What is it?

castaway is a simple library to add an as language construct to Objective-C.

NSObject* obj = /* get an object */;

obj.as(^(id<Drawable> drawable) {
    [drawable draw];
});

The typical way to do this without castaway is:

NSObject* obj = /* get an object */;

if ([obj conformsToProtocol:@protocol(Drawable)]) {
    [(id<Drawable>)obj draw];
}

It's verbose, repetitive, and the compiler isn't checking that you cast to the same protocol you checked for.

castaway makes it dead simple to write concise and declarative code that's safe.

castaway also includes a match construct to write safe type-dependent code for multiple cases.

id result = /* get a result */;

result.match(@[
    ^(NSArray* manyThings) {
        for (id thing in manyThings) { /* ... */ }
    },
    ^(NSString* str) {
        NSLog(@"The string: %@", str);
    },
    ^(UIView* view) {
        [view removeFromSuperview];
    },
    ^(id somethingElse) {
        NSLog(@"wasn't expecting: %@", somethingElse);
    }
]);

The match construct returns the result of the executed block that matched.

NSString* description = obj.match(@[
    ^(NSArray* manyThings) {
        return [NSString stringWithFormat:@"%d things", manyThings.count];
    },
    ^(NSString* str) {
        return @"one string";
    },
    ^(id somethingElse) {
        return @"one random thing";
    }
]);

If obj is an NSArray of 3 items, description is "3 things". If obj is a string, it's "one string". Otherwise it's "one random thing".

Installation

Then, you just need to import the castaway header:

#import <castaway/castaway.h>

Calling as on id types

In the above examples, we used as and match with dot-syntax. You're not able to use property syntax on id typed variables, so you should use the message versions of as and match.

id obj = /* get an object */;

[obj as:^(id<Drawable> drawable) {
    [drawable draw];
}]; 

Author

Dustin Bachrach, [email protected]

Special thanks to PromiseKit by Max Howell for NSMethodSignatureForBlock.

License

castaway is available under the MIT license. See the LICENSE file for more info.