Support mixin of structs

Hi @essenciary, @abhimanyuaryan quite some time that the mixin topic was raised in StippleUI. Now I have a new use case in StipplePlotly to follow this up.

I propose to include the following in Stipple.

macro mixin(expr, prefix = "", postfix = "")
    x = eval(expr)
    pre = eval(prefix)
    post = eval(postfix)
    T = x isa DataType ? x : typeof(x)
    mix = x isa DataType ? x() : x
    values = [Stipple.Observables.to_value(getfield(mix, f)) for f in fieldnames(T)]
    output = quote end
    for (f, type, v) in zip(Symbol.(pre, fieldnames(T), post), fieldtypes(T), values)
        push!(output.args, :($(esc(f))::$type = $v) )
    end
    
    :($output)
end

and the following in StipplePlotly

const PlotlyEvent = Dict{String, Any}
@kwdef struct PlotlyEvents
    _selected::R{PlotlyEvent} = PlotlyEvent()
    _hover::R{PlotlyEvent} = PlotlyEvent()
    _click::R{PlotlyEvent} = PlotlyEvent()
    _relayout::R{PlotlyEvent} = PlotlyEvent()
end

in order to support

using Stipple
using PlotlyBase

julia> @reactive! mutable struct PlotDemo <: ReactiveModel
           plot::R{Plot} = Plot()
           @mixin PlotlyEvents "plot"
       end
:PlotDemo

julia> fieldnames(PlotDemo)
(:plot, :plot_selected, :plot_hover, :plot_click, :plot_relayout, :channel__, :isready, :isprocessing)

I see some advanteage over the approach of Mixers.jl as there we cannot determine the position of the fields in the model, whereas here we can, and we can easily determine pre- and postfixes.

What do you think?

[hhaensel]

@hhaensel I get the error #119 on

julia> PlotlyEvents
PlotlyEvents

julia> Stipple.@mixin PlotlyEvents
ERROR: LoadError: UndefVarError: PlotlyEvents not defined
Stacktrace:
 [1] top-level scope
   @ :0
 [2] eval
   @ ./boot.jl:360 [inlined]
 [3] eval(x::Symbol)
   @ Stipple ~/.julia/dev/Stipple/src/Stipple.jl:16
 [4] var"@mixin"(__source__::LineNumberNode, __module__::Module, expr::Any, prefix::Any, postfix::Any) (repeats 2 times)
   @ Stipple ~/.julia/dev/Stipple/src/Stipple.jl:1312
in expression starting at REPL[9]:1

I know where it comes from, it’s from the fact the the macro evaluates the expression within its name space. Let me work on it a bit…

[hhaensel]

StipplePlotly also has a new version with mixin support.

@reactive! mutable struct PlotDemo <: ReactiveModel
    @mixin plot::PlotWithEvents
end

@reactive! mutable struct PBPlotDemo <: ReactiveModel
    @mixin plot::PBPlotWithEvents
end

julia> fieldnames(PlotDemo)
(:plot_data, :plot_layout, :plot_selected, :plot_hover, :plot_click, :plot_relayout, :channel__, :isready, :isprocessing)

julia> fieldnames(PBPlotDemo)
(:plot, :plot_selected, :plot_hover, :plot_click, :plot_relayout, :channel__, :isready, :isprocessing)