Flip animations in Svelte - Step-by-Step Execution

Concept Flow - Flip animations

Initial Render

↓

Element Positioned

↓

State Change Trigger

↓

Measure Old Position

↓

DOM Update

↓

Measure New Position

↓

Calculate Delta

↓

Apply Transform to Offset

↓

Animate Transform to Zero

↓

Remove Transform

↓

Final Position

Flip animation measures element position before and after a change, then animates the difference to create smooth movement.

Execution Sample

Svelte

import { flip } from 'svelte/animate';

<ul>
  {#each items as item (item.id)}
    <li animate:flip>{item.text}</li>
  {/each}
</ul>

This code animates list items smoothly when their order or content changes using Svelte's flip animation.

Execution Table

Step	Action	Old Position	New Position	Delta (Translate)	Animation Applied
1	Initial render of items	N/A	Positions set by DOM	N/A	No animation
2	Trigger state change (reorder items)	Positions before reorder	Positions after reorder	Calculated difference	Transform applied to offset element
3	Start animation	Same as step 2	Same as step 2	Transform animates from delta to zero	Smooth movement visible
4	Animation ends	N/A	Final positions	Transform removed	Element at new position without transform
5	Ready for next change	Final positions	N/A	N/A	No animation

💡 Animation completes and transform is removed, element stays at new position.

Variable Tracker

Variable	Start	After State Change	During Animation	After Animation
oldPosition	undefined	measured before DOM update	constant	undefined
newPosition	undefined	measured after DOM update	constant	undefined
delta	undefined	calculated as oldPosition - newPosition	animates from delta to 0	0
transform	none	set to translate(delta)	animates to none	none

Key Moments - 3 Insights

Why do we measure positions before and after the DOM update?

Measuring before and after lets us find the exact movement needed to animate smoothly, as shown in execution_table step 2.

What does the 'delta' represent in the animation?

Delta is the difference in position between old and new states; it is the amount the element moves during animation (execution_table step 2 and 3).

Why is the transform removed after the animation ends?

Removing the transform resets the element to its natural position in the DOM, preventing layout issues (execution_table step 4).

Visual Quiz - 3 Questions

Look at the execution_table, what happens at step 3 during the animation?

A Transform is removed and animation ends

B Positions are measured for the first time

C Transform animates from delta to zero creating smooth movement

D State change is triggered

💡 Hint

Refer to execution_table row 3 under 'Animation Applied' and 'Delta (Translate)' columns.

According to variable_tracker, what is the value of 'delta' after the animation?

A 0

B calculated as oldPosition - newPosition

C undefined

D animates from delta to 0

💡 Hint

Check variable_tracker row for 'delta' under 'After Animation' column.

If the transform was not removed after animation, what would happen?

A Animation would restart automatically

B Element would stay visually offset from its new position

C Positions would be measured incorrectly

D No visible effect

💡 Hint

See key_moments explanation about transform removal and execution_table step 4.

Concept Snapshot

Flip animations in Svelte:
- Measure element position before and after change
- Calculate position difference (delta)
- Apply transform to offset element by delta
- Animate transform from delta to zero
- Remove transform after animation
This creates smooth movement for reordered or changed elements.

Full Transcript

Flip animations in Svelte work by measuring an element's position before and after a change, such as reordering a list. The difference in position, called delta, is calculated. The element is then temporarily moved back to its old position using a CSS transform. An animation runs that moves the element from the old position to the new one by animating the transform from delta to zero. After the animation finishes, the transform is removed so the element stays naturally in its new place. This process creates a smooth visual transition that helps users see how elements move on the page.