用 OC 开发 iOS 应用的人都知道 OC 是一门动态语言。我们能够在运行时通过 Runtime API 获取并替换任意方法。所以，在 AOP 和 热更新的实现上都经常可以看到 Runtime 的身影。这虽然看似美好，但运用不当的话通常会造成重大 bug。Runtime 对于 OC 来说就像是魔力强大但又十分危险的黑魔法。
黑魔法.jpg
1 消息转发
iOS 的方法执行是通过 SEL 找到 对应的 IMP，然后执行相应的函数。如果 SEL 找不到对应的 IMP 就会启动消息转发机制。消息转发会经过三个阶段。
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第一阶段：在类的内部进行处理
//处理实例方法 + (BOOL)resolveInstanceMethod:(SEL)sel //处理类方法 + (BOOL)resolveClassMethod:(SEL)sel重写该方法可以对相应的 SEL 进行处理，返回 YES 说明处理完成，不进行下一步转发。
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第二阶段：寻找备援接收者
- (id)forwardingTargetForSelector:(SEL)aSelector该方法会返回一个能够处理改消息的备援接收者。如果返回 nil 则继续下一步。
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第三阶段：全局处理
- (void)forwardInvocation:(NSInvocation *)invocation将消息包装成一个 NSInvocation 对象，让系统对其进行处理。如果这里还是没有进行，则会抛出相应的异常。
整个流程如下图所示：
消息转发.jpg
2 Aspects
Aspects 通过 swizzling method 和 swizzling isa 实现对原方法的 hook ，以此来帮助使用者完成 AOP 编程。
2.1 Aspect 主要主要类介绍：
@interface AspectInfo : NSObject <AspectInfo>
- (id)initWithInstance:(__unsafe_unretained id)instance invocation:(NSInvocation *)invocation;
@property (nonatomic, unsafe_unretained, readonly) id instance;
@property (nonatomic, strong, readonly) NSArray *arguments;
@property (nonatomic, strong, readonly) NSInvocation *originalInvocation;
@end
AspectInfo：aspect 的信息，被 hook 方法的 IMP 信息存放在 NSInvocation 中。
@interface AspectIdentifier : NSObject
+ (instancetype)identifierWithSelector:(SEL)selector object:(id)object options:(AspectOptions)options block:(id)block error:(NSError **)error;
- (BOOL)invokeWithInfo:(id<AspectInfo>)info;
@property (nonatomic, assign) SEL selector;
@property (nonatomic, strong) id block;
@property (nonatomic, strong) NSMethodSignature *blockSignature;
@property (nonatomic, weak) id object;
@property (nonatomic, assign) AspectOptions options;
@end
AspectIdentifier：一个 aspect 的具体信息，相应的 sel，options，block
@interface AspectsContainer : NSObject
- (void)addAspect:(AspectIdentifier *)aspect withOptions:(AspectOptions)injectPosition;
- (BOOL)removeAspect:(id)aspect;
- (BOOL)hasAspects;
@property (atomic, copy) NSArray *beforeAspects;
@property (atomic, copy) NSArray *insteadAspects;
@property (atomic, copy) NSArray *afterAspects;
@end
AspectsContainer：本类所有的 aspect 的容器
@interface AspectTracker : NSObject
- (id)initWithTrackedClass:(Class)trackedClass;
@property (nonatomic, strong) Class trackedClass;
@property (nonatomic, readonly) NSString *trackedClassName;
@property (nonatomic, strong) NSMutableSet *selectorNames;
@property (nonatomic, strong) NSMutableDictionary *selectorNamesToSubclassTrackers;
- (void)addSubclassTracker:(AspectTracker *)subclassTracker hookingSelectorName:(NSString *)selectorName;
- (void)removeSubclassTracker:(AspectTracker *)subclassTracker hookingSelectorName:(NSString *)selectorName;
- (BOOL)subclassHasHookedSelectorName:(NSString *)selectorName;
- (NSSet *)subclassTrackersHookingSelectorName:(NSString *)selectorName;
@end
AspectTracker：用来追踪 aspect 的相关信息
2.2 Aspects API调用之后发生的事情
下面是 aspect 相关方法调用之后，主要的执行逻辑
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)) {
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;
}
aspect 方法入口，先是断言判断传入的参数。然后 aspect_performLocked 加锁同步操作，通过 aspect_isSelectorAllowedAndTrack 判断该方法是否允许 hook。判断逻辑如下：
- 实例方法的"retain", "release", "autorelease", "forwardInvocation:" 不允许 hook , "dealloc" 方法只能是 AspectPositionBefore 的方式操作
- 类方法要判断父类，子类中没有 hook 过该方法。
如果可以 hook，则根据参数生成一个 AspectIdentifier 对象，之后放在相应的 AspectsContainer 容器中，然后调用 aspect_prepareClassAndHookSelector 进行 class swizzling 和 method swizzling。
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));
}
}
调用 aspect_hookClass 进行相应的 class swizzling 操作。
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;
}
class swizzling 的主要逻辑如下：
- 若 class 是元类，swizzling 相应的 ForwardInvocation 方法
- 若 class 是普通的类， 先判断是否是 KVO 对象，是的话 swizzling 相应的 ForwardInvocation 方法
- 若不是 KVO 对象的话，先创建一个子类，并 swizzling 子类的 ForwardInvocation，同时将本类的 isa 指针，指向创建的子类。
hook 过的 class 都保存在 swizzledClasses 对象中。接下去进行 method swizzling， 调用 aspect_isMsgForwardIMP 确认实现函数不是消息转发相关的。然后让 aliasSelector 指向 targetMethodIMP，本来的 OriginalSel 指向 msgForwardIMP。至此设置阶段就完成了。
2.3 被 Hook 的方法被调用之后发生的事情
在调用 aspect 的 api 进行相关设置之后，程序执行到刚被 hook 过的方法就会进入 aspect 的执行逻辑。
由于上面的设置，我们知道调用实例方法的类的 isa 会指向了一个子类，然后子类的 OriginalSel 指向了 msgForwardIMP, 而子类相应的 ForwardInvocation 也被替换了，指向了 __ASPECTS_ARE_BEING_CALLED__,所以，一旦调用相应的方法就会进入 __ASPECTS_ARE_BEING_CALLED__ 里面。
如果是类方法，则是 OriginalSel 指向了 msgForwardIMP ，本身的 ForwardInvocation 被替换指向了__ASPECTS_ARE_BEING_CALLED__ ，所以一样进入到 __ASPECTS_ARE_BEING_CALLED__ 方法里。
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)];
}
__ASPECTS_ARE_BEING_CALLED__ 内部的逻辑是：根据 aliasSelector 获取之前存储的类或者元类的 AspectsContainer 对象，之后根据 AspectOptions 在适当的时机获取 AspectsContainer 中对应的所有 AspectIdentifier 并执行相应的 block 。
同时处理被 hook 方法原来的实现( IMP ) , 被封装在 originalInvocation 中（若有 AspectIdentifier 的 AspectOptions 是 insteadAspects ，则 originalInvocation 将不会被执行）。若 AspectIdentifier 的 AspectOptions 是 AspectOptionAutomaticRemoval ，则该 AspectIdentifier 会被存入 aspectsToRemove 中，执行结束之后会被移除。
3 总结
以上就是本人对于 Aspects 这个强大的 AOP 库的一个大致理解。同时这里面还有很多细节很值得学习。
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