Hook Objective-C blocks with libffi. It's a powerful AOP tool for blocks. BlockHook can run your code before/instead/after invoking a block. BlockHook can even notify you when a block dealloc. You can trace the whole lifecycle of a block using BlockHook!

Want to hook blocks passing to methods? Try BlockTracker!

• Easy to use. Keep your code clear.
• Support 4 hook modes: Before, Instead, After and Dead.
• Let you modify return value and arguments.
• Support invoking original implementation.
• Remove hook at any time.
• Traverse all hook tokens of block.
• Provide block mangle name.
• Self-managed tokens.
• Support custom struct.
• Support Carthage & CocoaPods.

BlockHook needs libffi, which supports iOS, tvOS and macOS. You can run BlockHookSample iOS, BlockHookSample tvOS or BlockHookSample macOS target.

You can hook a block using 4 modes (before/instead/after/dead). This method returns a BHToken instance for more control. You can remove a BHToken, or set custom return value to its retValue property. Calling invokeOriginalBlock method will invoke original implementation of the block.

- (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.

This is an example for hooking block in all modes. You can change block return value from 8 to 15. Then remove some hook and check if it is successful. Finally we get callback when block dealloc.

NSObject *z = NSObject.new;
int(^block)(int x, int y) = ^int(int x, int y) {
    int result = x + y;
    NSLog(@"%d + %d = %d, z is a NSObject: %@", x, y, result, z);
    return result;
};
    
BHToken *token = [block block_hookWithMode:BlockHookModeDead|BlockHookModeBefore|BlockHookModeInstead|BlockHookModeAfter usingBlock:^(BHInvocation *invocation, int x, int y) {
    int ret = 0;
    [invocation getReturnValue:&ret];
    switch (invocation.mode) {
        case BlockHookModeBefore:
            // BHInvocation has to be the first arg.
            NSLog(@"hook before block! invocation:%@", invocation);
            break;
        case BlockHookModeInstead:
            [invocation invokeOriginalBlock];
            NSLog(@"let me see original result: %d", ret);
            // change the block imp and result
            ret = x * y;
            [invocation setReturnValue:&ret];
            NSLog(@"hook instead: '+' -> '*'");
            break;
        case BlockHookModeAfter:
            // print args and result
            NSLog(@"hook after block! %d * %d = %d", x, y, ret);
            break;
        case BlockHookModeDead:
            // BHInvocation is the only arg.
            NSLog(@"block dead! token:%@", invocation.token);
            break;
        default:
            break;
    }
}];
    
NSLog(@"hooked block");
int ret = block(3, 5);
NSLog(@"hooked result:%d", ret);
// remove token.
[token remove];
NSLog(@"remove tokens, original block");
ret = block(3, 5);
NSLog(@"original result:%d", ret);

Here is the log:

hooked block
hook before block! invocation:<BHInvocation: 0x60b00003c370>
3 + 5 = 8, z is a NSObject: <NSObject: 0x6020000279d0>
let me see original result: 0
hook instead: '+' -> '*'
hook after block! 3 * 5 = 15
hooked result:15
block dead! token:<BHToken: 0x60d000004bd0>
remove tokens, original block
3 + 5 = 8, z is a NSObject: <NSObject: 0x6020000279d0>
original result:8

Sometimes you want user login first before routing to other components. To intercept a block without hacking into code of routers, you can use block interceptor.

NSObject *testArg = [NSObject new];
NSObject *testArg1 = [NSObject new];
    
NSObject *(^testblock)(NSObject *) = ^(NSObject *a) {
    return [NSObject new];
};
    
[testblock block_interceptor:^(BHInvocation *invocation, IntercepterCompletion  _Nonnull completion) {
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSObject * __unsafe_unretained arg;
        [invocation getArgument:&arg atIndex:1];
        NSLog(@"Original argument:%@", arg);
        [invocation setArgument:(void *)&testArg1 atIndex:1];
        completion();
    });
}];
    
testblock(testArg);

CocoaPods is a dependency manager for Cocoa projects. You can install it with the following command:

$ gem install cocoapods

To integrate BlockHook into your Xcode project using CocoaPods, specify it in your Podfile:

source 'https://github.com/CocoaPods/Specs.git'
platform :ios, '8.0'
use_frameworks!
target 'MyApp' do
	pod 'BlockHook'
end

You need replace "MyApp" with your project's name.

Then, run the following command:

$ pod install

Carthage is a decentralized dependency manager that builds your dependencies and provides you with binary frameworks.

You can install Carthage with Homebrew using the following command:

$ brew update
$ brew install carthage

To integrate BlockHook into your Xcode project using Carthage, specify it in your Cartfile:

github "yulingtianxia/BlockHook"

Run carthage update to build the framework and drag the built BlockHook.framework into your Xcode project.

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

• If you need help or you'd like to ask a general question, open an issue.
• If you found a bug, open an issue.
• If you have a feature request, open an issue.
• If you want to contribute, submit a pull request.

• Hook Objective-C Block with Libffi
• BlockHook with Struct
• BlockHook with Revocation
• BlockHook with Private Data
• BlockHook with Invocation(1)
• BlockHook with Invocation(2)
• BlockHook and Memory Safety

yulingtianxia, yulingtianxia@gmail.com

BlockHook is available under the MIT license. See the LICENSE file for more info.

Thanks to MABlockClosure and Aspects!