gistfile1.m

#import "NSURLSession+Swizzle.h"
#import <objc/runtime.h>

@implementation NSURLSession (Swizzle)

+ (void)load{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Class class = self.class;

        SEL originalSelector = @selector(downloadTaskWithURL:);
        SEL swizzledSelector = @selector(xxx_downloadTaskWithURL:);

        Method originalMethod = class_getInstanceMethod([NSURLSession sharedSession].class, originalSelector)
        Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);

        IMP swizzledImp = method_getImplementation(swizzledMethod);
        IMP originalImp = method_getImplementation(originalMethod);

        BOOL didAddMethod =
        class_addMethod(class,
                        originalSelector,
                        swizzledImp,
                        method_getTypeEncoding(swizzledMethod));

        if (didAddMethod) {
            class_replaceMethod(class,
                                swizzledSelector,
                                originalImp,
                                method_getTypeEncoding(originalMethod));
        } else {
            method_exchangeImplementations(originalMethod, swizzledMethod);
        }

    });
}

- (NSURLSessionDownloadTask *)xxx_downloadTaskWithURL:(NSURL *)url
{
    NSLog(@"%s", __PRETTY_FUNCTION__);

    return [self xxx_downloadTaskWithURL:url];
}

@end