Blocks/Closures

Blocks are another way that Objective C performs callbacks. Flash and Swift uses this technique as well, where they are called closures. Basically this involves passing in a function/method/message to the child, whilst maintaining the focus of the parent. So it would be a method that the child can call, and local variables within the closure are based on local variables where the closure is defined. I find it makes your code hard to read, (especially in Objective C where the syntax can be a struggle to get right too!) but it’s still a valid technique.
One caveat – ‘this’ in an ActionScript closure is actually assumed to be in the global scope.

ActionScript3:

Parent.as (Document class)

package  {
	import flash.display.MovieClip;
	
	public class Parent extends MovieClip {
		private var string:String;
		public function Parent() {
			string="I am Parent";
// function closure
			var parentMethod:Function=function():String {  
				return(string);
			}  
		
			var child:Child=new Child();
			child.runThis(parentMethod);
		}

	}
	
}

Child.as

package  {
	public class Child {
		private var string:String;
		public function Child() {
			string="I am Child";
		}
		public function runThis(functionToRun:Function):void {
			functionToRun();
		}
	}
	
}
//traces I am Parent

Objective C:

In Parent.m

    NSString *string = @"I am Parent";
    NSString * (^parentMethod)() =
    ^ NSString * () {
        return string;
    };
    Child *child = [[Child alloc] init];
    NSLog(@"%@", [child runThis:parentMethod]);

In Child.m

-(NSString *)runThis:(NSString *(^)())blockToRun {
    NSString *string = @"I am Child";
    return(blockToRun());
}
//NSLogs I am Parent

Swift:

The following works well in playgrounds:

import Cocoa
var parent:Parent=Parent()
parent.go()  //Note that this outputs I am Parent
class Parent {
    var string="I am Parent"
    var child = Child()
    func parentMethod()->NSString {
        return string
    }
    func go()->NSString {
        return child.runThis(parentMethod)
    }
}
class Child {
    var string="I am Child"
    func runThis(closure: () -> NSString)->NSString {
        return closure()
    }
}

About craiggrummitt

iOS development with Swift - book: https://manning.com/books/ios-development-with-swift video course: https://www.manning.com/livevideo/ios-development-with-swift-lv