Using method swizzling and blocks to test Class methods in Objective-C.

ExampleTest.m

#import "SenTestCase+MethodSwizzling.m"

@interface ExampleTest : SenTestCase {}
+ (BOOL)trueMethod;
+ (BOOL)falseMethod;
@end

@implementation ExampleTest

+ (BOOL)trueMethod  { return YES; }
+ (BOOL)falseMethod { return NO; }

- (void)testMethodSwizzlingShouldCallAlternativeImplementation {
    [self swizzleMethod:@selector(trueMethod)  inClass:[self class]
             withMethod:@selector(falseMethod) fromClass:[self class]
           executeBlock:^{
                assertThatBool([[self class] trueMethod], isEqualToBool(NO));
            }]
}

@end

SenTestCase+MethodSwizzling.m

#include <objc/runtime.h>

@interface SenTestCase (MethodSwizzling)
- (void)swizzleMethod:(SEL)aOriginalMethod
              inClass:(Class)aOriginalClass
           withMethod:(SEL)aNewMethod
            fromClass:(Class)aNewClass
         executeBlock:(void (^)(void))aBlock;
@end

@implementation SenTestCase (MethodSwizzling)
- (void)swizzleMethod:(SEL)aOriginalMethod
              inClass:(Class)aOriginalClass
           withMethod:(SEL)aNewMethod
            fromClass:(Class)aNewClass
         executeBlock:(void (^)(void))aBlock {
    Method originalMethod = class_getClassMethod(aOriginalClass, aOriginalMethod);
    Method mockMethod = class_getInstanceMethod(aNewClass, aNewMethod);
    method_exchangeImplementations(originalMethod, mockMethod);
    aBlock();
    method_exchangeImplementations(mockMethod, originalMethod);
}
@end

Hi there,

cool trick, thanks for sharing. The example given should not work unless you extract the target method as
Method mockMethod = class_getClassMethod(aNewClass, aNewMethod);
If it's working then there is some trouble going on with equality of BOOLs. To test it, try to
[NSException raise:@"DEBUG" format:@"##### %@", mockMethod];
right after line 18. It should be null.

Thx reborg, that helped a lot!

As a newbie to objc, this gist is really going to make my life easier. Thanks!

the problem with this is if the test case you put into the block fails, you will not "deswizzle" the method. this is likely to mess up your other test cases. that's why i prefer to do the swizzling and cleanup stuff in the -setUp and -tearDown...