Add dynamic deserialization of AbstractTypes

[gryumov]

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• Did you add a description to the Pull Request?
• Did you add new tests?
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using Serde

abstract type AbstractFoo end

struct Foo1 <: AbstractFoo
    name::String
    num::Int64
end

struct Foo2 <: AbstractFoo
    name::String
    str::String
end

Serde.ClassType(::Type{AbstractFoo}) = Serde.AbstractType()

#___

Serde.subtype_key(::Type{AbstractFoo}) = :name
Serde.subtypes(::Type{AbstractFoo}) = InteractiveUtils.subtypes(AbstractFoo)

#___

h1 = " {\"name\": \"Foo1\" ,\"num\":100}      "
h2 = " {\"name\": \"Foo2\" ,\"str\":\"test\"} "
h3 = " {\"name\": \"Foo2\" ,\"str\":\"test\"} "
h4 = " {\"name\": \"Foo2\" ,\"num\":100}      "

julia> deser_json(AbstractFoo, h1)
Foo1("Foo1", 100)

julia> deser_json(AbstractFoo, h2)
Foo2("Foo2", "test")

julia> @time deser_json(AbstractFoo, h1)
  0.010999 seconds (1.28 k allocations: 1.197 MiB)
Foo1("Foo1", 100)

julia> @time deser_json(Foo1, h1)
  0.000033 seconds (14 allocations: 824 bytes)
Foo1("Foo1", 100)

julia> @time InteractiveUtils.subtypes(AbstractFoo)
  0.013692 seconds (1.26 k allocations: 1.196 MiB)
2-element Vector{Any}:
 Foo1
 Foo2

Serde.subtypes(::Type{AbstractFoo}) = (Foo1, Foo2)

julia> @time deser_json(AbstractFoo, h1)
  0.000050 seconds (14 allocations: 816 bytes)
Foo1("Foo1", 100)

julia> @time deser_json(AbstractFoo, h2)
  0.000041 seconds (16 allocations: 864 bytes)
Foo2("Foo2", "test")

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