EnumPoetry

1. ViewState.swift

// Sources/ViewState.swift
// Given an enum with some cases

protocol EnumPoetry {}

enum ViewState<T>: EnumPoetry {
    case start(T)
    case loading
    case completed(Int, String)
}

2. EnumPoetry.stencil

// Templates/EnumPoetry.stencil
// When run this stencil template code with:
// sourcery --sources Sources/ --templates Templates/ --output Sources/

{% macro associatedValueTypes associatedValues %}{% for associatedValue in associatedValues %}{% if forloop.first %}{{ associatedValue.typeName }}{% else %}, {{ associatedValue.typeName }}{% endif %}{% endfor %}{% endmacro %}
{% macro associatedValueNames associatedValues %}{% for associatedValue in case.associatedValues %}{% if forloop.first %}value{{forloop.counter}}{% else %}, value{{forloop.counter}}{% endif %}{% endfor %}{% endmacro %}

{% for enum in types.enums where enum.implements.EnumPoetry %}
{{ enum.accessLevel }} extension {{ enum.name }} {
    {% for case in enum.cases %}
    {% if case.hasAssociatedValue %}
    func on{{ case.name|capitalize }}(_ fn: ({% call associatedValueTypes case.associatedValues %}) -> Void) {
        guard case let .{{ case.name }}({% call associatedValueNames case.associatedValues %}) = self else { return }
        fn({% call associatedValueNames case.associatedValues %})
    }
    {% else %}
    func on{{ case.name|capitalize }}(_ fn: () -> Void) {
        guard case .{{ case.name }} = self else { return }
        fn()
    }
    {% endif %}
    
    {% endfor %}
    
}
{% endfor %}

3. EnumPoetry.generated.swift

// Sources/EnumPoetry.generated.swift
// Then you will get this

internal extension ViewState {
    func onStart(_ fn: (T) -> Void) {
        guard case let .start(value1) = self else { return }
        fn(value1)
    }
    func onLoading(_ fn: () -> Void) {
        guard case .loading = self else { return }
        fn()
    }
    func onCompleted(_ fn: (Int, String) -> Void) {
        guard case let .completed(value1, value2) = self else { return }
        fn(value1, value2)
    }
}

4. UsingState.swift

// Sources/UsingState.swift
// And you can use this way:

func handle(state: ViewState<Int>) {
    state.onStart { number in
        print("start: \(number)")
    }

    state.onLoading { print("loading: ∞") }

    state.onCompleted { (number, text) in
        print("completed: \(number) \(text)")
    }
}