RxSwiftMapViewDelegateProxyExample

This is a follow up to the Swift Meetup on RxSwift (#SLUG)

A lot of people asked me how to encapsulate delegate implementations into Rx nicely. In other languages, you're usually forced to implement an entire class but Swift allows us to add extensions!

This example app shows how to migrate from Delegate implementations to Rx Observables<T> using MKMapView and MKMapViewDelegate

We will be turning func mapView(_ mapView: MKMapView, regionDidChangeAnimated animated: Bool) method into an Observable<Bool> property

In addition we'll add a simple var rx_centerDidChange : Observable<CLLocationCoordinate2D> extension property. This will fire when the center changes.

We'll out put some data to the console as well as a UILabel right up top.

The Example Source Code is Complete, but here are the steps if you want to recreate the magical experience

Step 1 - Proxy Setup

Create a DelegateProxy class for MKMapView. This is just an Object that inherits DelegateProxy, MKMapViewDelegate and DelegateProxyType

import RxSwift
import RxCocoa
classRxMKMapViewDelegateProxy:DelegateProxy,MKMapViewDelegate,DelegateProxyType{
//We need a way to read the current delegate classfunc currentDelegateFor(object:AnyObject)->AnyObject?{letmapView:MKMapView= object as!MKMapViewreturn mapView.delegate
}
//We need a way to set the current delegate classfunc setCurrentDelegate(delegate:AnyObject?, toObject object:AnyObject){letmapView:MKMapView= object as!MKMapView
mapView.delegate = delegate as!MKMapViewDelegate}}

Step 2 - Create the Extension Methods

Okay now let's create an Extension for MKMapView and add the methods that we want.

1. We'll need a property of rx_delegate which is of the type we made above.
2. Create methods that listen to the delegates respective selectors

To translate func mapView(_ mapView: MKMapView, regionDidChangeAnimated animated: Bool)

We will need to convert this function's selector into its string format. Here it is:

"mapView:regionDidChangeAnimated:"

The delegate proxy we made has a method called: rx_observe which takes a selector string like above and returns an Array<Any?> This array contains the parameters passed down from that method. You will have to cast it appropriately based off of the documentation.

The full extension code is below:

extensionMKMapView{publicvarrx_delegate:DelegateProxy{returnproxyForObject(self)asRxMKMapViewDelegateProxy}publicvarrx_regionDidChangeAnimated:Observable<Bool>{return rx_delegate.observe("mapView:regionDidChangeAnimated:").map{ params inreturnparams[1]as!Bool}}publicvarrx_centerDidChange:Observable<CLLocationCoordinate2D>{return rx_regionDidChangeAnimated.map({(animated)->CLLocationCoordinate2Dinreturnself.centerCoordinate
})}}

Step 3 - Listen for the Changes

Now you don't have to implement the delegate! Just observe the property right on the spot.

varmapView=MKMapView();
mapView.rx_centerDidChange
.subscribeNext{ newCenterCoord in
// so do something with it already
}

Going forward

I hope this example shows you how you can leverage extensions to encapsulate Delegate listeners into consumable RxSwift Methods.

Follow this pattern and enjoy your bliss!