Aspects源码分析

Posted 2021-12-02 FarmGuo

Aspects是一个用来切片编程的开源框架，提供了丰富接口，可以Hook类和单个对象的方法，并提供了原实现前Hook，替换原实现，原实现后Hook等选项。

1 补充知识点

Objective-C里面的方法调用，我们看到的第1个参数，实际上是第3个参数，前2个参数是隐形参数，第1个是self（encode后是@），即实例对象或者类对象，第2个是Selector（encode后是#），即方法名。
Block有些类似，我们看到的第1个参数，实际上是第2个参数，前1个参数是隐形参数，即Block自身，encode后是@?。

2 简单流程

不管是Hook类的还是Hook某个对象的，基本流程都是将原SEL指向objc_msgForward，使其走消息转发的流程，然后将原实现保存在aliseSEL中。然后将forwardInnovation的实现设为自定义的函数，如果原来有forwardInnovation的实现，则将其保存在AspectsForwardInvocationSelectorName这个SEL中。

Aspects做了很多其他的操作来保证安全性和稳定性。

1 先判断方法是否能被Hook

这部分位于aspect_isSelectorAllowedAndTrack中

static BOOL aspect_isSelectorAllowedAndTrack(NSObject *self, SEL selector, AspectOptions options, NSError **error) {
    static NSSet *disallowedSelectorList;
    static dispatch_once_t pred;
    dispatch_once(&pred, ^{
        disallowedSelectorList = [NSSet setWithObjects:@"retain", @"release", @"autorelease", @"forwardInvocation:", nil];
    });

    // Check against the blacklist.
    NSString *selectorName = NSStringFromSelector(selector);
    if ([disallowedSelectorList containsObject:selectorName]) {
        NSString *errorDescription = [NSString stringWithFormat:@"Selector %@ is blacklisted.", selectorName];
        AspectError(AspectErrorSelectorBlacklisted, errorDescription);
        return NO;
    }

    // Additional checks.
    AspectOptions position = options&AspectPositionFilter;
    if ([selectorName isEqualToString:@"dealloc"] && position != AspectPositionBefore) {
        NSString *errorDesc = @"AspectPositionBefore is the only valid position when hooking dealloc.";
        AspectError(AspectErrorSelectorDeallocPosition, errorDesc);
        return NO;
    }

    if (![self respondsToSelector:selector] && ![self.class instancesRespondToSelector:selector]) {
        NSString *errorDesc = [NSString stringWithFormat:@"Unable to find selector -[%@ %@].", NSStringFromClass(self.class), selectorName];
        AspectError(AspectErrorDoesNotRespondToSelector, errorDesc);
        return NO;
    }

    // Search for the current class and the class hierarchy IF we are modifying a class object
    if (class_isMetaClass(object_getClass(self))) {
        Class klass = [self class];
        NSMutableDictionary *swizzledClassesDict = aspect_getSwizzledClassesDict();
        Class currentClass = [self class];
        do {
            AspectTracker *tracker = swizzledClassesDict[currentClass];
            if ([tracker.selectorNames containsObject:selectorName]) {

                // Find the topmost class for the log.
                if (tracker.parentEntry) {
                    AspectTracker *topmostEntry = tracker.parentEntry;
                    while (topmostEntry.parentEntry) {
                        topmostEntry = topmostEntry.parentEntry;
                    }
                    NSString *errorDescription = [NSString stringWithFormat:@"Error: %@ already hooked in %@. A method can only be hooked once per class hierarchy.", selectorName, NSStringFromClass(topmostEntry.trackedClass)];
                    AspectError(AspectErrorSelectorAlreadyHookedInClassHierarchy, errorDescription);
                    return NO;
                }else if (klass == currentClass) {
                    // Already modified and topmost!
                    return YES;
                }
            }
        }while ((currentClass = class_getSuperclass(currentClass)));

        // Add the selector as being modified.
        currentClass = klass;
        AspectTracker *parentTracker = nil;
        do {
            AspectTracker *tracker = swizzledClassesDict[currentClass];
            if (!tracker) {
                tracker = [[AspectTracker alloc] initWithTrackedClass:currentClass parent:parentTracker];
                swizzledClassesDict[(id<NSCopying>)currentClass] = tracker;
            }
            [tracker.selectorNames addObject:selectorName];
            // All superclasses get marked as having a subclass that is modified.
            parentTracker = tracker;
        }while ((currentClass = class_getSuperclass(currentClass)));
    }

    return YES;
}
首先就是判断retain、release、autorelease等等这些禁止hook的方法。
对于dealloc则只能替换或后Hook。
对于不响应的方法不Hook。
最后是判断是否已经Hook过了该方法。但个人认为这里有点瑕疵，会产生一个问题，当2个分别继承自NSObject的类有着相同的方法名，Hook了其中一个类的方法后，另一个类的就不能Hook了。

2 替换后的Block和原实现的参数是否匹配

这部分判断位于aspect_isCompatibleBlockSignature中，
首先Block是一种特殊的结构体，首个变量是和其他对象一样都是isa，这使得Block也可以响应方法的调用。在Block结构体的最后保存着Block的签名，这个签名和方法的签名是一样的。参数的匹配就是比较2个签名的参数个数和参数类型的匹配。

static BOOL aspect_isCompatibleBlockSignature(NSMethodSignature *blockSignature, id object, SEL selector, NSError **error) {
    BOOL signaturesMatch = YES;
    NSMethodSignature *methodSignature = [[object class] instanceMethodSignatureForSelector:selector];
    if (blockSignature.numberOfArguments > methodSignature.numberOfArguments) {
        signaturesMatch = NO;
    }else {
        if (blockSignature.numberOfArguments > 1) {
            const char *blockType = [blockSignature getArgumentTypeAtIndex:1];
            if (blockType[0] != '@') {
                signaturesMatch = NO;
            }
        }
        // Argument 0 is self/block, argument 1 is SEL or id<AspectInfo>. We start comparing at argument 2.
        // The block can have less arguments than the method, that's ok.
        if (signaturesMatch) {
            for (NSUInteger idx = 2; idx < blockSignature.numberOfArguments; idx++) {
                const char *methodType = [methodSignature getArgumentTypeAtIndex:idx];
                const char *blockType = [blockSignature getArgumentTypeAtIndex:idx];
                // Only compare parameter, not the optional type data.
                if (!methodType || !blockType || methodType[0] != blockType[0]) {
                    signaturesMatch = NO; break;
                }
            }
        }
    }

    if (!signaturesMatch) {
        NSString *description = [NSString stringWithFormat:@"Blog signature %@ doesn't match %@.", blockSignature, methodSignature];
        AspectError(AspectErrorIncompatibleBlockSignature, description);
        return NO;
    }
    return YES;
}

3 进行Hook

对于类的Hook走的是简单流程，但对于对象的Hook，因为为了不影响其他的对象，处理流程上有些不同。
以要Hook的实例对象的类为父类，新生成一个子类，然后将这个子类的forwardInvocation实现改为自定义的函数，同时替换这个子类的-class和+class方法，让其都返回要Hook的实例对象的类。然后修改将要Hook的实例对象的isa，将其isa指向这个子类。然后再将这个子类的原方法的实现改为objc_msgSend，并将原实现保存在aliseSEL中。总结起来就是Hook类和Hook某个对象的方法类似，不同的是，Hook对象需要额外生成一个子类，而后所有的修改操作都在这个子类上进行。