Extend DFImageManaging API

DFProxyImageManager

// DFProxyImageManager.h

#import "DFImageManaging.h"
#import "DFImageRequest.h"
#import <Foundation/Foundation.h>

/*! The DFProxyRequestTransforming protocol defines an interface for transforming application-specific classes or protocols to the resources supported by the DFImageManager.
 */
@protocol DFProxyRequestTransforming <NSObject>

/*! Returns the result of transforming a given request.
 @param request The copy of the original request, users can modify passed request.
 */
- (nonnull DFImageRequest *)transformedRequest:(nonnull DFImageRequest *)request;

@end


/*! The DFProxyImageManager is used to modify image requests before they are received by the actual image manager. For example, the users can use it to transform application-specific classes or protocols to the resources supported by the DFImageManager.
 */
@interface DFProxyImageManager : NSProxy <DFImageManaging>

/*! Image manager that the receiver was initialized with.
 */
@property (nonnull, nonatomic) id<DFImageManaging> imageManager;

/*! Initializes proxy with the image manager.
 */
- (nonnull instancetype)initWithImageManager:(nonnull id<DFImageManaging>)imageManager;

/*! Sets the request transformer.
 */
@property (nullable, nonatomic) id<DFProxyRequestTransforming> transformer;

/*! Sets request transformer with a given block. Overwrites transformer value.
 */
- (void)setRequestTransformerWithBlock:(DFImageRequest *__nonnull (^__nullable)(DFImageRequest *__nonnull request))block;

@end


// DFProxyImageManager.m

#import "DFProxyImageManager.h"
#import "DFImageRequest.h"
#import "DFImageTask.h"

@interface _DFProxyRequestTransformer : NSObject <DFProxyRequestTransforming>
@end

@implementation _DFProxyRequestTransformer {
    DFImageRequest *(^_block)(DFImageRequest *);
}

- (instancetype)initWithBlock:(DFImageRequest * __nonnull (^ __nonnull)(DFImageRequest * __nonnull))block {
    if (self = [super init]) {
        _block = [block copy];
    }
    return self;
}

- (nonnull DFImageRequest *)transformedRequest:(nonnull DFImageRequest *)request {
    return _block(request);
}

@end


#define _DF_TRANSFORMED_REQUEST(request) (_transformer ? [_transformer transformedRequest:request] : (request))

@implementation DFProxyImageManager

- (nonnull instancetype)initWithImageManager:(nonnull id<DFImageManaging>)imageManager {
    _imageManager = imageManager;
    return self;
}

- (void)forwardInvocation:(NSInvocation *)anInvocation {
    [anInvocation invokeWithTarget:_imageManager];
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    return [(NSObject *)_imageManager methodSignatureForSelector:aSelector];
}

- (void)setRequestTransformerWithBlock:(DFImageRequest * __nonnull (^ __nullable)(DFImageRequest * __nonnull))block {
    self.transformer = [[_DFProxyRequestTransformer alloc] initWithBlock:block];
}

#pragma mark <DFImageManaging>

- (BOOL)canHandleRequest:(nonnull DFImageRequest *)request {
    return [_imageManager canHandleRequest:_DF_TRANSFORMED_REQUEST(request)];
}

- (nonnull DFImageTask *)imageTaskForResource:(nonnull id)resource completion:(nullable DFImageTaskCompletion)completion {
    return [self imageTaskForRequest:[DFImageRequest requestWithResource:resource] completion:completion];
}

- (nonnull DFImageTask *)imageTaskForRequest:(nonnull DFImageRequest *)request completion:(nullable DFImageTaskCompletion)completion {
    return [_imageManager imageTaskForRequest:_DF_TRANSFORMED_REQUEST(request) completion:completion];
}

- (void)startPreheatingImagesForRequests:(nonnull NSArray *)requests {
    [_imageManager startPreheatingImagesForRequests:[self _transformedRequests:requests]];
}

- (void)stopPreheatingImagesForRequests:(nonnull NSArray *)requests {
    [_imageManager stopPreheatingImagesForRequests:[self _transformedRequests:requests]];
}

- (nonnull NSArray *)_transformedRequests:(nonnull NSArray *)requests {
    NSMutableArray *transformedRequests = [NSMutableArray new];
    for (DFImageRequest *request in requests) {
        [transformedRequests addObject:_DF_TRANSFORMED_REQUEST(request)];
    }
    return [transformedRequests copy];
}

@end