Enter and leave animations in Angular - Step-by-Step Execution

Concept Flow - Enter and leave animations

Component loads

↓

Element added to DOM

↓

Trigger enter animation

↓

Animation runs

↓

Animation ends

↓

Element visible

↓

Trigger leave animation

↓

Animation runs

↓

Animation ends

↓

Element removed from DOM

When an element appears, Angular triggers an enter animation before showing it. When it disappears, Angular triggers a leave animation before removing it from the DOM.

Execution Sample

Angular

import { Component } from '@angular/core';
import { trigger, transition, style, animate } from '@angular/animations';

@Component({
  selector: 'app-fade',
  template: `<div *ngIf="isVisible" @fade>Fade me in and out</div>`,
  animations: [
    trigger('fade', [
      transition(':enter', [style({opacity: 0}), animate('500ms', style({opacity: 1}))]),
      transition(':leave', [animate('500ms', style({opacity: 0}))])
    ])
  ]
})
export class FadeComponent {
  isVisible = true;
}

Defines a fade animation that runs when an element enters or leaves the DOM.

Execution Table

Step	Trigger	Animation State	Action	Element Visibility
1	Component loads	No animation	Element not in DOM	Hidden
2	Element added	Enter animation starts	Opacity from 0 to 1 over 500ms	Fading in
3	After 500ms	Enter animation ends	Element fully visible	Visible
4	Element removed	Leave animation starts	Opacity from 1 to 0 over 500ms	Fading out
5	After 500ms	Leave animation ends	Element removed from DOM	Hidden

💡 Element is removed from DOM after leave animation finishes

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5
elementOpacity	N/A	0 -> animating to 1	1	1 -> animating to 0	N/A (removed)
elementInDOM	false	true	true	true	false

Key Moments - 3 Insights

Why does the element stay visible after the enter animation ends?

Because after the enter animation finishes (step 3), the element's opacity is fully 1 and it remains in the DOM visible as shown in the execution_table.

When exactly is the element removed from the DOM?

The element is removed only after the leave animation completes (step 5), not immediately when the element is removed from the template (step 4).

What happens if the animation duration is changed?

The animation runs for the new duration, so the fading in or out takes longer or shorter, reflected in the timing between steps 2-3 or 4-5 in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the elementOpacity value after step 3?

A Animating from 0 to 1

B 0

C 1

D Animating from 1 to 0

💡 Hint

Check the 'elementOpacity' row in variable_tracker after step 3

At which step does the element get removed from the DOM?

A Step 5

B Step 2

C Step 4

D Step 3

💡 Hint

Look at the 'elementInDOM' variable in variable_tracker and the 'Action' column in execution_table

If the enter animation duration is increased, how does it affect the execution_table?

A Leave animation runs longer

B Step 3 happens later, enter animation runs longer

C Step 2 is skipped

D Element is removed earlier

💡 Hint

Refer to the timing between steps 2 and 3 in execution_table

Concept Snapshot

Angular enter and leave animations:
- Use trigger() with :enter and :leave transitions
- :enter runs when element is added to DOM
- :leave runs before element is removed
- Animations run before visibility changes
- Element removed only after leave animation ends

Full Transcript

This visual execution shows how Angular handles enter and leave animations. When a component loads and an element is added, Angular triggers the enter animation, gradually changing opacity from 0 to 1 over 500 milliseconds. The element becomes fully visible after the animation ends. When the element is removed, Angular triggers the leave animation, fading opacity from 1 to 0 over 500 milliseconds before removing the element from the DOM. Variables like elementOpacity and elementInDOM track the element's state through these steps. Key moments include understanding that the element stays visible after enter animation ends and is only removed after leave animation finishes. Changing animation duration affects how long these transitions take. The quizzes test understanding of opacity values, removal timing, and animation duration effects.