前言
众所周知，Aspects框架运用了AOP（面向切面编程）的思想，这里解释下AOP的思想：AOP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。也许大家用Aspects提供的两个方法用着很爽，有没有想揭开它的神秘面纱，看一下它的庐山真面目？接下来主要讲下Aspects的主要实现原理。
实现原理
其实主要涉及到两点，用过runtime的同学都知道swizzling和消息转发，如果还不是很清楚的同学可以看下我之前写的一篇文章OC中swizzling的“移魂大法”,“移魂大法”我这边就不累赘了，提下消息转发。
消息转发
当向一个对象发送消息的时候，在这个对象的类继承体系中没有找到该方法的时候，就会Crash掉，抛出* unrecognized selector sent to …异常信息，但是在抛出这个异常前其实还是可以拯救的，Crash之前会依次执行下面的方法：
-
resolveInstanceMethod(或resolveClassMethod)：实现该方法，可以通过class_addMethod添加方法，返回YES的话系统在运行时就会重新启动一次消息发送的过程，NO的话会继续执行下一个方法。
2.forwardingTargetForSelector：实现该方法可以将消息转发给其他对象，只要这个方法返回的不是nil或self，也会重启消息发送的过程，把这消息转发给其他对象来处理。
methodSignatureForSelector：会去获取一个方法签名，如果没有获取到的话就回直接挑用doesNotRecognizeSelector，如果能获取的话系统就会创建一个NSlnvocation传给forwardInvocation方法。forwardInvocation：该方法是上一个步传进来的NSlnvocation，然后调用NSlnvocation的invokeWithTarget方法，转发到对应的Target。doesNotRecognizeSelector：抛出unrecognized selector sent to …异常。
整体原理
上面讲了几种消息转发的方法，Aspects主要是利用了forwardInvocation进行转发，Aspects其实利用和kvo类似的原理，通过动态创建子类的方式，把对应的对象isa指针指向创建的子类，然后把子类的forwardInvocation的IMP替成__ASPECTS_ARE_BEING_CALLED__，假设要hook的方法名XX,在子类中添加一个Aspects_XX的方法，然后将Aspects_XX的IMP指向原来的XX方法的IMP，这样方便后面调用原始的方法，再把要hook的方法XX的IMP指向_objc_msgForward，这样就进入了消息转发流程，而forwardInvocation的IMP被替换成了__ASPECTS_ARE_BEING_CALLED__，这样就会进入__ASPECTS_ARE_BEING_CALLED__进行拦截处理，这样整个流程大概结束。
代码解析
代码解析这块的话主要讲下核心的模块，一些边边角角的东西去看了自然就明白了。
typedef NS_OPTIONS(NSUInteger, AspectOptions) {
AspectPositionAfter = 0, /// Called after the original implementation (default)
AspectPositionInstead = 1, /// Will replace the original implementation.
AspectPositionBefore = 2, /// Called before the original implementation.
AspectOptionAutomaticRemoval = 1 << 3 /// Will remove the hook after the first execution.
};
AspectOptions提供各种姿势，前插、后插、整个方法替换都行，简直就是爽。
static void aspect_performLocked(dispatch_block_t block) {
static OSSpinLock aspect_lock = OS_SPINLOCK_INIT;
OSSpinLockLock(&aspect_lock);
block();
OSSpinLockUnlock(&aspect_lock);
}
在aspect_add、aspect_remove方法里面用了aspect_performLocked, 而aspect_performLocked方法用了OSSpinLockLock加锁机制，保证线程安全并且性能高。不过这种锁已经不在安全，主要原因发生在低优先级线程拿到锁时，高优先级线程进入忙等(busy-wait)状态，消耗大量 CPU 时间，从而导致低优先级线程拿不到 CPU 时间，也就无法完成任务并释放锁。这种问题被称为优先级反转，有兴趣的可以点击任意门不再安全的 OSSpinLock
static Class aspect_hookClass(NSObject *self, NSError **error) {
NSCParameterAssert(self);
Class statedClass = self.class;
Class baseClass = object_getClass(self);
NSString *className = NSStringFromClass(baseClass);
// Already subclassed
if ([className hasSuffix:AspectsSubclassSuffix]) {
return baseClass;
// We swizzle a class object, not a single object.
}else if (class_isMetaClass(baseClass)) {
return aspect_swizzleClassInPlace((Class)self);
// 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) {
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;
}
aspect_swizzleForwardInvocation(subclass);
aspect_hookedGetClass(subclass, statedClass);
aspect_hookedGetClass(object_getClass(subclass), statedClass);
objc_registerClassPair(subclass);
}
object_setClass(self, subclass);
return subclass;
}
aspect_hookClass这个方法顾名思义就是要hook对应的Class，这在里创建子类，通过aspect_swizzleForwardInvocation方法把forwardInvocation的实现替换成__ASPECTS_ARE_BEING_CALLED__, aspect_hookedGetClass修改了 subclass 以及其 subclass metaclass 的 class 方法,使他返回当前对象的 class，最后通过object_setClass把isa指针指向subclass
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));
}
}
通过aspect_isMsgForwardIMP方法判断要hook的方法时候被hook了，这里如果有使用JSPatch的话会冲突掉，不过JSPatch已经被苹果禁止使用了，苹果爸爸说的算。如果没有被hook走的话会先创建一个有自己别名的方法，然后把这个带有别名方法的IMP指向原始要hook方法的实现，最后要hook的方法的IMP指向_objc_msgForward或者_objc_msgForward_stret,为什么还会有一个_objc_msgForward_stret，详细原因可以参考JSPatch实现原理详解<二>里面的Special Struct
// 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)];
}
该函数为swizzle后, 实现新IMP统一处理的核心方法 , 完成一下几件事：
- 处理调用逻辑, 有before, instead, after, remove四种option
- 将block转换成一个
NSInvocation对象以供调用。 - 从
AspectsContainer根据aliasSelector取出对象, 并组装一个AspectInfo, 带有原函数的调用参数和各项属性, 传给外部的调用者 (在这是block) . - 调用完成后销毁带有
removeOption的hook逻辑, 将原selector挂钩到原IMP上, 删除别名selector
总结
-forwardInvocation:的消息转发虽然看起来很神奇，但是平时没什么需求的话尽量不要去碰这个东西，一般的话swizzling基本可以满足我们项目的大部分需求了，还有就是JSPatch既然不能用了，但是Aspects这个框架还是正常上AppStore的，利用这个库的话还是可以弄出轻量级Hotfix,感兴趣的同学可以点击轻量级低风险 iOS 热更新方案
