juliagridvoodoo

import Base
using Test
struct Grid
    _components::Dict{DataType, Any}
end


function makecomponent(x, g)
    return nothing
end

function makecomponent(::Type{Val{:b}}, g)
    return 13
end

function makecomponent(::Type{Val{:x}}, g)
    return 19
end

function Base.getproperty(g::Grid, s::Symbol)
    if s == :_components
        return getfield(g, s)
    else
        components = getfield(g, :_components)
        if !haskey(components, Val{s})
            c = makecomponent(Val{s}, g)
            if isnothing(c)
                @warn "cannot make component $s"
            else
                @info "made component $s"
                components[Val{s}] = c
            end
        end
        getindex(components, Val{s})
    end
end

function Base.setproperty!(g::Grid, s::Symbol, v)
    return if s == :_components
        error("wrong access")
    else
        setindex!(getfield(g, :_components), v, Val{s})
    end
end

g = Grid(Dict{DataType, Any}())

@testset "xgrid" begin
    g.a = 1
    @test g.a == 1
    g.c = 3
    @test g.c == 3
    y = g.b
    @test y == 13
end

try
    local y = g.y
catch e
    @warn "detected error"
end

abstract type AbstractGridComponent end
struct AType <: AbstractGridComponent end
struct BType <: AbstractGridComponent  end
struct XType <: AbstractGridComponent  end
tsym(::Type{AType}) = :a
tsym(::Type{BType}) = :b
tsym(::Type{XType}) = :x
Base.getindex(g::Grid, T::Type{<:AbstractGridComponent}) = getproperty(g, tsym(T))

@testset "xgrid[Type]" begin
    @test g[AType] == 1
    xx = g[XType]
    @test xx == 19
end

g