PropertyMethods

Suppose a type needs to define non-field properties. This might be because it needs to implement some of the properties of another type to whose interface it must conform. The Delegation pattern is an example. There could be other reasons why some properties need to be computed rather than directly read.

Here is an example, though somewhat contrived:

using PropertyMethods

mutable struct Rectangle
    x
    y
    width
    height
end

mutable struct OffsetRectangle
    rect
    x_offset
    y_offset
end

Base.getproperty(r::OffsetRectangle, ::Val{:x}) =
    r.x_offset + r.rect.x

Base.getproperty(r::OffsetRectangle, ::Val{:y}) =
    r.y_offset + r.rect.y

Base.getproperty(r::OffsetRectangle, ::Val{:width}) =
    r.rect.width

Base.getproperty(r::OffsetRectangle, ::Val{:height}) =
    r.rect.height

@property_trampolines OffsetRectangle

@property_trampolines is necessary to define some additional methods:

using MacroTools

MacroTools.striplines(@macroexpand @property_trampolines OffsetRectangle)

quote
    (PropertyMethods.Base).getproperty(var"#15#o"::OffsetRectangle, var"#16#prop"::PropertyMethods.Symbol) = begin
            (PropertyMethods.Base).getproperty(var"#15#o", PropertyMethods.Val(var"#16#prop"))
        end
    ((PropertyMethods.Base).getproperty(var"#17#o"::OffsetRectangle, var"#18#prop"::PropertyMethods.Val{var"#19#T"}) where var"#19#T") = begin
            PropertyMethods.getfield(var"#17#o", var"#19#T")
        end
    (PropertyMethods.Base).propertynames(var"#20#o"::OffsetRectangle, var"#21#private"::PropertyMethods.Bool = false) = begin
            PropertyMethods.propertynames_from_val_methods(PropertyMethods.typeof(var"#20#o"), var"#21#private")
        end
    (PropertyMethods.Base).hasproperty(var"#22#o"::OffsetRectangle, var"#23#prop"::PropertyMethods.Symbol) = begin
            var"#23#prop" in PropertyMethods.propertynames_from_val_methods(PropertyMethods.typeof(var"#22#o"), true)
        end
end

rect1 = Rectangle(0, 1, 2, 4)

orect = OffsetRectangle(rect1, 2, 2)

for prop in propertynames(orect)
    println(prop, '\t', getproperty(orect, prop))
end

rect	Main.Rectangle(0, 1, 2, 4)
x_offset	2
y_offset	2
height	4
width	2
y	3
x	2

Some of the getproperty methods we defined just trampoline to the value of the OffsetRectangle's rect field. This is just boilerplate. We can do better:

MacroTools.striplines(@macroexpand @delegate Rectangle rect width height)

quote
    Base.getproperty(o::Rectangle, ::Val{:width}) = o.rect.width
    Base.getproperty(o::Rectangle, ::Val{:height}) = o.rect.height
end

So, instead of explicitly coding the OffsetRectangle methods for the width and height fields, we can write

@delegate Rectangle rect width height

orect.width

2

Index

• PropertyMethods.propertynames_from_val_methods
• PropertyMethods.@delegate
• PropertyMethods.@property_trampolines

Definitions

propertynames_from_val_methods(t::Type, private::Bool)

@delegate from_type to_field properties...

@property_trampolines MyStruct