Input signals and model signals in Angular - Step-by-Step Execution

Concept Flow - Input signals and model signals

Parent Component

↓

Child Component

↓

Child Model Signal

↓

Child Component

↓

Parent Component

↓

Parent Model

The parent sends input signals to the child. The child updates its model and emits signals back. The parent listens and updates accordingly.

Execution Sample

Angular

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

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

A child component receives a model signal as input and displays its value.

Execution Table

Step	Action	Input Signal Value	Model Signal Value	Output/Effect
1	Parent sets input signal to 'Hello'	Hello	'' (initial)	Child receives 'Hello' as input
2	Child model signal updates to input value	Hello	Hello	Child displays 'Hello'
3	Child emits model signal change	Hello	Hello	Parent listens and updates its model
4	Parent updates model signal to 'Hello'	Hello	Hello	Parent model updated
5	Parent changes input signal to 'World'	World	Hello	Child receives new input 'World'
6	Child model signal updates to 'World'	World	World	Child displays 'World'
7	Child emits model signal change	World	World	Parent listens and updates its model
8	Parent updates model signal to 'World'	World	World	Parent model updated
9	No further changes	World	World	Execution stops
10	Exit	N/A	N/A	No more input changes

💡 No more input changes; signals stabilized

Variable Tracker

Variable	Start	After 1	After 2	After 5	After 6	Final
Input Signal	''	'Hello'	'Hello'	'World'	'World'	'World'
Model Signal	''	''	'Hello'	'Hello'	'World'	'World'

Key Moments - 2 Insights

Why does the child model signal update after the input signal changes?

Because the child listens to the input signal and updates its model signal accordingly, as shown in execution_table rows 2 and 6.

How does the parent know when to update its model signal?

The child emits a model signal change event that the parent listens to, triggering the update, as seen in rows 3 and 7.

Visual Quiz - 3 Questions

Look at the execution table, what is the model signal value at step 2?

A 'World'

B '' (empty string)

C 'Hello'

D undefined

💡 Hint

Check the 'Model Signal Value' column at step 2 in the execution_table.

At which step does the input signal change from 'Hello' to 'World'?

A Step 4

B Step 5

C Step 6

D Step 7

💡 Hint

Look at the 'Input Signal Value' column in the execution_table.

If the parent never updates the input signal, what happens to the model signal?

A It stays the same as the initial value

B It updates automatically to 'World'

C It updates to 'Hello'

D It becomes undefined

💡 Hint

Refer to variable_tracker and execution_table exit_note about signal stabilization.

Concept Snapshot

Input signals flow from parent to child components.
Child components receive input signals and update their model signals.
Model signals can emit changes back to the parent.
Parent listens and updates its own model accordingly.
This creates a reactive data flow between parent and child.
Signals keep UI in sync without manual event handling.

Full Transcript

In Angular, input signals are passed from a parent component to a child component. The child receives these signals and updates its internal model signals accordingly. When the child model signal changes, it emits a signal back to the parent. The parent listens to these emitted signals and updates its own model. This cycle keeps the data synchronized between parent and child components. The execution table shows how the input signal changes from empty to 'Hello' and then to 'World', and how the model signal updates in response. The variable tracker records these changes step-by-step. Key moments clarify why the child updates its model after input changes and how the parent listens to child signals. The visual quiz tests understanding of signal values at specific steps and the effect of no input changes. This reactive pattern simplifies data flow and UI updates in Angular applications.