Resource manager rudiments

ResourceManager.h

#define FILE(file) \
    ([[NSBundle mainBundle] pathForResource:file \
                                     ofType:nil])
#define FILE_IN_DIR(file, dir) \
    ([[NSBundle mainBundle] pathForResource:file \
                                     ofType:nil \
                                inDirectory:dir])

ResourceManager.m

#import "ResourceManager.h"
#import <objc/runtime.h>

void swizzle(id target, SEL origSel_, SEL altSel_);

@implementation NSBundle(ResourceManagerExtension)

+ (void)initialize {
    swizzle(self,
            @selector(pathForResource:ofType:inDirectory:),
            @selector(resourcemanager_pathForResource:ofType:inDirectory:));
}

- (NSString *)pathForResource:(NSString *)name ofType:(NSString *)ext {
    return [self pathForResource:name ofType:ext inDirectory:nil];
}

- (NSString *)resourcemanager_pathForResource:(NSString *)name ofType:(NSString *)ext inDirectory:(NSString *)subpath
    // First, we look in ~/Library/AltBundle/, then in ~/Library/ExtResources/.
    // If the subpath is not found there, we call the original implementation
    // of 'pathForResource:ofType:inDirectory:'
{
    NSString *filename =
        ([ext length]
         ? [name stringByAppendingFormat:@".%@", ext]
         : name);
    NSArray *paths =
        [NSArray arrayWithObjects:
         [NSString pathWithComponents:
          [NSArray arrayWithObjects:
           [self bundlePath], @"Library", @"AltBundle", subpath, filename, nil]],
         [NSString pathWithComponents:
          [NSArray arrayWithObjects:
           [self bundlePath], @"Library", @"ExtResources", subpath, filename, nil]],
         nil];

    NSFileManager *fm = [NSFileManager defaultManager];
    for (NSString *path in paths) {
        if ([fm fileExistsAtPath:path])
            return path;
    }
    return [self resourcemanager_pathForResource:name ofType:ext inDirectory:subpath];
}

@end


void swizzle(id target, SEL origSel_, SEL altSel_)
    // This is an adapted version of the code from https://github.com/rentzsch/jrswizzle
{
    Method origMethod = class_getInstanceMethod(target, origSel_);
    if (!origMethod)
        return;

    Method altMethod = class_getInstanceMethod(target, altSel_);
    if (!altMethod)
        return;

    class_addMethod(target,
                origSel_,
                class_getMethodImplementation(target, origSel_),
                method_getTypeEncoding(origMethod));
    class_addMethod(target,
                altSel_,
                class_getMethodImplementation(target, altSel_),
                method_getTypeEncoding(altMethod));

    method_exchangeImplementations(class_getInstanceMethod(target, origSel_), class_getInstanceMethod(target, altSel_));
}