我们知道iOS程序的入口函数在main.其实mian只是苹果给我们的”直观能够感受”的入口,在执行main之前,编译器已经帮我们做了相当多的事情.具体可以参考objc-os.h文件.Objective-C的Runtime库也是在main之前创建好的.我们关注sel_init()
SEL:
/************************************************************************ _objc_init* Bootstrap initialization. Registers our image notifier with dyld.* Called by libSystem BEFORE library initialization time**********************************************************************/void _objc_init(void){ static bool initialized = false; if (initialized) return; initialized = true; // fixme defer initialization until an objc-using image is found? environ_init(); tls_init(); static_init(); lock_init(); exception_init(); _dyld_objc_notify_register(&map_images, load_images, unmap_image);}
接口我们可以到sel_init()调用栈:
_| _objc_init() _| _dyld_objc_notify_register _| map_images_nolock() _| sel_init()
在map_images_nolock()方法中我们看到在sel_init()下面有arr_init():
void arr_init(void) { AutoreleasePoolPage::init(); SideTableInit();}
这个函数就是我们熟悉的AutoreleasePoolPage的初始化和全局SideTable的初始化,这个以后再分析.这里我们看一下sel_init()所做的工作:
/************************************************************************ sel_init* Initialize selector tables and register selectors used internally.**********************************************************************/void sel_init(size_t selrefCount){ // save this value for later SelrefCount = selrefCount; #if SUPPORT_PREOPT builtins = preoptimizedSelectors(); if (PrintPreopt && builtins) { uint32_t occupied = builtins->occupied; uint32_t capacity = builtins->capacity; _objc_inform("PREOPTIMIZATION: using selopt at %p", builtins); _objc_inform("PREOPTIMIZATION: %u selectors", occupied); _objc_inform("PREOPTIMIZATION: %u/%u (%u%%) hash table occupancy", occupied, capacity, (unsigned)(occupied/(double)capacity*100)); }#endif // Register selectors used by libobjc #define s(x) SEL_##x = sel_registerNameNoLock(#x, NO)#define t(x,y) SEL_##y = sel_registerNameNoLock(#x, NO) mutex_locker_t lock(selLock); s(load); s(initialize); t(resolveInstanceMethod:, resolveInstanceMethod); t(resolveClassMethod:, resolveClassMethod); t(.cxx_construct, cxx_construct); t(.cxx_destruct, cxx_destruct); s(retain); s(release); s(autorelease); s(retainCount); s(alloc); t(allocWithZone:, allocWithZone); s(dealloc); s(copy); s(new); t(forwardInvocation:, forwardInvocation); t(_tryRetain, tryRetain); t(_isDeallocating, isDeallocating); s(retainWeakReference); s(allowsWeakReference); #undef s#undef t}
我们可以看到一些常见系统内置方法分别调用__sel_registerName()这个方法:
static SEL __sel_registerName(const char *name, bool shouldLock, bool copy) { SEL result = 0; if (shouldLock) selLock.assertUnlocked(); else selLock.assertLocked(); if (!name) return (SEL)0; result = search_builtins(name); if (result) return result; conditional_mutex_locker_t lock(selLock, shouldLock); if (namedSelectors) { result = (SEL)NXMapGet(namedSelectors, name); } if (result) return result; // No match. Insert. if (!namedSelectors) { namedSelectors = NXCreateMapTable(NXStrValueMapPrototype, (unsigned)SelrefCount); } if (!result) { // 初始化sel_alloc result = sel_alloc(name, copy); // 将selector 插入到NXMapTable 表中 // fixme choose a better container (hash not map for starters) NXMapInsert(namedSelectors, sel_getName(result), result); } return result;}
这方法的作用是将selector注册到NXMapTable表中,如果selector不存在则调用selector初始化方法,然后将selector作为selector作为key,selector作为value存到NXMapTable哈希表中:
static SEL sel_alloc(const char *name, bool copy){ selLock.assertLocked(); return (SEL)(copy ? strdupIfMutable(name) : name); }
在objc.h文件中我们可以看到SEL的声明:
/// An opaque type that represents a method selector.typedef struct objc_selector *SEL;
至此我们可以理解了,SEL就是一个表示方法的selector指针,映射方法的名字.
IMP:
/// A pointer to the function of a method implementation. #if !OBJC_OLD_DISPATCH_PROTOTYPEStypedef void (*IMP)(void /* id, SEL, ... */ ); #elsetypedef id _Nullable (*IMP)(id _Nonnull, SEL _Nonnull, ...); #endif
从定义来看,IMP是一个指向实现函数的指针.IMP也是实现函数的入口,其和SEL的关系等以后将消息发送在细说.
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Method:
method的声明结构:
typedef struct method_t *Method;
继续查看method_t的定义:
struct method_t { SEL name; const char *types; MethodListIMP imp; struct SortBySELAddress : public std::binary_function { bool operator() (const method_t& lhs, const method_t& rhs) { return lhs.name < rhs.name; } };};
method_t中有两个我们熟悉的成员变量:SEL和MethodListIMP,看一下MethodListIMP:
#if __has_feature(ptrauth_calls)// Method lists use process-independent signature for compatibility.// Method caches use process-dependent signature for extra protection.// (fixme not yet __ptrauth(...) because of `stp` inline asm in objc-cache.mm)using MethodListIMP = IMP __ptrauth_objc_method_list_imp;using MethodCacheIMP = StorageSignedFunctionPointer; #elseusing MethodListIMP = IMP;using MethodCacheIMP = IMP;#endif
MethodListIMP其实就是IMP,method可以理解为SEL(方法名称)和IMP(方法实现)相互对应的集合体.正常的情况一个SEL对应一个IMP,而SEL和IMP的绑定到运行时才确定的.
相关API
下面是NSObject.h和runtime.h文件为我们提供的有关SEL,IMP和Method相关的接口及说明:
runtim.h文件:
根据SEL获取实例Method指针
// 获取Method声明OBJC_EXPORT Method _Nullableclass_getInstanceMethod(Class _Nullable cls, SEL _Nonnull name) OBJC_AVAILABLE(10.0, 2.0, 9.0, 1.0, 2.0); // 获取Method实现/************************************************************************ class_getInstanceMethod. Return the instance method for the* specified class and selector.**********************************************************************/Method class_getInstanceMethod(Class cls, SEL sel){ if (!cls || !sel) return nil; // This deliberately avoids +initialize because it historically did so. // This implementation is a bit weird because it's the only place that // wants a Method instead of an IMP. #warning fixme build and search caches // Search method lists, try method resolver, etc. lookUpImpOrNil(cls, sel, nil, NO/*initialize*/, NO/*cache*/, YES/*resolver*/); #warning fixme build and search caches return _class_getMethod(cls, sel);}
这里面调用了_class_getMethod()私有函数:
/************************************************************************ _class_getMethod* fixme* Locking: read-locks runtimeLock**********************************************************************/static Method _class_getMethod(Class cls, SEL sel){ mutex_locker_t lock(runtimeLock); return getMethod_nolock(cls, sel);}/************************************************************************ getMethod_nolock* fixme* Locking: runtimeLock must be read- or write-locked by the caller**********************************************************************/static method_t *getMethod_nolock(Class cls, SEL sel){ method_t *m = nil; runtimeLock.assertLocked(); // fixme nil cls? // fixme nil sel? assert(cls->isRealized()); while (cls && ((m = getMethodNoSuper_nolock(cls, sel))) == nil) { cls = cls->superclass; } return m;}
这个函数内部调用的是getMethodNoSuper_nolock():
static method_t *getMethodNoSuper_nolock(Class cls, SEL sel){ runtimeLock.assertLocked(); assert(cls->isRealized()); // fixme nil cls? // fixme nil sel? for (auto mlists = cls->data()->methods.beginLists(), end = cls->data()->methods.endLists(); mlists != end; ++mlists) { method_t *m = search_method_list(*mlists, sel); if (m) return m; } return nil;}
查找方法的过程是先从本class的方法列表中查看是否存在,不存再在看父类递归这个过程( cls = cls->superclass).这个我们在以后讲消息发送,转发时在细看.
根据SEL获取类Method指针
OBJC_EXPORT Method _Nullableclass_getClassMethod(Class _Nullable cls, SEL _Nonnull name) OBJC_AVAILABLE(10.0, 2.0, 9.0, 1.0, 2.0);
类对象的方法列表存放在元类中,所以获取类方法要去元类中查找,其在获取的时候参数已经指明元类:
/************************************************************************ class_getClassMethod. Return the class method for the specified* class and selector.**********************************************************************/Method class_getClassMethod(Class cls, SEL sel){ if (!cls || !sel) return nil; //这里cls获取的是元类 return class_getInstanceMethod(cls->getMeta(), sel);}
返回一个函数的实现指针:
OBJC_EXPORT IMP _Nullableclass_getMethodImplementation(Class _Nullable cls, SEL _Nonnull name) OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);
其实现过程:
IMP class_getMethodImplementation(Class cls, SEL sel){ IMP imp; if (!cls || !sel) return nil; imp = lookUpImpOrNil(cls, sel, nil, YES/*initialize*/, YES/*cache*/, YES/*resolver*/); // Translate forwarding function to C-callable external version if (!imp) { return _objc_msgForward; } return imp;}
其内部我们可以看到两个主要的调用函数:lookUpImpOrNil()和_objc_msgForward.前面函数是获取SEL对应的IMP,其实现过程如下:
/************************************************************************ lookUpImpOrNil.* Like lookUpImpOrForward, but returns nil instead of _objc_msgForward_impcache**********************************************************************/IMP lookUpImpOrNil(Class cls, SEL sel, id inst, bool initialize, bool cache, bool resolver){ IMP imp = lookUpImpOrForward(cls, sel, inst, initialize, cache, resolver); if (imp == _objc_msgForward_impcache) return nil; else return imp;}// MARK: - 获取imp,先从缓存中查找imp,如果存在直接返回imp./************************************************************************ lookUpImpOrForward.* The standard IMP lookup. * initialize==NO tries to avoid +initialize (but sometimes fails)* cache==NO skips optimistic unlocked lookup (but uses cache elsewhere)* Most callers should use initialize==YES and cache==YES.* inst is an instance of cls or a subclass thereof, or nil if none is known. * If cls is an un-initialized metaclass then a non-nil inst is faster.* May return _objc_msgForward_impcache. IMPs destined for external use * must be converted to _objc_msgForward or _objc_msgForward_stret.* If you don't want forwarding at all, use lookUpImpOrNil() instead.**********************************************************************/IMP lookUpImpOrForward(Class cls, SEL sel, id inst, bool initialize, bool cache, bool resolver){ IMP imp = nil; bool triedResolver = NO; runtimeLock.assertUnlocked(); // Optimistic cache lookup if (cache) { imp = cache_getImp(cls, sel); if (imp) return imp; } // runtimeLock is held during isRealized and isInitialized checking // to prevent races against concurrent realization. // runtimeLock is held during method search to make // method-lookup + cache-fill atomic with respect to method addition. // Otherwise, a category could be added but ignored indefinitely because // the cache was re-filled with the old value after the cache flush on // behalf of the category. runtimeLock.lock(); checkIsKnownClass(cls); if (!cls->isRealized()) { realizeClass(cls); } if (initialize && !cls->isInitialized()) { runtimeLock.unlock(); _class_initialize (_class_getNonMetaClass(cls, inst)); runtimeLock.lock(); // If sel == initialize, _class_initialize will send +initialize and // then the messenger will send +initialize again after this // procedure finishes. Of course, if this is not being called // from the messenger then it won't happen. 2778172 } retry: runtimeLock.assertLocked(); // 从缓存中尝试查找IMP // Try this class's cache. imp = cache_getImp(cls, sel); if (imp) goto done; // 从本类的方法列表中尝试查找IMP // Try this class's method lists. { Method meth = getMethodNoSuper_nolock(cls, sel); if (meth) { log_and_fill_cache(cls, meth->imp, sel, inst, cls); imp = meth->imp; goto done; } } // 从父类的方法列表中尝试查找IMP // Try superclass caches and method lists. { unsigned attempts = unreasonableClassCount(); for (Class curClass = cls->superclass; curClass != nil; curClass = curClass->superclass) { // Halt if there is a cycle in the superclass chain. if (--attempts == 0) { _objc_fatal("Memory corruption in class list."); } // Superclass cache. imp = cache_getImp(curClass, sel); if (imp) { if (imp != (IMP)_objc_msgForward_impcache) { // Found the method in a superclass. Cache it in this class. log_and_fill_cache(cls, imp, sel, inst, curClass); goto done; } else { // Found a forward:: entry in a superclass. // Stop searching, but don't cache yet; call method // resolver for this class first. break; } } // Superclass method list. Method meth = getMethodNoSuper_nolock(curClass, sel); if (meth) { log_and_fill_cache(cls, meth->imp, sel, inst, curClass); imp = meth->imp; goto done; } } } // 如果以上过程都没有找到,尝试一次动态方法解析 // No implementation found. Try method resolver once . if (resolver && !triedResolver) { runtimeLock.unlock(); _class_resolveMethod(cls, sel, inst); runtimeLock.lock(); // Don't cache the result; we don't hold the lock so it may have // changed already. Re-do the search from scratch instead. triedResolver = YES; goto retry; } // 如果方法解析也没有IMP,启动消息转发 // No implementation found, and method resolver didn't help. // Use forwarding. imp = (IMP)_objc_msgForward_impcache; cache_fill(cls, sel, imp, inst); done: runtimeLock.unlock(); return imp;}
根据SEL在缓存中如果没有找到IMP,则在本类的Method获取找IMP,如果没有找到去父类中查找.以上过程都没有找到IMP的话,启动一次方法解析,方法解析也没有IMP的话就启动消息转发.
一个对象是否响应某个方法
// MARK: - 一个实例对象是否响应某个方法/** * Returns a Boolean value that indicates whether instances of a class respond to a particular selector. * * @param cls The class you want to inspect. * @param sel A selector. * * @return \c YES if instances of the class respond to the selector, otherwise \c NO. * * @note You should usually use \c NSObject's \c respondsToSelector: or \c instancesRespondToSelector: * methods instead of this function. */OBJC_EXPORT BOOLclass_respondsToSelector(Class _Nullable cls, SEL _Nonnull sel) OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0); // MARK: - 是否响应某个方法BOOL class_respondsToSelector(Class cls, SEL sel){ return class_respondsToSelector_inst(cls, sel, nil);}// MARK: - 是否响应某个方法,其内部是通过获取IMP是否存在来判断// inst is an instance of cls or a subclass thereof, or nil if none is known.// Non-nil inst is faster in some cases. See lookUpImpOrForward() for details.bool class_respondsToSelector_inst(Class cls, SEL sel, id inst){ IMP imp; if (!sel || !cls) return NO; // Avoids +initialize because it historically did so. // We're not returning a callable IMP anyway. imp = lookUpImpOrNil(cls, sel, inst, NO/*initialize*/, YES/*cache*/, YES/*resolver*/); return bool(imp);}
其最终还是痛SEL是否有对应IMP来判断对象能否响应某个方法.
获取一个类的方法列表
// MARK: - 获取一个类的方法列表/** * Describes the instance methods implemented by a class. * * @param cls The class you want to inspect. * @param outCount On return, contains the length of the returned array. * If outCount is NULL, the length is not returned. * * @return An array of pointers of type Method describing the instance methods * implemented by the class—any instance methods implemented by superclasses are not included. * The array contains *outCount pointers followed by a NULL terminator. You must free the array with free(). * * If cls implements no instance methods, or cls is Nil, returns NULL and *outCount is 0. * * @note To get the class methods of a class, use \c class_copyMethodList(object_getClass(cls), &count). * @note To get the implementations of methods that may be implemented by superclasses, * use \c class_getInstanceMethod or \c class_getClassMethod. */OBJC_EXPORT Method _Nonnull * _Nullableclass_copyMethodList(Class _Nullable cls, unsigned int * _Nullable outCount) OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);// MARK: - 获取一个类所有实现的方法列表/************************************************************************ class_copyMethodList* fixme* Locking: read-locks runtimeLock**********************************************************************/Method *class_copyMethodList(Class cls, unsigned int *outCount){ unsigned int count = 0; Method *result = nil; if (!cls) { if (outCount) *outCount = 0; return nil; } mutex_locker_t lock(runtimeLock); assert(cls->isRealized()); count = cls->data()->methods.count(); if (count > 0) { result = (Method *)malloc((count + 1) * sizeof(Method)); count = 0; for (auto& meth : cls->data()->methods) { result[count++] = &meth; } result[count] = nil; } if (outCount) *outCount = count; return result;}
获取一个方法名称
// MARK: - 获取一个方法名称/** * Returns the name of a method. * * @param m The method to inspect. * * @return A pointer of type SEL. * * @note To get the method name as a C string, call \c sel_getName(method_getName(method)). */OBJC_EXPORT SEL _Nonnullmethod_getName(Method _Nonnull m) OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0); // MARK: - 获取一个方法名称/************************************************************************ method_getName* Returns this method's selector.* The method must not be nil.* The method must already have been fixed-up.* Locking: none**********************************************************************/SEL method_getName(Method m){ if (!m) return nil; assert(m->name == sel_registerName(sel_getName(m->name))); return m->name;}
获取一个方法名字最终还是通过sel_registerName()来获取,也是我们最开始的部分sel_init()提到.
获取一个方法的IMP
// MARK: - 获取一个方法的实现/** * Returns the implementation of a method. * * @param m The method to inspect. * * @return A function pointer of type IMP. */OBJC_EXPORT IMP _Nonnullmethod_getImplementation(Method _Nonnull m) OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);// MARK: - 获取一个方法的IMPIMP method_getImplementation(Method m){ return m ? m->imp : nil;}
通过直接Method中的imp成员变量
获取一个方法的参数和返回值类型
// MARK: - 获取一个方法的参数和返回值类型/** * Returns a string describing a method's parameter and return types. * * @param m The method to inspect. * * @return A C string. The string may be \c NULL. */OBJC_EXPORT const char * _Nullablemethod_getTypeEncoding(Method _Nonnull m) OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0); // MARK: - 获取一个方法的参数和返回值类型/************************************************************************ method_getTypeEncoding* Returns this method's old-style type encoding string.* The method must not be nil.* Locking: none**********************************************************************/const char *method_getTypeEncoding(Method m){ if (!m) return nil; return m->types;}
获取一个方法的参数数量
// MARK: - 获取一个方法参数数量/** * Returns the number of arguments accepted by a method. * * @param m A pointer to a \c Method data structure. Pass the method in question. * * @return An integer containing the number of arguments accepted by the given method. */OBJC_EXPORT unsigned intmethod_getNumberOfArguments(Method _Nonnull m) OBJC_AVAILABLE(10.0, 2.0, 9.0, 1.0, 2.0); // MARK: - 获取一个方法的参数数量/************************************************************************ method_getNumberOfArguments.**********************************************************************/unsigned int method_getNumberOfArguments(Method m){ if (!m) return 0; return encoding_getNumberOfArguments(method_getTypeEncoding(m));}
获取一个方法的返回值类型
// MARK: - 获取一个方法的返回值类型/** * Returns a string describing a method's return type. * * @param m The method to inspect. * * @return A C string describing the return type. You must free the string with \c free(). */OBJC_EXPORT char * _Nonnullmethod_copyReturnType(Method _Nonnull m) OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0); // MARK: - 获取方法的返回值类型char * method_copyReturnType(Method m){ return encoding_copyReturnType(method_getTypeEncoding(m));}
获取方法某个参数的类型
// MARK: - 获取方法某个参数的类型/** * Returns a string describing a single parameter type of a method. * * @param m The method to inspect. * @param index The index of the parameter to inspect. * * @return A C string describing the type of the parameter at index \e index, or \c NULL * if method has no parameter index \e index. You must free the string with \c free(). */OBJC_EXPORT char * _Nullablemethod_copyArgumentType(Method _Nonnull m, unsigned int index) OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0); // MARK: - 获取方法某个参数类型char * method_copyArgumentType(Method m, unsigned int index){ return encoding_copyArgumentType(method_getTypeEncoding(m), index);}
获取方法返回值类型
// MARK: - 获取方法返回值类型/** * Returns by reference a string describing a method's return type. * * @param m The method you want to inquire about. * @param dst The reference string to store the description. * @param dst_len The maximum number of characters that can be stored in \e dst. * * @note The method's return type string is copied to \e dst. * \e dst is filled as if \c strncpy(dst, parameter_type, dst_len) were called. */OBJC_EXPORT voidmethod_getReturnType(Method _Nonnull m, char * _Nonnull dst, size_t dst_len) OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0); // MARK: - 获取方法返回值类型void method_getReturnType(Method m, char *dst, size_t dst_len){ encoding_getReturnType(method_getTypeEncoding(m), dst, dst_len);}
更新设置某个方法的IMP
// MARK: - 更新设置某个方法的IMP/** * Sets the implementation of a method. * * @param m The method for which to set an implementation. * @param imp The implemention to set to this method. * * @return The previous implementation of the method. */OBJC_EXPORT IMP _Nonnullmethod_setImplementation(Method _Nonnull m, IMP _Nonnull imp) OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0); // MARK: - 更新设置某个方法IMPIMP method_setImplementation(Method m, IMP imp){ // Don't know the class - will be slow if RR/AWZ are affected // fixme build list of classes whose Methods are known externally? mutex_locker_t lock(runtimeLock); return _method_setImplementation(Nil, m, imp);}// MARK: - 更新设置某个方法的IMP/************************************************************************ method_setImplementation* Sets this method's implementation to imp.* The previous implementation is returned.**********************************************************************/static IMP _method_setImplementation(Class cls, method_t *m, IMP imp){ runtimeLock.assertLocked(); if (!m) return nil; if (!imp) return nil; IMP old = m->imp; m->imp = imp; // Cache updates are slow if cls is nil (i.e. unknown) // RR/AWZ updates are slow if cls is nil (i.e. unknown) // fixme build list of classes whose Methods are known externally? flushCaches(cls); updateCustomRR_AWZ(cls, m); return old;}
先找到old imp,然后把old imp 替换成 new imp.
交换两个方法的实现即交换两个方法的IMP
// MARK: - 交换两个方法的实现即交换两个方法的IMP/** * Exchanges the implementations of two methods. * * @param m1 Method to exchange with second method. * @param m2 Method to exchange with first method. * * @note This is an atomic version of the following: * \code * IMP imp1 = method_getImplementation(m1); * IMP imp2 = method_getImplementation(m2); * method_setImplementation(m1, imp2); * method_setImplementation(m2, imp1); * \endcode */OBJC_EXPORT voidmethod_exchangeImplementations(Method _Nonnull m1, Method _Nonnull m2) OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0); // MARK: - 交换两个方法的实现即交换两个方法的IMPvoid method_exchangeImplementations(Method m1, Method m2){ if (!m1 || !m2) return; mutex_locker_t lock(runtimeLock); IMP m1_imp = m1->imp; m1->imp = m2->imp; m2->imp = m1_imp; // RR/AWZ updates are slow because class is unknown // Cache updates are slow because class is unknown // fixme build list of classes whose Methods are known externally? flushCaches(nil); updateCustomRR_AWZ(nil, m1); updateCustomRR_AWZ(nil, m2);}
NSObject.h文件
执行某个方法
执行某个方法,一般用于方法解析中动态添加方法,下面这几个方法功能类似:
// MARK: - 执行某个方法 一般用于方法解析中动态添加方法- (id)performSelector:(SEL)aSelector;- (id)performSelector:(SEL)aSelector withObject:(id)object;- (id)performSelector:(SEL)aSelector withObject:(id)object1 withObject:(id)object2;// MARK: - 执行某个方法 一般用于方法解析中动态添加方法- (id)performSelector:(SEL)sel { if (!sel) [self doesNotRecognizeSelector:sel]; return ((id(*)(id, SEL))objc_msgSend)(self, sel);}- (id)performSelector:(SEL)sel withObject:(id)obj { if (!sel) [self doesNotRecognizeSelector:sel]; return ((id(*)(id, SEL, id))objc_msgSend)(self, sel, obj);}- (id)performSelector:(SEL)sel withObject:(id)obj1 withObject:(id)obj2 { if (!sel) [self doesNotRecognizeSelector:sel]; return ((id(*)(id, SEL, id, id))objc_msgSend)(self, sel, obj1, obj2);}
先判断这个方法是否存在doesNotRecognizeSelector(),不存在报错”unrecognized selector sent to instance”.然后调用了message.h中的消息发送函数.
内省方法:是否响应某个方法
// MARK: - 内省方法:是否响应某个方法- (BOOL)respondsToSelector:(SEL)aSelector;// MARK: - 是否响应某个方法- (BOOL)respondsToSelector:(SEL)sel { if (!sel) return NO; return class_respondsToSelector_inst([self class], sel, self);}// MARK: - 是否响应某个方法,其内部是通过获取IMP是否存在来判断// inst is an instance of cls or a subclass thereof, or nil if none is known.// Non-nil inst is faster in some cases. See lookUpImpOrForward() for details.bool class_respondsToSelector_inst(Class cls, SEL sel, id inst){ IMP imp; if (!sel || !cls) return NO; // Avoids +initialize because it historically did so. // We're not returning a callable IMP anyway. imp = lookUpImpOrNil(cls, sel, inst, NO/*initialize*/, YES/*cache*/, YES/*resolver*/); return bool(imp);}
一个实例对象是否响应某个方法
// MARK: - 一个对象是否响应某个方法+ (BOOL)instancesRespondToSelector:(SEL)aSelector;// MARK: - 一个对象是否响应某个方法+ (BOOL)instancesRespondToSelector:(SEL)sel { if (!sel) return NO; return class_respondsToSelector(self, sel);}
获取一个SEL对应的IMP
// MARK: - 获取一个SEL对应的IMP- (IMP)methodForSelector:(SEL)aSelector;// MARK: - 获取一个SEL对应的IMP- (IMP)methodForSelector:(SEL)sel { if (!sel) [self doesNotRecognizeSelector:sel]; return object_getMethodImplementation(self, sel);}
获取一个实例方法SEL对应的IMP
// MARK: - 获取一个实例方法SEL对应的IMP+ (IMP)instanceMethodForSelector:(SEL)aSelector;// MARK: - 获取一个实例方法SEL对应的IMP+ (IMP)instanceMethodForSelector:(SEL)sel { if (!sel) [self doesNotRecognizeSelector:sel]; return class_getMethodImplementation(self, sel);}
不能响应某个SEL
// MARK: - 不能响应某个SEL,报错- (void)doesNotRecognizeSelector:(SEL)aSelector;// MARK: - 不能响应某个方法// Replaced by CF (throws an NSException)- (void)doesNotRecognizeSelector:(SEL)sel { _objc_fatal("-[%s %s]: unrecognized selector sent to instance %p", object_getClassName(self), sel_getName(sel), self);}
动态方法解析
动态方法解析,一般用来给某个没有实现的方法添加一个实现;返回false时,执行消息转发流程
// MARK: - 动态方法解析+ (BOOL)resolveClassMethod:(SEL)sel OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);+ (BOOL)resolveInstanceMethod:(SEL)sel OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);// MARK: - 动态方法解析+ (BOOL)resolveClassMethod:(SEL)sel { return NO;}+ (BOOL)resolveInstanceMethod:(SEL)sel { return NO;}
消息转发相关
// MARK: - 消息转发相关- (id)forwardingTargetForSelector:(SEL)aSelector OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);- (void)forwardInvocation:(NSInvocation *)anInvocation OBJC_SWIFT_UNAVAILABLE("");- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector OBJC_SWIFT_UNAVAILABLE("");+ (NSMethodSignature *)instanceMethodSignatureForSelector:(SEL)aSelector OBJC_SWIFT_UNAVAILABLE("");
本文主要介绍了SEL,IMP,Method的定义和实现,以及系统为我们提供的常用API.这里面涉及到到向一个对象发送消息的流程及转发过程,这个会在以后的章节中讲到.
编译后的源码放在Github,里面有相关阅读注释.
作者：偶尔登南山
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