals

Solution

1. Step 1: Check signal initialization

   The signal data is declared without an initial value, which is invalid.

2. Step 2: Understand signal requirements

   Signals must have an initial value when created, so signal() without arguments causes an error.

3. Final Answer:

   Missing initial value for signal() -> Option B

4. Quick Check:

   Signals need initial values [OK]

Hint: Always initialize signals with a value [OK]

Common Mistakes:

• Leaving signals uninitialized
• Thinking set() is disallowed on inputs
• Confusing lifecycle hooks usage

5. You want to create a component that receives a reactive input signal userName and also maintains a local model signal greeting that updates automatically when userName changes. Which approach correctly implements this behavior?

hard

A. Use @Input() userName: string; and create greeting = signal(''); updated by a setter

B. Use @Input() userName = signal(''); and update greeting manually in ngOnChanges

C. Use @Input() userName = signal(''); and inside the component create greeting = computed(() => `Hello, ${this.userName()}`);

D. Use @Input() userName = signal(''); and assign greeting = signal('Hello'); once in constructor

Solution

1. Step 1: Understand reactive input and model signals

   The input signal userName provides reactive data from parent. The local greeting should react automatically to changes.

2. Step 2: Use computed signal for automatic updates

   Using computed creates a signal that updates whenever userName() changes, keeping greeting in sync.

3. Final Answer:

   Use @Input() userName = signal(''); and greeting = computed(() => `Hello, ${this.userName()}`); -> Option C

4. Quick Check:

   Computed signals auto-update from input signals [OK]

Hint: Use computed() for dependent reactive signals [OK]

Common Mistakes:

• Manually updating signals instead of computed
• Using plain string input instead of signal
• Assigning greeting only once without reactivity