__attribute__((noescape)) In Swift

Using Swift
1

Is there a way to add a __attribute__((noescape)) to a parameter in Swift? I'm running into an issue with an Objective-C method that requires it.

In a large mixed project, I'm overriding UITableView.performBatchUpdates(_:completion) in a UITableView subclass in Swift.:

class MyCustomTableView: UITableView {
   // how to annotate the updates parameter as NS_NOESCAPE?
   public override func performBatchUpdates(_ updates: (() -> Void)?, completion: ((Bool) -> Void)? = nil) {
        // do some bookkeeping
       super.performBatchUpdates(updates, completion: completion)
   } 
}

This subclass is also used in the Objective-C parts of the project, so it get's added to the Project-Swift.h header and Swift automatically converts it to the following:

@interface MyCustomTableView : UITableView
- (void)performBatchUpdates:(void (^ _Nullable)(void))updates completion:(void (^ _Nullable)(BOOL))completion;
@end

However, Objective-C requires that this method is annotation with NS_NOESCAPE ( = __attribute__((noescape))).

I don't see a direct way to add this in Swift? How do I solve this?

2

Swiss has this limitation: you can't have a closure non-escaping and optional at the same time, Optional<()->Void> is always escaping. With non optional closures (not your case) you just use @escaping to make the closure escaping and by default the closure is non escaping.

Are you getting some compilation error or what?

3

We're getting warnings (which we also cannot turn off) in the Project-Swift.h generated header file, because Objective-C requires an NS_NOESCAPE.

4

If you don't fear missing it in other possibly important cases, you may switch that warning off by adding -Wno-missing-noescape to Xcode's "Other Warning Flags".

5

Yes we don’t wanna do that. So there’s no swift solution for this?

6

Not sure, possibly no pure Swift solution.

As you are using Obj-C anyway, as a workaround you may create an obj-c wrapper:

Wrong code 
@interface UITableView(Extras)
- (void)performBatchUpdates2:(void (^ _Nullable)(void))updates completion:(void (^ _Nullable)(BOOL finished))completion;
@end

@implementation UITableView(Extras)
- (void)performBatchUpdates2:(void (^ _Nullable)(void))updates completion:(void (^ _Nullable)(BOOL finished))completion {
    [self performBatchUpdates:updates completion:completion];
}
@end

and override performBatchUpdates2 instead of performBatchUpdates from Swift. You'll get no warning in this case.

Edit: no, that won't work, scratch that.

7

This will work, a sketch:

Obj-C:

@interface ObjcTableView: UITableView
-(void)performBatchUpdates2: ...
@end

@implementation ObjcTableView
-(void)performBatchUpdates: ... {
    [self performBatchUpdates2: ...];
}
-(void)performBatchUpdates2: ... {
    [super performBatchUpdates: ...];
}
@end

Swift:

class MyCustomTableView: ObjcTableView {
    override func performBatchUpdates2(...) {
        ....
    }
}

Or just live with the warning!

1 Like