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Swift Usage Tips

Background

Today was a rough day.

Keep moving forward.

Back to mixed-language development. Don't think that just because you're familiar with OC and UIKit, skimming through Swift syntax is enough for a smooth transition — it took me quite a while to fill in the gaps. Anyone who has never actually written Swift would think it's simple. Compared to OC, Swift has a more concise syntax and borrows the best ideas from many popular languages. Optional binding and guard caught me off guard at first, but I'm eager to start using them.

Calling OC Code

You need to create a bridging header file ProjectName-Bridging-Header.h, then update the Objective-C Bridging Header setting under Build Settings in the target to ProjectName/ProjectName-Bridging-Header.h.

#ifndef SwiftTipsDemo_Bridging_Header_h
#define SwiftTipsDemo_Bridging_Header_h

#import "OCViewController.h"

#endif /* SwiftTipsDemo_Bridging_Header_h */

Since the entire project is within the same namespace, you can use OCViewController directly in Swift.

navigationController?.pushViewController(OCViewController(), animated: true)

Importing Third-Party OC Libraries from CocoaPods

Using RAC as an example: after adding pod 'ReactiveCocoa', '4.0.1' to your Podfile and running an update, add the import to your bridging header. If you only add #import "ReactiveCocoa-Bridging-Header.h", you'll get a "header file not found" error.

#ifndef SwiftTipsDemo_Bridging_Header_h
#define SwiftTipsDemo_Bridging_Header_h

#import <ReactiveCocoa/ReactiveCocoa-Bridging-Header.h>

#endif /* SwiftTipsDemo_Bridging_Header_h */

The existing codebase used Xibs, so newly created Swift files needed to follow the same pattern. In general, this is how you do it: when creating a new OC file, check the box to create an associated Xib, and then you're good to go.

MyViewController *my = [[MyViewController alloc] init];
[self.navigationController pushViewController:my animated:YES];

In Swift, if you check the box to create a Xib when creating a new Swift file, using the following code will actually only load an empty controller. That is, the system does not automatically load a Xib file with the same name as the controller.

navigationController?.pushViewController(Example2ViewController(), animated: true)

To load it this way, you need to write a convenience initializer in Swift:

convenience init() {
   let nibNameOrNil = String?("SwiftViewController")
   self.init(nibName: nibNameOrNil, bundle: nil)
}

Swift's Dynamic Dispatch

While answering a question on stackoverflow, I came across an issue involving using RACObserve in Controller0 (OC) to observe property changes in a ViewModel (Swift). I knew that RACObserve uses KVO under the hood, and for a Swift object to support KVO it must inherit from NSObject — so I gave an answer in English. But the poster had indeed inherited from NSObject. I pulled out my computer late at night to test it, and sure enough, it didn't work. Another person had answered that the property must be marked with dynamic. Adding dynamic fixed the issue. My answer wasn't accepted, but it was a good reminder to dig deeper into how Swift handles dynamic dispatch.

Code:

All code from this article can be found on my GitHub: SwiftTipsDemo.