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