Investigate having support for union

[williambailey]

Following up my own question in #9. I'm currently trying to think of the cleanest way to add union support to Graphiti.

This PR enables union support by way of a marker protocol. Usage is something like the following (trying to stick to the StarWars example)...

// define union marker protocol
protocol DroidAndPlanetUnion {}

// associate other members of the union
extension Droid: DroidAndPlanetUnion {}
extension Planet: DroidAndPlanetUnion {}

// building the unions
let starWarsSchema = try! Schema<NoRoot, NoContext> { schema in
    // ...
    try schema.union(type: DroidAndPlanetUnion.self) { union in
        union.description = "an unholy union of droids and planets"
        union.types = [ Droid.self, Planet.self ]
    }
    // alternatively if you don't need a description.
    try schema.union(type: DroidAndPlanetUnion.self, members: [ Droid.self, Planet.self ])
    // ...
}

I wasn't able to figure out a way to validate that the types provided in union.types implement the DroidAndPlanetUnion protocol.

Tests etc. can follow should we agree on the way forward.

[williambailey]

Build failure is related to needing to first declare the init function on GraphQLUnionType public in the GraphQL package.

[paulofaria]

have you thought about any other option? I think we could go with that if we're not able to come up with something else.

[williambailey]

I tried using [Type.Type] here (and in the union<Type> function members arguments) in an attempt to make it a bit more type safe however I was then unable to actually produce any code that would compile without errors like... Cannot convert value of type ... to expected argument type '[_.Type]'

[williambailey]

I toyed with using enums to define the union but this resulted in more complication both with the implementation and on the call side. The marker protocol approach seemed the most elegant. Open to other suggestions however.

[paulofaria]

CI is not passing due to some ACL issues.. can you please fix it? 😊

[paulofaria]

By the way, merged and tagged GraphQL PR. Let's see if this alone fixes it.

[paulofaria]

NICE™. CI is passing. think it's ready to merge?

[williambailey]

I think so. 😀

[codecov-io]

Codecov Report

Merging #10 into master will increase coverage by 0.64%.
The diff coverage is 91.37%.

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
+ Coverage   82.69%   83.34%   +0.64%     
==========================================
  Files          13       13              
  Lines        1745     1855     +110     
==========================================
+ Hits         1443     1546     +103     
- Misses        302      309       +7

Impacted Files	Coverage Δ
...s/GraphitiTests/StarWarsTests/StarWarsSchema.swift	80% <ø> (ø)	⬆️
...aphitiTests/StarWarsTests/StarWarsQueryTests.swift	96.6% <100%> (+0.21%)	⬆️
...sts/StarWarsTests/StarWarsIntrospectionTests.swift	97.66% <100%> (+0.1%)	⬆️
...sts/GraphitiTests/StarWarsTests/StarWarsData.swift	85% <100%> (+9.32%)	⬆️
Sources/Graphiti/Schema/Schema.swift	59.02% <74.35%> (+1.67%)	⬆️

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