技术教程 编程教程 iOS 开源库系列 Aspects核心源码分析---面向切面编程之疯狂的 Aspects 正文

iOS 开源库系列 Aspects核心源码分析---面向切面编程之疯狂的 Aspects

2023-03-09,,

Aspects的源码学习，我学到的有几下几点

    Objective-C Runtime
    理解OC的消息分发机制
    KVO中的指针交换技术
    Block 在内存中的数据结构
    const 的修饰区别
    block 中常量在特定情况下的三种处理方法
    断言语句，
    自旋锁 使用注意

    _objc_msgForward_stret 和 _objc_msgForward 前者存在的必要

    Type Encoding

https://www.cnblogs.com/DafaRan/p/8192069.html

简介

Aspects是一个面向切面编程的库。
如果想深入了解iOS Runtime中的消息发送机制，Aspects的源码是值得分析的。

项目主页
Aspects

整体分析

阅读Aspects的源码需要以下知识作为基础

    Objective-C Runtime
    理解OC的消息分发机制
    KVO中的指针交换技术

阅读本文之前，建议应该先断点调试下Aspects的Demo，了解大致的过程。

核心实现

Aspects的核心实现就是利用Runtime中的消息分发机制如图：

Aspects通过把selector的方法替换为msg_forward方法转发 转而调用 forwardInvocation（forwardInvocation的实现被Aspects替换，将原来的方法实现与添加的实现组合在了一起）

核心源码分析

这是Aspects 面向切面编程的入口方法

- (id<AspectToken>)aspect_hookSelector:(SEL)selector

                      withOptions:(AspectOptions)options

                       usingBlock:(id)block

                            error:(NSError **)error {

    return aspect_add(self, selector, options, block, error);

}复制代码

这段代码可以分三部分来看

    aspect_isSelectorAllowedAndTrack 这个方法 对父子类同时hook一个方法进行了一些限制
    aspect_getContainerForObject 通过Runtime添加关联值的方式 管理hook的方法
    aspect_prepareClassAndHookSelector 这是核心的实现，涉及到动态生成子类，改变isa指针的指向，改变方法的实现 一系列操作
static id aspect_add(id self, SEL selector, AspectOptions options, id block, NSError **error) {

    NSCParameterAssert(self);

    NSCParameterAssert(selector);

    NSCParameterAssert(block);

    __block AspectIdentifier *identifier = nil;

    aspect_performLocked(^{

        if (aspect_isSelectorAllowedAndTrack(self, selector, options, error)) {

            //一个实例 只有一个container

            //这是区分实例对象和类对象的关键

            //实例对象可以有很多个，但是同一个类的类对象只能有一个

            AspectsContainer *aspectContainer = aspect_getContainerForObject(self, selector);

            //原来的selector block

            identifier = [AspectIdentifier identifierWithSelector:selector object:self options:options block:block error:error];

            if (identifier) {

                //container 里 存有 identifier (selector,block)

                [aspectContainer addAspect:identifier withOptions:options];

                // Modify the class to allow message interception.

                aspect_prepareClassAndHookSelector(self, selector, error);

            }

        }

    });

    return identifier;

}复制代码

核心方法

aspect_prepareClassAndHookSelector这是核心的实现，涉及到动态生成子类，改变isa指针，改变方法的实现 一系列操作

static void aspect_prepareClassAndHookSelector(NSObject *self, SEL selector, NSError **error) {

    NSCParameterAssert(selector);

    //动态创建子类，改变forwardInvocation方法的实现

    Class klass = aspect_hookClass(self, error);

    Method targetMethod = class_getInstanceMethod(klass, selector);

    IMP targetMethodIMP = method_getImplementation(targetMethod);

    if (!aspect_isMsgForwardIMP(targetMethodIMP)) {

        // Make a method alias for the existing method implementation, it not already copied.

        const char *typeEncoding = method_getTypeEncoding(targetMethod);

        SEL aliasSelector = aspect_aliasForSelector(selector);

        if (![klass instancesRespondToSelector:aliasSelector]) {

            //子类的aliasSelector的实现为 当前类的selector

            __unused BOOL addedAlias = class_addMethod(klass, aliasSelector, method_getImplementation(targetMethod), typeEncoding);

            NSCAssert(addedAlias, @"Original implementation for %@ is already copied to %@ on %@", NSStringFromSelector(selector), NSStringFromSelector(aliasSelector), klass);

        }

        //selector方法替换为_objc_msgForward

        // We use forwardInvocation to hook in.

        class_replaceMethod(klass, selector, aspect_getMsgForwardIMP(self, selector), typeEncoding);

        AspectLog(@"Aspects: Installed hook for -[%@ %@].", klass, NSStringFromSelector(selector));

    }

}复制代码

动态生成子类，改变isa指针

#pragma mark - Hook Class

static Class aspect_hookClass(NSObject *self, NSError **error) {

    NSCParameterAssert(self);

    //这里可以思考一下 class 方法 和 isa 的区别

    //[self class] KVO可能改变了isa指针的指向

    Class statedClass = self.class;

    // object_getClass 能准确的找到isa指针

    Class baseClass = object_getClass(self);

    NSString *className = NSStringFromClass(baseClass);

    // Already subclassed

    //如果已经子类化了 就返回

    if ([className hasSuffix:AspectsSubclassSuffix]) {

        return baseClass;

        //如果是类 就改掉类的forwardInvocation 而不是一个子类对象

        // We swizzle a class object, not a single object.

    }else if (class_isMetaClass(baseClass)) {

        return aspect_swizzleClassInPlace((Class)self);

        //考虑到KVO,KVO的底层实现,交换了isa指针

        // Probably a KVO'ed class. Swizzle in place. Also swizzle meta classes in place.

    }else if (statedClass != baseClass) {

        return aspect_swizzleClassInPlace(baseClass);

    }

    // Default case. Create dynamic subclass.

    const char *subclassName = [className stringByAppendingString:AspectsSubclassSuffix].UTF8String;

    Class subclass = objc_getClass(subclassName);

    if (subclass == nil) {

        // 通过创建新子类的方式

        subclass = objc_allocateClassPair(baseClass, subclassName, 0);

        if (subclass == nil) {

            NSString *errrorDesc = [NSString stringWithFormat:@"objc_allocateClassPair failed to allocate class %s.", subclassName];

            AspectError(AspectErrorFailedToAllocateClassPair, errrorDesc);

            return nil;

        }

        // forwardInvocation 替换成 (IMP)_ASPECTS_ARE_BEING_CALLED__

        aspect_swizzleForwardInvocation(subclass);

        //子类的class方法返回当前被hook的对象的class

        aspect_hookedGetClass(subclass, statedClass);

        aspect_hookedGetClass(object_getClass(subclass), statedClass);

        objc_registerClassPair(subclass);

    }

    //将当前self设置为子类，这里其实只是更改了self的isa指针而已, 这里hook了子类的forwardInvocation方法，再次使用当前类时，其实是使用了子类的forwardInvocation方法。

    object_setClass(self, subclass);

    return subclass;

}复制代码

作者：Junyiii
链接：https://juejin.im/post/58f6d1675c497d006ca638fe
来源：掘金
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。
 
 

Aspects初始化工作核心部分的解析

aspect_add方法:

 

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static id aspect_add(id self, SEL selector, AspectOptions options, id block, NSError **error) {
    NSCParameterAssert(self);
    NSCParameterAssert(selector);
    NSCParameterAssert(block);
    //声明AspectIdentifier实例
    __block AspectIdentifier *identifier = nil;
    aspect_performLocked(^{
        //判断当前XX方法是否允许被Hook,1."retain"、"release"、"autorelease"、"forwardInvocation"这几个方法是不被允许的，所谓的黑名单。2.如果方法是dealloc,则他的切入点必须是Before。3.判断当前实例对象和类对象是否能响应方法4.是否是类对象，如果是则判断继承体系中方法是否已经被Hook,而实例则不用。
        if (aspect_isSelectorAllowedAndTrack(self, selector, options, error)) {
            //获得当前aspects__XX方法的AspectsContainer容器
            AspectsContainer *aspectContainer = aspect_getContainerForObject(self, selector);
            //初始化AspectIdentifier变量,方法内部通过toll-free bridged获取Block方法签名，并判断其兼容性
            identifier = [AspectIdentifier identifierWithSelector:selector object:self options:options block:block error:error];
            if (identifier) {
                //通过options选项分别添加到容器中的beforeAspects,insteadAspects,afterAspects这三个数组
                [aspectContainer addAspect:identifier withOptions:options];
 
                //HookSelector的过程和HookClass的过程
                aspect_prepareClassAndHookSelector(self, selector, error);
            }
        }
    });
    return identifier;
}

HookClass过程:

 
 

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static Class aspect_hookClass(NSObject *self, NSError **error) {
    NSCParameterAssert(self);
    Class statedClass = self.class;
    Class baseClass = object_getClass(self);
    NSString *className = NSStringFromClass(baseClass);
 
    // 如果类名有_Aspects_前缀，说明Class已被Hook
    if ([className hasSuffix:AspectsSubclassSuffix]) {
        return baseClass;
 
        // 判断是否为类对象，如果是，则直接在当前类中进行swizzle
    }else if (class_isMetaClass(baseClass)) {
        return aspect_swizzleClassInPlace((Class)self);
        // 判断是否为KVO过的对象，因为KVO的对象ISA指针会指向一个中间类，则直接在这个中间类中进行swizzle
    }else if (statedClass != baseClass) {
        return aspect_swizzleClassInPlace(baseClass);
    }
 
    // 默认则会动态创建一个子类
    const char *subclassName = [className stringByAppendingString:AspectsSubclassSuffix].UTF8String;
    Class subclass = objc_getClass(subclassName);
 
    if (subclass == nil) {
        subclass = objc_allocateClassPair(baseClass, subclassName, 0);
        if (subclass == nil) {
            NSString *errrorDesc = [NSString stringWithFormat:@"objc_allocateClassPair failed to allocate class %s.", subclassName];
            AspectError(AspectErrorFailedToAllocateClassPair, errrorDesc);
            return nil;
        }
        //实现替换当前类forwardInvocation方法的实现为__ASPECTS_ARE_BEING_CALLED__
        aspect_swizzleForwardInvocation(subclass);
        //实现当前类的isa指针指向原生的类
        aspect_hookedGetClass(subclass, statedClass);
        //实现当前类的元类的isa指针指向原生的类
        aspect_hookedGetClass(object_getClass(subclass), statedClass);
        //注册当前类
        objc_registerClassPair(subclass);
    }
    //将当前对象的isa指针指向刚生成的类
    object_setClass(self, subclass);
    return subclass;
}

HookSelector过程

 
 

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static void aspect_prepareClassAndHookSelector(NSObject *self, SEL selector, NSError **error) {
    NSCParameterAssert(selector);
    //HookClass过程
    Class klass = aspect_hookClass(self, error);
    //此时的klass类为刚创建的具有_Aspects_后缀的子类，在创建的时候指定类他的父类，所以我们可以获取到selector这个方法
    Method targetMethod = class_getInstanceMethod(klass, selector);
    IMP targetMethodIMP = method_getImplementation(targetMethod);
    //判断是否为消息转发
    if (!aspect_isMsgForwardIMP(targetMethodIMP)) {
        //获得原生方法的类型编码
        const char *typeEncoding = method_getTypeEncoding(targetMethod);
        SEL aliasSelector = aspect_aliasForSelector(selector);
        if (![klass instancesRespondToSelector:aliasSelector]) {
            //为klass添加aspects__XX方法，方法的实现为原生方法的实现。
            __unused BOOL addedAlias = class_addMethod(klass, aliasSelector, method_getImplementation(targetMethod), typeEncoding);
            NSCAssert(addedAlias, @"Original implementation for %@ is already copied to %@ on %@", NSStringFromSelector(selector), NSStringFromSelector(aliasSelector), klass);
        }
        // 将原生方法实现替换为_objc_msgForward或_objc_msgForward_stret，用来实现消息转发
        class_replaceMethod(klass, selector, aspect_getMsgForwardIMP(self, selector), typeEncoding);
        AspectLog(@"Aspects: Installed hook for -[%@ %@].", klass, NSStringFromSelector(selector));
    }
}

Aspects执行工作核心部分的解析

当我们正式向某个接受者发送消息的时候,会进行消息转发，而之前HookClass的过程当中我们已经对forwardInvocation的实现替换为了__ASPECTS_ARE_BEING_CALLED__

 
 

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static void __ASPECTS_ARE_BEING_CALLED__(__unsafe_unretained NSObject *self, SEL selector, NSInvocation *invocation) {
    NSCParameterAssert(self);
    NSCParameterAssert(invocation);
    //获取原始方法XX
    SEL originalSelector = invocation.selector;
    //获取含有前缀的方法aspects_XX
    SEL aliasSelector = aspect_aliasForSelector(invocation.selector);
    //替换Sel
    invocation.selector = aliasSelector;
    //获得实例对象容器
    AspectsContainer *objectContainer = objc_getAssociatedObject(self, aliasSelector);
    //获得类对象容器
    AspectsContainer *classContainer = aspect_getContainerForClass(object_getClass(self), aliasSelector);
    //初始化AspectInfo，传入self、invocation参数
    AspectInfo *info = [[AspectInfo alloc] initWithInstance:self invocation:invocation];
    NSArray *aspectsToRemove = nil;
 
    // 执行before切入点的调用
    aspect_invoke(classContainer.beforeAspects, info);
    aspect_invoke(objectContainer.beforeAspects, info);
 
    // 执行Instead切入点的调用，判断当前insteadAspects是否有数据，如果没有数据则判断当前继承链是否能响应aspects_XX方法,如果能，则直接调用,此时的aspects_XX则为原生的实现。
    BOOL respondsToAlias = YES;
    if (objectContainer.insteadAspects.count || classContainer.insteadAspects.count) {
        aspect_invoke(classContainer.insteadAspects, info);
        aspect_invoke(objectContainer.insteadAspects, info);
    }else {
        Class klass = object_getClass(invocation.target);
        do {
            if ((respondsToAlias = [klass instancesRespondToSelector:aliasSelector])) {
                [invocation invoke];
                break;
            }
        }while (!respondsToAlias & (klass = class_getSuperclass(klass)));
    }
 
    // 执行after切入点的调用
    aspect_invoke(classContainer.afterAspects, info);
    aspect_invoke(objectContainer.afterAspects, info);
 
    // 若Hook未被正确执行，则调用原生消息转发。
    if (!respondsToAlias) {
        invocation.selector = originalSelector;
        SEL originalForwardInvocationSEL = NSSelectorFromString(AspectsForwardInvocationSelectorName);
        if ([self respondsToSelector:originalForwardInvocationSEL]) {
            ((void( *)(id, SEL, NSInvocation *))objc_msgSend)(self, originalForwardInvocationSEL, invocation);
        }else {
            [self doesNotRecognizeSelector:invocation.selector];
        }
    }
 
    // 对需要被移除的切面执行remove方法
    [aspectsToRemove makeObjectsPerformSelector:@selector(remove)];
}

执行block的代码

 
 

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- (BOOL)invokeWithInfo:(id)info {
    //根据blockSignature获取Invocation
    NSInvocation *blockInvocation = [NSInvocation invocationWithMethodSignature:self.blockSignature];
    //获取原生方法的Invocation
    NSInvocation *originalInvocation = info.originalInvocation;
    //获取blockInvocation参数个数
    NSUInteger numberOfArguments = self.blockSignature.numberOfArguments;
 
    // 判断blockInvocation参数个数是否大于originalInvocation参数个数
    if (numberOfArguments > originalInvocation.methodSignature.numberOfArguments) {
        AspectLogError(@"Block has too many arguments. Not calling %@", info);
        return NO;
    }
 
    // blockInvocation给索引为1的参数赋值
    if (numberOfArguments > 1) {
        [blockInvocation setArgument:&info atIndex:1];
    }
    //当所以大于1的时候进行遍历，把原生的参数值赋值给相应的blockInvocation中的参数
    void *argBuf = NULL;
    for (NSUInteger idx = 2; idx

流程图:

http://ios.jobbole.com/84522/

iOS 开源库系列 Aspects核心源码分析---面向切面编程之疯狂的 Aspects的相关教程结束。

《iOS 开源库系列 Aspects核心源码分析---面向切面编程之疯狂的 Aspects.doc》

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