Enum plus plus

[pietrocaselani]

This PR adds Sourcery/Stencil templates to give Swift enum more powers!
Now we have the protocol EnumClosures, that when implemented in an enum will create closures for each enum case.

Using the following enum as example:

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

you can implement EnumClosures, by doing extension ViewState: EnumClosures, and Sourcery will generate the following code:

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

There is also the EnumProperties that will generate accessors for each case. So using the same enum, ViewState, you can extend ViewState to use EnumProperties by doing extension ViewState: EnumProperties which will generate the following code:

extension ViewState {
    internal var isStart: Bool {
        guard case .start = self else { return false }
        return true
    }
    internal var isLoading: Bool {
        guard case .loading = self else { return false }
        return true
    }
    internal var isCompleted: Bool {
        guard case .completed = self else { return false }
        return true
    }

    internal var start: T? {
        guard case let .start(data) = self else { return nil }
        return (data)
    }
    internal var completed: (count: Int, message: String)? {
        guard case let .completed(count, message) = self else { return nil }
        return (count, message)
    }
}

And by doing that, we are now able to write such code:

func render(viewState: ViewState<User>) {
    viewState.onLoading {
        // show loading indicator
    }

    viewState.onStart { user in
        // show the user
    }

    viewState.onCompleted { (count, message) in
        // show the cout and message
    }

    // Or

    view.loading = viewState.isLoading
    view.user = viewState.start
    view.countAndMessage = viewState.completed
}

[codecov-io]

Codecov Report

Merging #199 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #199   +/-   ##
=======================================
  Coverage   34.54%   34.54%           
=======================================
  Files         292      292           
  Lines        7292     7292           
=======================================
  Hits         2519     2519           
  Misses       4773     4773

Impacted Files	Coverage Δ
...ackerCore/MovieDetails/MovieDetailsViewState.swift	40% <ø> (ø)	⬆️

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