UIAppearance proxy for custom objects

Question

I have a custom object, it inherits from NSObject. This object does "some things", one of it is creating an UIView with some UIKit objects (UILabel, UIButtons ecc ecc...). This object has some properties like: textColor, font, backgroundColor... that are used to customize the appearance of the contained UIKit objects.

I would like to customize this properties "one shot" for all created instances of this object, and I've looked at the UIAppearance protocol.

Standard UIKit objects already conforms to UIAppearance protocol, but I don't want to apply the style on ALL UILabels or UIButtons. I want to apply styles only to the UILabels and UIButtons contained inside my object instances. Moreover, I can't (and I don't want) use appearanceWhenContainedIn: because the developer using my custom object may not know what kind of objects are "contained" inside it.

So, I was looking at how to make my custom object conforms to UIAppearance protocol.

AFAIK it must implement the

+ (id)appearance

method. This method should return a proxy object where you can send all your customizations. But, looking at the appearance method of UIKit objects, I see that a private object is returned. An object of class _UIAppearance.

So, it seems that Apple doesn't give me a standard proxy object for customizing my own, and I have to create if from scratch. Is it right or I'm loosing something?

Thanks

Answer 1

After some reserach I "give up" about using a standard Apple object. It doesn't exists, for now. I've created my own proxy, it's quite simple (works only with "appearance:" by now).

Let's explain it. I want to set the appearance of "textColor" on a NSObject subclass, let's call it "FLObject". Make FLObject conforms to UIAppearance protocol and override the appearance method. In this method, you should return a proxy class (the one I created):

+ (id)appearance
{
    return [FLAppearance appearanceForClass:[self class]];
}

How it works? FLAppearance creates a single instance of itself for each class passed by the appearanceForClass: method. If you call it two times for the same class, the same instance is returned.

Then, you can do something like this:

[[FLObject appearance] setTextColor:[UIColor redColor]]; 

FLAppearance overrides the forwardInvocation: method, so it accepts all methods sent. Then, it puts all invocations in an array. When FLObject is initialized, a simple call to

[(FLAppearance *)[FLAppearance appearanceForClass:[self class]] startForwarding:self];

will start to send invocations and set the appearance. Sure, this needs some tuning and error checking, but I think it's a good start.

@interface FLAppearance ()

@property (strong, nonatomic) Class mainClass;
@property (strong, nonatomic) NSMutableArray *invocations;

@end

static NSMutableDictionary *dictionaryOfClasses = nil;

@implementation FLAppearance

// this method return the same object instance for each different class
+ (id) appearanceForClass:(Class)thisClass
{
    // create the dictionary if not exists
    // use a dispatch to avoid problems in case of concurrent calls
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        if (!dictionaryOfClasses)
            dictionaryOfClasses = [[NSMutableDictionary alloc]init];
    });



    if (![dictionaryOfClasses objectForKey:NSStringFromClass(thisClass)])
    {
        id thisAppearance = [[self alloc]initWithClass:thisClass];
        [dictionaryOfClasses setObject:thisAppearance forKey:NSStringFromClass(thisClass)];
        return thisAppearance;
    }
    else
        return [dictionaryOfClasses objectForKey:NSStringFromClass(thisClass)];
}

- (id)initWithClass:(Class)thisClass
{
    self = [self initPrivate];
    if (self) {
        self.mainClass = thisClass;
        self.invocations = [NSMutableArray array];
    }
    return self;
}

- (id)init
{
    [NSException exceptionWithName:@"InvalidOperation" reason:@"Cannot invoke init. Use appearanceForClass: method" userInfo:nil];
    return nil;
}

- (id)initPrivate
{
    if (self = [super init]) {

    }
    return self;
}

-(void)forwardInvocation:(NSInvocation *)anInvocation;
{
    // tell the invocation to retain arguments
    [anInvocation retainArguments];

    // add the invocation to the array
    [self.invocations addObject:anInvocation];
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    return [self.mainClass instanceMethodSignatureForSelector:aSelector];
}

-(void)startForwarding:(id)sender
{
    for (NSInvocation *invocation in self.invocations) {
        [invocation setTarget:sender];
        [invocation invoke];
    }
}

Answer 2

For the purposes of my own project, I collect everything together and released custom UIApperance proxy as open source project MZApperance

Answer 3

Nice implementation, I slightly modified the code and created the class as a subclass of NSProxy. Using it in a project I found a memory leak:

For example: using the proxy to set global settings/appearance, each instance of that class will never reach refCount 0, so dealloc will never be called.

Leak code:

-(void)forwardInvocation:(NSInvocation *)anInvocation;
{
    [...]

    // !! This will retain also the target

    [anInvocation retainArguments];

    [...]
}

Fix:

-(void)forwardInvocation:(NSInvocation *)anInvocation
{
     [anInvocation setTarget:nil];
     [anInvocation retainArguments];

     // add the invocation to the array
     [self.invocations addObject:anInvocation];
}

-(void)startForwarding:(id)sender
{
     for (NSInvocation *invocation in self.invocations) {

         // Create a new copy of the stored invocation,
         // otherwise setting the new target, this will never be released
         // because the invocation in the array is still alive after the call

         NSInvocation *targetInvocation = [invocation copy];
         [targetInvocation setTarget:sender];
         [targetInvocation invoke];
         targetInvocation = nil;
     }
}

Copy category for NSInvocation

-(id)copy
{
     NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:[self methodSignature]];
     NSUInteger numberOfArguments = [[self methodSignature] numberOfArguments];

     [invocation setTarget:self.target];
     [invocation setSelector:self.selector];

     if (numberOfArguments > 2) {
         for (int i = 0; i < (numberOfArguments - 2); i++) {
             char buffer[sizeof(intmax_t)];
             [self getArgument:(void *)&buffer atIndex:i + 2];
             [invocation setArgument:(void *)&buffer atIndex:i + 2];
         }
     }

     return invocation;
}

Answer 4

Check out http://logicalthought.co/blog/2012/10/8/uiappearance-and-custom-views

Basically you just need to tag your properties with UI_APPEARANCE_SELECTOR and everything works as long as your class is a subclass of UIView which will handle the actual vending of the private _UIAppearance class.

---

Edit:

You're probably better off just rolling your own solution using a singleton and some class methods rather than attempting to do something scary with the runtime. It doesn't look like UIAppearance supports your use case.

On the other hand, you could stick each object you vend in a private UIView subclass and then vend instances of that subclass instead. Then you can forward appearance messages sent to your NSObject to the instances you vend and use appearanceWhenContainedIn:<your private subclass>. That could get messy though and could be confusing for consumers of your class.