AppDelegate Class not Available to Obj-C Embedded Library in iOS Swift App

Question

I have a library written in Objective C, published as a Cocoapod. Part of what the library does is some swizzling of the AppDelegate class of any app that it is added to. When the library is added to a simple Objective-C app (a single-page app with nothing in it), the callback methods are swizzled properly and everyone is happy. However, when the library is added to a similar Swift app, the swizzling fails because the AppDelegate is null (or nil).

Here is a peek at the code within the library:

(Note: this method is called from the UIResponder's load: method.)

UIResponder+MyCustomMethods.m

/**
 *  This class swizzles the methods needed to observe protocol methods
 */
@implementation UIResponder (WTAutomatics)

...

+ (void)swizzleAppDelegate:(SEL)original with:(SEL)replacement forProtocol:(Protocol *)protocol
{
    id appDel = [[UIApplication sharedApplication] delegate];
    Class appDelegate = [appDel class];
    // Alternatively, we can do this. Both work the same.
    // Class appDelegate = [[[UIApplication sharedApplication] delegate] class];

    NSLog(@"Starting swizzle: %@", NSStringFromSelector(original));    
    if (!appDelegate) {
        WTLog(@"Failed to swizzle because appDelegate is null.");
        return;
    }

    if (class_conformsToProtocol(appDelegate, protocol)) {
        // Do the method swizzling
        ...
    }
}

In the code above, the value of appDelegate is valid and the swizzling works in any Objective C app. However, appDelegate is null when this is run within a Swift app and the swizzle fails.

Is there some difference in the order of initialization of the application delegate between the two languages? Is there something else I'm missing?

Answer 1

I'm not certain how this is working for you in the Obj-C application case. In my test, the +load is called before the application's main() method, which is where UIApplication is created and initialized. In my test, the following prints (null) for both lines:

@interface UIResponder (blah)
@end
@implementation UIResponder (blah)
+ (void) load
{
    NSLog( @"application: %@", [UIApplication sharedApplication] );
    NSLog( @"delegate:    %@", [[UIApplication sharedApplication] delegate] );
}
@end

Here's a version that does what you want:

@interface UIResponder (blah)
@end
@implementation UIResponder (blah)
+ (void) load
{
    [[NSNotificationCenter defaultCenter] addObserver: self
                                             selector: @selector(didFinishLaunching:) name: UIApplicationDidFinishLaunchingNotification object: nil];

}
+ (void) didFinishLaunching: (NSNotification*) n
{
    NSLog( @"application: %@", [UIApplication sharedApplication] );
    NSLog( @"delegate:    %@", [[UIApplication sharedApplication] delegate] );
}
@end

Furthermore, I don't think it is advisable to implement +load in a category extension because you don't know if that is overriding some +load method defined on the class itself. I believe when that is the case it is undefined which one is called? I'd have to check. A better solution might be to create your own UIResponder-derived class and put your +load shim there.

Answer 2

Apple Docs: +load Invoked whenever a class or category is added to the Objective-C runtime; implement this method to perform class-specific behavior upon loading.

It looks like your AppDelegate isn't loaded yet. Try to call your swizzling from another place. E.g.

+(void)initialize{
[self swizzle...];

}