flexible box layout prototype

main.lua

local start_time = love.filesystem.getLastModified("main.lua")

function love.update(dt)
	if (love.filesystem.getLastModified("main.lua") > start_time) then
		love.event.quit("restart")
	end
end

local function round(value, precision)
	if precision then return round(value / precision) * precision end
	return value >= 0 and math.floor(value+0.5) or ceil(value-0.5)
end

local widgets = {
	layout = "row",
	{ label = "asdf", height = 25 },
	{
		layout = "column",
		label = "ghjk",
		height = 25,
		{ label = "OK", width = 100 },
		{ label = "Cancel", width = 100 },
	},
	{
		layout = "column",
		height = 50,
		{ label = "GGGG", width = 100 },
		{ label = "Baby Baby", width = 100 },
		{
			layout = "column",
			width = 100,
			height = 50,
			{ label = "OK", width = 100 },
			{ label = "Cancel", width = 100 },
			{
				layout = "row",
				width = 100,
				height = 50,
				{ label = "OK" },
				{ label = "Cancel" },
			}
		}
	}
}

local layout = {}

-- scale normalized ui layout to w*h, with care to fix gaps
function layout.scale(ui, w, h, do_rounding)
	for _, item in ipairs(ui) do
		if do_rounding then
			item.x = item.x * w
			item.y = item.y * h

			-- fudge for floating point inaccuracy
			item.x = (item.x + 0.01)
			item.y = (item.y + 0.01)

			-- carry adjusted item position into width
			item.x_diff = item.x - math.floor(item.x)
			item.y_diff = item.y - math.floor(item.y)
			item.x = item.x - item.x_diff
			item.y = item.y - item.y_diff
			item.width = math.floor(item.width * w + item.x_diff)
			item.height = math.floor(item.height * h + item.y_diff)
		else
			item.x = item.x * w
			item.y = item.y * h
			item.width = item.width * w
			item.height = item.height * h
		end
	end
end

function layout.pack(widgets, ui, parent)
	local ui = ui or {}
	local total = 0

	for i, v in ipairs(widgets) do
		-- fill in default of 1
		v.width = v.width or 1
		v.height = v.height or 1

		local size = v.width
		if widgets.layout == "row" then
			size = v.height
		end

		assert(size > 0)
		total = total + size
	end

	local advance = 0
	for i, v in ipairs(widgets) do
		local item = {
			label = v.label or string.format("<unknown %d>", i),
			x = parent and parent.x or 0,
			y = parent and parent.y or 0,
			width = parent and parent.width or 1,
			height = parent and parent.height or 1
		}
		if widgets.layout == "row" then
			local height = v.height / total
			item.y = item.y + item.height * advance
			item.height = item.height * height
			advance = advance + height
		else
			local width = v.width / total
			item.x = item.x + item.width * advance
			item.width = item.width * width
			advance = advance + width
		end
		if #v > 0 then
			layout.pack(v, ui, item)
		else
			table.insert(ui, item)
		end
	end

	return ui
end

function love.draw()
	local w, h = love.graphics.getDimensions()
	local ui = layout.pack(widgets)
	layout.scale(ui, w, h, true)

	for i, v in ipairs(ui) do
		local pad = 0
		love.graphics.setColor(0, i*30, 255, 127)
		love.graphics.rectangle("fill",
			v.x + pad,
			v.y + pad,
			v.width - pad * 2,
			v.height - pad * 2
		)
		love.graphics.setColor(255, 255, 255, 255)
		love.graphics.print(
			string.format("%d: %s", i, v.label),
			v.x + pad,
			v.y + pad
		)
	end
end