flutter-figma-constraint-center.md

How to accurately implement Figma's center constraint on Flutter (% positioning)

This document describes breifly about how figma handles the center alignment, and how it's different from flutter standard.

First, read figma-constraint-center

Why not Center()?

Why not Positioned() ?

Using Alignment

if vertical and horizontal are both biased and center-aligned, then we can use Align & Alignment under Stack.

Stack(
  children: [
    Align(
      alignment: Alignment(-1.0 + x * 2, -1.0 + y * 2), // where x and y percentage by x and y
      child: child,
    ),
    // ...
  ],
)

Using Center

If the item is both centered in vertical & horizontal, then we can use Center under Stack. This cannot be applied to biased elements. (only true center, 0.5, 0.5)

Stack(
  children: [
    Center(
      child: child,
    ),
  ],
)

Using Padding & Align

Stack(
  children: [
    Padding(
      padding: EdgeInsets.only(top: 80), // top
      child: Align(
        alignment: Alignment.topCenter, // horizontal center
        child: Container(
          width: 75,
          height: 75,
          decoration: BoxDecoration(
          color: Colors.red
          ),
        )
      )
    )
  ]
)

With same technique, we can also use Row / Column & mainAxisAlignment: MainAxisAlignment.center but this does not represent the original design as-is.

Using Container with margin & alignment

// if the item is horizontally centered (no bias) and aligned to top 100px.
Container(
        // margin replaces `Positioned`.
        margin: : EdgeInsets.only(top: 100, right: 272),
        // alignment replaces `Align`
        alignment: Alignment.topCenter,
        child: child
);

Using Positioned.fill and LayoutBuilder

Fallbacks

• Center centered center-aligned item to
• Fallback left-sided center-aligned item to Positioned(left: x)
• Fallback right-sided center-aligned item to Positioned(right: x)
• Fallback top-sided center-aligned item to Positioned(top: y)
• Fallback bottom-sided center-aligned item to Positioned(bottom: y)

Related lints

To prevent this from happening (unless intended) is to run lints on figma design first, check if any layers have wrong center alignment while they actually should be having left or right alignment.