Replacing "performSelector" with a closure

In some code that I am porting to Swift I used the operation "performSelector". This is no longer available and has to be replaced by a closure. In this article I want to present an example of how I did this.

In Objective-C I had a piece of code that would call an operation on an array of objects that contained strings. Each string-containing object would report back if a change occurred. And each object that reported a change had to be reported in a notification.

This is the Objective-C code:

-(void)forAllObjectsDo:(SEL)method {

    // The line numbers that are send with the notification (if any)

    NSMutableArray* arr = [[NSMutableArray alloc] init];

    

    // For all objects do

    for (int i=0; i<[datamodel count]; i++) {

        

        // Get the object to process

        BRObject* obj = [datamodel objAtIndex:i];

        

        // Does the object respond to the given selector?

        if ([obj respondsToSelector:method]) {

            

            // Supress the "leak" warning, we don't want unneccessary warnings.

            #pragma clang diagnostic push

            #pragma clang diagnostic ignored "-Warc-performSelector-leaks"

            

            // The object implements the selector, call the selector and

            // check if the object was changed

            if ([obj performSelector:method]) {

                

                // Something changed.

                // Add this line to de notification info.

                [arr addObject:[NSNumber numberWithInt:i]];

            }

            

            // Restore the old diagnostics

            #pragma clang diagnostic pop

            

        }

    }

    

    // If there are changes, post the notification

    if ([arr count] > 0) {

        // Generate and send a notification

        NSDictionary* dict = [[NSDictionary alloc] initWithObjectsAndKeys:arr, UPDATED_OBJECTS_KEY, nil];

        [[NSNotificationCenter defaultCenter] postNotificationName:OBJECTS_UPDATED_NOTIFICATION

                                                            object:self

                                                          userInfo:dict];

    }

}

Even though I had used closures before, I still was at a loss how to convert the above code until I had some inspiration earlier this week. This is what I came up with:

    private func forAllObjectsDo(closure: (Int) -> Bool) {

        var arr: Array<Int> = []

        for i in 0 ..< dataModel.objects.count {

            if closure(i) { arr.append(i) }

        }

        fireViewUpdatedNotificationIfNotEmpty(arr)

    }

    

    private func fireViewUpdatedNotificationIfNotEmpty(arr: Array<Int>) {

        if arr.count > 0 {

            let dict = [UPDATED_OBJECTS_KEY: arr]

            NSNotificationCenter.defaultCenter().postNotificationName(OBJECT_UPDATED_NOTIFICATION, object:self, userInfo:dict)

        }

    }

The Objective-C routine was called as follows:

[self forAllObjectsDo:@selector(deleteHalf)];

The Swift code is called like this:

forAllObjectsDo({ [unowned self] (i) in return self.datamodel.objAtIndex(i).deleteHalf() })

All in al the new way is much better looking than the old way. And once I grokked the usage of closures, it seems silly that it was so hard to do before. The key was to realise that the old code cannot be replaced 1 to 1 with new code. As you can see above the separation is slightly different. In Objective-C I could iterate over the objects and call the selector, in the Swift code I have to include the selection of the object to call into the closure.

Happy coding...

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