How do I replace my KVO code with RAC3 and keep my existing models?

Question

I am looking into ReactiveCocoa to improve our Swift code. As a starting point, I would like to bind the text of a label to the transformed value of a property. Basically, I would like to replace some KVO code. So, I have the following variables:

@IBOutlet weak var myLabel: UILabel!
var myModel: MyModel

Since our models were developed in Objective-C, MyModel looks like this:

@interface MyModel : NSManagedObject
@property (nonatomic, retain) NSNumber * value;
@end

Thus, I would like to do something like this:

myLabel.text <~ myProperty.rac_signalForSelector("value")
    |> map { (value: NSNumber) in
        return "\(value.integerValue + 1)"
    }

However, this does obviously not work. How shall I approach the problem? And how is KVO generally realised with normal model properties?

I already found a similar post regarding this topic. The accepted answer there suggests to use ViewModels. However, instead of replacing my existing NSManagedObject models with ViewModels I want to keep my models since I like to manage them via XCode and store them via CoreData. Or is this somehow possible for ViewModels, as well? Am I missing anything crucial?

Answer 1

How about something like this (built with Xcode 6.4 using RAC 3.0.0):

/// Send changes in the property value of a source object to another property on a target object,
/// optionally using a mapping function to convert values as necessary.
func bind<S, T>(propertyWithKey targetKey: String, on target: NSObject, toPropertyWithKey sourceKey: String, on source:NSObject, usingValueConversion sourceToTargetValueConversion: ((S) -> T)? ) {

    var (source_signal, source_sink) = Signal<T, NSError>.pipe()

    var sourceSignalProducer = RACObserve(source, sourceKey).toSignalProducer()
        |> start(next: {value in
            if (value != nil) {
                if (sourceToTargetValueConversion == nil) {
                    sendNext(source_sink, value as! T)
                } else {
                    let mappedValue = sourceToTargetValueConversion!(value as! S)
                    sendNext(source_sink, mappedValue)
                }
            }
        })

    var convertedValueRACSignal = toRACSignal(
        source_signal
            |> map {(value: T) in value as! AnyObject}
    )

    convertedValueRACSignal ~> RAC(target, targetKey)
}

/// Send changes in the property value of a source object to another property on a target object.
/// No conversion of values or value types is performed between source and target.
func bind(propertyWithKey targetKey: String, on target: NSObject, toPropertyWithKey sourceKey: String, on source:NSObject) {

    var (source_signal, source_sink) = Signal<AnyObject, NSError>.pipe()

    var sourceSignalProducer = RACObserve(source, sourceKey).toSignalProducer()
        |> start(next: {value in
            if (value != nil) {
                sendNext(source_sink, value!)
            }
        })

    var convertedValueRACSignal = toRACSignal(source_signal)

    convertedValueRACSignal ~> RAC(target, targetKey)
}



// From Colin Eberhardt's post at http://blog.scottlogic.com/2014/07/24/mvvm-reactivecocoa-swift.html
// a struct that replaces the RAC macro
struct RAC  {
    var target : NSObject!
    var keyPath : String!
    var nilValue : AnyObject!

    init(_ target: NSObject!, _ keyPath: String, nilValue: AnyObject? = nil) {
        self.target = target
        self.keyPath = keyPath
        self.nilValue = nilValue
    }


    func assignSignal(signal : RACSignal) {
        signal.setKeyPath(self.keyPath, onObject: self.target, nilValue: self.nilValue)
    }
}

// From Colin Eberhardt's post at http://blog.scottlogic.com/2014/07/24/mvvm-reactivecocoa-swift.html
infix operator ~> {}
func ~> (signal: RACSignal, rac: RAC) {
    rac.assignSignal(signal)
}

// From Colin Eberhardt's post at http://blog.scottlogic.com/2014/07/24/mvvm-reactivecocoa-swift.html
func RACObserve(target: NSObject!, keyPath: String) -> RACSignal  {
    return target.rac_valuesForKeyPath(keyPath, observer: target)
}

In your example you'd call it:

bind(propertyWithKey: "text", on: myTextField, toPropertyWithKey: "value", on: myProperty)
            { (number: NSNumber) in
                return "\(number.integerValue)" as NSString
            }

I'm somewhat new to ReactiveCocoa, so the above is probably a naive implementation, but it may point you in the right direction.

Update This implementation of bind using channels is somewhat more compact and leverages RAC's built in KVO support:

func bind<S, T>(propertyWithKey targetKey: String, on target: NSObject, toPropertyWithKey sourceKey: String, on source:NSObject, usingValueConversion sourceToTargetValueConversion: ((S) -> T)? ) {

    var kvoChannelSource = RACKVOChannel(target: source, keyPath: sourceKey, nilValue: nil)
    var sourceSignalWithoutNils = kvoChannelSource.followingTerminal
        .filter { (var value:AnyObject?) -> Bool in
            return (value != nil)
        }

    var mappedSourceSignal = sourceSignalWithoutNils
    if (sourceToTargetValueConversion != nil)
    {
        mappedSourceSignal = sourceSignalWithoutNils.map { (var s: AnyObject!) -> AnyObject! in
            var result : T = sourceToTargetValueConversion!(s as! S)
            return result as! AnyObject
        }
    }

    var kvoChannelTarget = RACKVOChannel(target: target, keyPath: targetKey, nilValue: nil)
    mappedSourceSignal.subscribe(kvoChannelTarget.followingTerminal)
}