FlexibleGridLayout.cs

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class FlexibleGridLayout : LayoutGroup 
{
    public enum FitType {
        Uniform,
        Width,
        Height
    }

    public FitType fitType;

    public int rows;
    public int columns;
    public Vector2 cellSize;
    public Vector2 spacing;


    public override void CalculateLayoutInputHorizontal() {

        base.CalculateLayoutInputHorizontal();

        float sqrRt = Mathf.Sqrt(rectChildren.Count);
        rows = Mathf.CeilToInt(sqrRt);
        columns = Mathf.CeilToInt(sqrRt);

        if (fitType == FitType.Width)
        {
            rows = Mathf.CeilToInt(rectChildren.Count / (float)columns);
        }
        if (fitType == FitType.Height)
        {
            rows = Mathf.CeilToInt(rectChildren.Count / (float)rows);
        }

        float parentWidth = rectTransform.rect.width;
        float parentHeight = rectTransform.rect.height;

        float cellWidth = (parentWidth / (float)columns) - ((spacing.x / ((float)columns)) * (columns - 1)) - (padding.left / (float)columns) - (padding.right / (float)columns);
        float cellHeight = (parentHeight / (float)rows) - ((spacing.y / ((float)rows)) * (rows - 1)) - (padding.top / (float)rows) - (padding.bottom / (float)rows);

        cellSize.x = cellWidth;
        cellSize.y = cellHeight;

        int columnCount = 0;
        int rowCount = 0;

        for (int i = 0; i < rectChildren.Count; i++) {
            
            rowCount = i / columns;
            columnCount = i % columns;

            var item = rectChildren[i];

            var xPos = (cellSize.x * columnCount) + (spacing.x * columnCount) + padding.left;
            var yPos = (cellSize.y * rowCount) + (spacing.y * rowCount) + padding.top;

            SetChildAlongAxis(item, 0, xPos, cellSize.x);
            SetChildAlongAxis(item, 1, yPos, cellSize.y);
        }

    }

    public override void CalculateLayoutInputVertical() { }

    public override void SetLayoutHorizontal() { }

    public override void SetLayoutVertical() { }
}