AOP---Aspects

 

1. AOP简介

AOP: Aspect Oriented Programming 面向切面编程。

面向切面编程(也叫面向方面)：Aspect Oriented Programming(AOP),是目前软件开发中的一个热点。利用AOP可以对业务逻辑的各个部分进行隔离，从而使得业务逻辑各部分之间的耦合度降低，提高程序的可重用性，同时提高了开发的效率。

AOP是OOP的延续，是（Aspect Oriented Programming）的缩写，意思是面向切面（方面）编程。

主要的功能是：日志记录，性能统计，安全控制，事务处理，异常处理等等。

主要的意图是：将日志记录，性能统计，安全控制，事务处理，异常处理等代码从业务逻辑代码中划分出来，通过对这些行为的分离，我们希望可以将它们独立到非指导业务逻辑的方法中，进而改 变这些行为的时候不影响业务逻辑的代码。

可以通过预编译方式和运行期动态代理实现在不修改源代码的情况下给程序动态统一添加功能的一种技术。AOP实际是GoF设计模式的延续，设计模式孜孜不倦追求的是调用者和被调用者之间的解耦，AOP可以说也是这种目标的一种实现。

假设把应用程序想成一个立体结构的话，OOP的利刃是纵向切入系统，把系统划分为很多个模块（如：用户模块，文章模块等等），而AOP的利刃是横向切入系统，提取各个模块可能都要重复操作的部分（如：权限检查，日志记录等等）。由此可见，AOP是OOP的一个有效补充。

注意：AOP不是一种技术，实际上是编程思想。凡是符合AOP思想的技术，都可以看成是AOP的实现

2. iOS中的AOP

利用 Objective-C 的 Runtime 特性，我们可以给语言做扩展，帮助解决项目开发中的一些设计和技术问题。这一篇，我们来探索一些利用 Objective-C Runtime 的黑色技巧。这些技巧中最具争议的或许就是 Method Swizzling 。

其次，用不用就看项目规模和团队规模。有些业务确实非常适合使用AOP,比如log，AOP还可以用来debug

AOP的优势：

1. 减少切面业务的开发量，“一次开发终生使用”，比如日志
2. 减少代码耦合，方便复用。切面业务的代码可以独立出来，方便其他应用使用
3. 提高代码review的质量，比如我可以规定某些类的某些方法才用特定的命名规范，这样review的时候就可以发现一些问题

AOP的弊端：

1. 它破坏了代码的干净整洁。
   （因为 Logging 的代码本身并不属于 ViewController 里的主要逻辑。随着项目扩大、代码量增加，你的 ViewController 里会到处散布着 Logging 的代码。这时，要找到一段事件记录的代码会变得困难，也很容易忘记添加事件记录的代码）

3. iOS AOP实战

玩转 Method Swizzling

1.事务拦截，安全可变容器

iOS中有各类容器的概念，容器分可变容器和非可变容器，可变容器一般内部在实现上是一个链表，在进行各类（insert 、remove、 delete、 update ）难免有空操作、指针越界的问题。
最粗暴的方式就是在使用可变容器的时间，每次操作都必须手动做空判断、索引比较这些操作:

 NSMutableDictionary *dic = [[NSMutableDictionary alloc] init];
    if (obj) {
        [dic setObject:obj forKey:@"key"];
    }

 NSMutableArray *array = [[NSMutableArray alloc] init];
    if (index < array.count) {
        NSLog(@"%@",[array objectAtIndex:index]);
    }

在代码中大量的使用这鞋操作实在是太过于繁琐了，试想如果可变容器自身如何能做这些兼容岂不是更好。可能会想到继承的方法来解决，但是项目中尽可能的避免过多的派生（至于派生的弊端这里就不多说了）；或者想到分类，这里也不尽人意。

Method Swizzling 移花接木

runtime 这里就不多多说了（swift里面已经对这个概念的说法从心转变成了 Reflection<反射>），objective c中每个方法的名字(SEL)跟函数的实现(IMP)是一一对应的，Swizzle的原理只是在这个地方做下手脚，将原来方法名与实现的指向交叉处理，就能达到一个新的效果。

废话少说，直接上代码：

这里使用NSMutableArray 做实例，为NSMutableArray追加一个新的方法

@implementation NSMutableArray (safe)

+ (void)load
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        id obj = [[self alloc] init];
        [obj swizzleMethod:@selector(addObject:) withMethod:@selector(safeAddObject:)];
        [obj swizzleMethod:@selector(objectAtIndex:) withMethod:@selector(safeObjectAtIndex:)];
        [obj swizzleMethod:@selector(insertObject:atIndex:) withMethod:@selector(safeInsertObject:atIndex:)];
        [obj swizzleMethod:@selector(removeObjectAtIndex:) withMethod:@selector(safeRemoveObjectAtIndex:)];
        [obj swizzleMethod:@selector(replaceObjectAtIndex:withObject:) withMethod:@selector(safeReplaceObjectAtIndex:withObject:)];
    });
}

- (void)safeAddObject:(id)anObject
{
    if (anObject) {
        [self safeAddObject:anObject];
    }else{
        NSLog(@"obj is nil");
        
    }
}

- (id)safeObjectAtIndex:(NSInteger)index
{
    if(index<[self count]){
        return [self safeObjectAtIndex:index];
    }else{
        NSLog(@"index is beyond bounds ");
    }
    return nil;
}

- (void)swizzleMethod:(SEL)origSelector withMethod:(SEL)newSelector
{
    Class class = [self class];
    
    Method originalMethod = class_getInstanceMethod(class, origSelector);
    Method swizzledMethod = class_getInstanceMethod(class, newSelector);
    
    BOOL didAddMethod = class_addMethod(class,
                                        origSelector,
                                        method_getImplementation(swizzledMethod),
                                        method_getTypeEncoding(swizzledMethod));
    if (didAddMethod) {
        class_replaceMethod(class,
                            newSelector,
                            method_getImplementation(originalMethod),
                            method_getTypeEncoding(originalMethod));
    } else {
        method_exchangeImplementations(originalMethod, swizzledMethod);
    }
}

这里唯一可能需要解释的是 class_addMethod 。要先尝试添加原 selector 是为了做一层保护，因为如果这个类没有实现 originalSelector ，但其父类实现了，那 class_getInstanceMethod 会返回父类的方法。这样 method_exchangeImplementations 替换的是父类的那个方法，这当然不是你想要的。所以我们先尝试添加 orginalSelector ，如果已经存在，再用 method_exchangeImplementations 把原方法的实现跟新的方法实现给交换掉。

safeAddObject 代码看起来可能有点奇怪，像递归不是么。当然不会是递归，因为在 runtime 的时候，函数实现已经被交换了。调用 objectAtIndex: 会调用你实现的 safeObjectAtIndex:，而在 NSMutableArray: 里调用 safeObjectAtIndex: 实际上调用的是原来的 objectAtIndex: 。

如此以来，一直担心的问题就迎刃而解了，不仅在可变数组、可变字典等容器内都可以做自己想做的事情。

Demo

作者：风之痕_
链接：https://www.jianshu.com/p/addd4eac54ed
來源：简书
简书著作权归作者所有，任何形式的转载都请联系作者获得授权并注明出处。

2.aspect 框架流程技术点解析

@interface NSObject (Aspects)

+ (id<AspectToken>)aspect_hookSelector:(SEL)selector
                               withOptions:(AspectOptions)options
                                usingBlock:(id)block
                                     error:(NSError **)error;


/// Adds a block of code before/instead/after the current `selector` for a specific instance.
- (id<AspectToken>)aspect_hookSelector:(SEL)selector
                               withOptions:(AspectOptions)options
                                usingBlock:(id)block
                                     error:(NSError **)error;

框架是对NSObject的扩展，入口方法一个类方法，一个实例方法，这里面要注意的点有：

1.block转为了__strong修饰的id类型，相应的业务的block会由栈区转变为堆区；Block会复制到堆；

• 调用Block的copy实例方法时
• Block作为函数返回值返回时
• 将Block赋值给附有__strong修饰符id类型的类或者Block类型成员变量时
• 在方法名中含有usingBlock的Cocoa框架方法或Grand Central Dispatch 的API中传递Block时

2.为什么此处用id类型不用真实的block类型，回调要咋处理？后续会细讲，主要是因为可以动态的拿到block的签名，入口处就不用关心block的参数个数的问题了，后续可以动态的把参数调用block的时候传回去。

接下来就是具体hook的流程

static id aspect_add(id self, SEL selector, AspectOptions options, id block, NSError **error) {
    __block AspectIdentifier *identifier = nil;
    aspect_performLocked(^{
        if (aspect_isSelectorAllowedAndTrack(self, selector, options, error)) {
            AspectsContainer *aspectContainer = aspect_getContainerForObject(self, selector);
            identifier = [AspectIdentifier identifierWithSelector:selector object:self options:options block:block error:error];
            if (identifier) {
                [aspectContainer addAspect:identifier withOptions:options];

                // Modify the class to allow message interception.
                aspect_prepareClassAndHookSelector(self, selector, error);
            }
        }
    });
    return identifier;
}

首先通过自旋锁保证hook的流程都是串行的。ibireme 的 不再安全的 OSSpinLock 

static void aspect_performLocked(dispatch_block_t block) {
    static OSSpinLock aspect_lock = OS_SPINLOCK_INIT;
    OSSpinLockLock(&aspect_lock);
    block();
    OSSpinLockUnlock(&aspect_lock);
}

aspect_isSelectorAllowedAndTrack  主要是判断类的这个方法是否可以hook，不细说了。

// Loads or creates the aspect container.
static AspectsContainer *aspect_getContainerForObject(NSObject *self, SEL selector) {
    NSCParameterAssert(self);
    SEL aliasSelector = aspect_aliasForSelector(selector);
    AspectsContainer *aspectContainer = objc_getAssociatedObject(self, aliasSelector);
    if (!aspectContainer) {
        aspectContainer = [AspectsContainer new];
        objc_setAssociatedObject(self, aliasSelector, aspectContainer, OBJC_ASSOCIATION_RETAIN);
    }
    return aspectContainer;
}

AspectsContainer，使用关联对象对已经hook的类的相应方法进行管理，为什么需要这个对象呢，有可能某个类的某个方法会被多场景hook吧。注意关联对象的key是aliasSelector，可以保证唯一，而且是加了框架前缀的。

构造AspectIdentifier

+ (instancetype)identifierWithSelector:(SEL)selector object:(id)object options:(AspectOptions)options block:(id)block error:(NSError **)error {
    NSCParameterAssert(block);
    NSCParameterAssert(selector);
    NSMethodSignature *blockSignature = aspect_blockMethodSignature(block, error); // TODO: check signature compatibility, etc.
    if (!aspect_isCompatibleBlockSignature(blockSignature, object, selector, error)) {
        return nil;
    }

    AspectIdentifier *identifier = nil;
    if (blockSignature) {
        identifier = [AspectIdentifier new];
        identifier.selector = selector;
        identifier.block = block;
        identifier.blockSignature = blockSignature;
        identifier.options = options;
        identifier.object = object; // weak
    }
    return identifier;
}

AspectIdentifier的构造最重要在这一点，就是利用桥接做block内存模型映射，然后通过地址偏移获取block的签名。后续通过blcok签名就可以生成NSInvocation *blockInvocation ，进行block 的动态调用，多参数的block回调就自然支持了。

NSMethodSignature *blockSignature = aspect_blockMethodSignature(block, error);

static NSMethodSignature *aspect_blockMethodSignature(id block, NSError **error) {
    AspectBlockRef layout = (__bridge void *)block;
	if (!(layout->flags & AspectBlockFlagsHasSignature)) {
        NSString *description = [NSString stringWithFormat:@"The block %@ doesn't contain a type signature.", block];
        AspectError(AspectErrorMissingBlockSignature, description);
        return nil;
    }
	void *desc = layout->descriptor;
	desc += 2 * sizeof(unsigned long int);
	if (layout->flags & AspectBlockFlagsHasCopyDisposeHelpers) {
		desc += 2 * sizeof(void *);
    }
	if (!desc) {
        NSString *description = [NSString stringWithFormat:@"The block %@ doesn't has a type signature.", block];
        AspectError(AspectErrorMissingBlockSignature, description);
        return nil;
    }
	const char *signature = (*(const char **)desc);
	return [NSMethodSignature signatureWithObjCTypes:signature];
}

紧接着进行类的方法hook

static void aspect_prepareClassAndHookSelector(NSObject *self, SEL selector, NSError **error) {
    NSCParameterAssert(selector);
    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]) {
            __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);
        }

        // We use forwardInvocation to hook in.
        class_replaceMethod(klass, selector, aspect_getMsgForwardIMP(self, selector), typeEncoding);
        AspectLog(@"Aspects: Installed hook for -[%@ %@].", klass, NSStringFromSelector(selector));
    }
}

第一步，对forwardInvocation进行hook，接管消息转发流程

static NSString *const AspectsForwardInvocationSelectorName = @"__aspects_forwardInvocation:";
static void aspect_swizzleForwardInvocation(Class klass) {
    NSCParameterAssert(klass);
    // If there is no method, replace will act like class_addMethod.
    IMP originalImplementation = class_replaceMethod(klass, @selector(forwardInvocation:), (IMP)__ASPECTS_ARE_BEING_CALLED__, "v@:@");
    if (originalImplementation) {
        class_addMethod(klass, NSSelectorFromString(AspectsForwardInvocationSelectorName), originalImplementation, "v@:@");
    }
    AspectLog(@"Aspects: %@ is now aspect aware.", NSStringFromClass(klass));
}

第二步，对SEL selector 进行hook，流程就是创建新的aspects_开头 SEL aliasSelector，使用原来方法的实现加入类中，然后原方法SEL selector指向IMP msgForwardIMP = _objc_msgForward;也就是只要使用原selector直接走消息转发流程，走(IMP)__ASPECTS_ARE_BEING_CALLED__逻辑。

static void aspect_prepareClassAndHookSelector(NSObject *self, SEL selector, NSError **error) {
    NSCParameterAssert(selector);
    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]) {
            __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);
        }

        // We use forwardInvocation to hook in.
        class_replaceMethod(klass, selector, aspect_getMsgForwardIMP(self, selector), typeEncoding);
        AspectLog(@"Aspects: Installed hook for -[%@ %@].", klass, NSStringFromSelector(selector));
    }
}

当被hook当方法触发时会走到下面的逻辑，aspect_invoke处就会调用block回调，把AspectInfo传出去

// This is the swizzled forwardInvocation: method.
static void __ASPECTS_ARE_BEING_CALLED__(__unsafe_unretained NSObject *self, SEL selector, NSInvocation *invocation) {
    NSCParameterAssert(self);
    NSCParameterAssert(invocation);
    SEL originalSelector = invocation.selector;
	SEL aliasSelector = aspect_aliasForSelector(invocation.selector);
    invocation.selector = aliasSelector;
    AspectsContainer *objectContainer = objc_getAssociatedObject(self, aliasSelector);
    AspectsContainer *classContainer = aspect_getContainerForClass(object_getClass(self), aliasSelector);
    AspectInfo *info = [[AspectInfo alloc] initWithInstance:self invocation:invocation];
    NSArray *aspectsToRemove = nil;

    // Before hooks.
    aspect_invoke(classContainer.beforeAspects, info);
    aspect_invoke(objectContainer.beforeAspects, info);

    // Instead hooks.
    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)));
    }

    // After hooks.
    aspect_invoke(classContainer.afterAspects, info);
    aspect_invoke(objectContainer.afterAspects, info);

    // If no hooks are installed, call original implementation (usually to throw an exception)
    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];
        }
    }

    // Remove any hooks that are queued for deregistration.
    [aspectsToRemove makeObjectsPerformSelector:@selector(remove)];
}
#undef aspect_invoke

下面的逻辑就是要动态的触发block回调，把originalInvocation的参数传给block的使用声明处，就可以捕获方法调用参数了。

- (BOOL)invokeWithInfo:(id<AspectInfo>)info {
    NSInvocation *blockInvocation = [NSInvocation invocationWithMethodSignature:self.blockSignature];
    NSInvocation *originalInvocation = info.originalInvocation;
    NSUInteger numberOfArguments = self.blockSignature.numberOfArguments;

    // Be extra paranoid. We already check that on hook registration.
    if (numberOfArguments > originalInvocation.methodSignature.numberOfArguments) {
        AspectLogError(@"Block has too many arguments. Not calling %@", info);
        return NO;
    }

    // The `self` of the block will be the AspectInfo. Optional.
    if (numberOfArguments > 1) {
        [blockInvocation setArgument:&info atIndex:1];
    }
    
	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];
    }
    
    [blockInvocation invokeWithTarget:self.block];
    
    if (argBuf != NULL) {
        free(argBuf);
    }
    return YES;
}

整体大概流程至此结束，后续细节点再补充