Aspect Oriented Programming(AOP),面向切面编程,是一个比较热门的话题。AOP主要实现的目的是针对业务处理过程中的切面进行提取,它所面对的是处理过程中的某个步骤或阶段,以获得逻辑过程中各部分之间低耦合性的隔离效果。
主要用来处理一些具有横切性质的系统性服务,如日志记录、权限管理、缓存、对象池管理等,AOP 已经成为一种非常常用的解决方案。
比如说我们在实现权限验证的时候,需要在每个业务的执行前对权限进行相应的判断,从而导致了大量的重复代码,不利于模块的复用。AOP则通过将每个业务的公共行为进行抽离,封装成一个可复用的模块,这个模块就叫做『切面』。
Aspects是一个轻量级的面向切面编程的库。它主要提供了三个切入点:before(在原始的方法前执行)/instead(替换原始的方法执行)/after(在原始的方法后执行,默认),通过Runtime消息转发实现Hook,同时这也会带来一定的负担,所以它不适合循环多次调用的方法。
Aspects扩展了NSObject类,对外提供如下两个方法:
///为指定的类的当前方法的before/instead/after切入点添加block代码+ (id)aspect_hookSelector:(SEL)selector withOptions:(AspectOptions)options usingBlock:(id)block error:(NSError **)error;///为指定实例的当前方法的before/instead/after切入点添加block代码- (id)aspect_hookSelector:(SEL)selector withOptions:(AspectOptions)options usingBlock:(id)block error:(NSError **)error;
aspect_add方法:
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:
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
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)); }}
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__
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
- (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 < numberofarguments;="" idx++)="" {="" const="" char="" *type="[originalInvocation.methodSignature" getargumenttypeatindex:idx];="" nsuinteger="" argsize;="" nsgetsizeandalignment(type,="" &argsize,="" null);="" if="" (!(argbuf="reallocf(argBuf," argsize)))="" {="" aspectlogerror(@'failed="" to="" allocate="" memory="" for="" block="" invocation.');="" return="" no;="" }="" [originalinvocation="" getargument:argbuf="" atindex:idx];="" [blockinvocation="" setargument:argbuf="" atindex:idx];="" }="" 执行block="" [blockinvocation="" invokewithtarget:self.block];="" if="" (argbuf="" !="NULL)" {="" free(argbuf);="" }="" return="">
流程图:
著作权归作者所有
联系客服