Introduce unbind methods based binding identifiers

[mac-cain13]

This is an implementation of #70 (and thus also of #53). Feedback is very welcome, I had some spare time to code a solution. You probably have feedback on what is where, comments, tests and coding conventions. Feel free to update the PR or add comments so I can make some changes.

(Or even take this as inspiration/a starting point to start your own implementation.)

Most important changes:

• bind returns an opaque TweakBindingIdentifier as a reference to that binding
• unbind can be used to unregister a bound closure
• Introduced MultiTweakBinding to keep logic for bindMultiple simple and consistent with bind
• bindMultiple returns MultiTweakBindingIdentifier that can be used with unbindMultiple

Other small changes:

• tweakBindings are now based on hashable AnyTweak instead of the "arbitrary" tweak.persistenceIdentifier
• Removed unused tweak property from TweakBinding
• Made binding private in TweakBinding

Usage:

let colorTintBinding = ExampleTweaks.bind(ExampleTweaks.colorTint) { button.tintColor = $0 }
// Some time later
ExampleTweaks.unbind(colorTintBinding)

[bryanjclark]

Thanks so much!! I'll review this in a bit. Your notes sound great - excited to peek under the hood here.

[bryanjclark]

Just did a review - this looks wonderful! Sorry for this taking foooooorever, you've probably moved on by now. My bad!

Two things I noticed:

• I wish there were a single struct, like an AnyTweakBindingIdentifier, that could represent a TweakBindingIdentifier or a MultiTweakBindingIdentifier -- though I'm still puzzling through how we could do that. It'd make it easier for adopters to hold on to a single Set<AnyTweakBindingIdentifier> to use in deinit later on -- but that can wait as an enhancement!
• Need to update the ViewController.swift in the example project (though I've done that, so that'll be incoming here!)

This looks great! I'll merge it in. Thank you!!

[mac-cain13]

Don't worry, I know how hard it is to make time to review PRs on Open Source projects. Thanks for merging it in. :)

Agree on your BindingIdentifier comment, that would be a nice improvement!

[DanielAsher]

Thanks for this!

In case anyone uses RxSwift, here is an extension to Tweak that allows integration with Observables

import RxSwift

extension Tweak {
    public var rx : Observable<T> {
        return Observable<T>.create { observer in
            
            let binding = YourTweaks.bind(self, binding: { (value: T) -> Void in
                observer.onNext(value)                
            })
            return Disposables.create {
                YourTweaks.unbind(identifier: binding)
            }
        }
    }
}

This can be used so:

       Observable.combineLatest(YourTweaks.textViewFontscale.rx, YourTweaks.textViewLineHeightMultiple.rx) { ($0, $1) }
            .subscribe(onNext: { [weak self] fontScale, lineHeightMultiple in
                guard let this = self else { return }
                let fontSize = this.originalFontSize * fontScale
                let font = UIFont.getCustomFont(size: fontSize)
                this.textView.font = font
                // Uses BonMot attributed string API
                this.baseStyle = StringStyle(.lineHeightMultiple(lineHeightMultiple), .adapt(.control), .font(font))
                let fullStyle = this.baseStyle.byAdding(stringStyle: this.focusStyle)
                this.textView.attributedText = presenter.text.styled(with: fullStyle) 
            })
            .addDisposableTo(disposeBag)