baiyuqiu / BlockHook

Hook Objective-C block with libffi. (Developing)

BlockHook

Hook Objective-C block with libffi. (Developing!!!) Thanks to MABlockClosure and Aspects!

Article

Hook Objective-C Block with Libffi

Usage

You can hook a block using 3 modes (before/instead/after). This method returns a BHToken instance for more control. You can remove a BHToken, or set custom return value to its retValue property.

- (BHToken *)block_hookWithMode:(BlockHookMode)mode
                     usingBlock:(id)block

BlockHook is easy to use. Its APIs take example by Aspects. Here is a full set of usage of BlockHook.

int (^block)(int, int) = ^(int x, int y) {
   int result = x + y;
   NSLog(@"I'm here! result: %d", result);
   return result;
};
    
BHToken *tokenInstead = [block block_hookWithMode:BlockHookModeInstead usingBlock:^(BHToken *token, int x, int y){
   // change the block imp and result
   *(int *)(token.retValue) = x * y;
   NSLog(@"hook instead");
}];
    
BHToken *tokenAfter = [block block_hookWithMode:BlockHookModeAfter usingBlock:^(BHToken *token, int x, int y){
   // print args and result
   NSLog(@"hook after block! x:%d y:%d ret:%d", x, y, *(int *)(token.retValue));
}];

BHToken *tokenBefore = [block block_hookWithMode:BlockHookModeBefore usingBlock:^(id token){
   // BHToken has to be the first arg.
   NSLog(@"hook before block! token:%@", token);
}];
    
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
   NSLog(@"hooked block");
   int ret = block(3, 5);
   NSLog(@"hooked result:%d", ret);
   // remove all tokens when you don't need.
   // reversed order of hook.
   [tokenBefore remove];
   [tokenAfter remove];
   [tokenInstead remove];
   NSLog(@"original block");
   ret = block(3, 5);
   NSLog(@"original result:%d", ret);
});

Here is the log:

hooked block
hook before block! token:<BHToken: 0x1d00e0f80>
hook instead
hook after block! x:3 y:5 ret:15
hooked result:15
original block
I'm here! result: 8
original result:8

Installation

BlockHook needs libffi : https://github.com/libffi/libffi You must build libffi for every architecture you need. The sample project "BlockHookSample" just support arm64.

After importing libffi, just add the two files BlockHook.h/m to your project.