SEL IMP Method解读

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(↦_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_PROTOTYPES
typedef void (*IMP)(void /* id, SEL, ... */ ); 
#else
typedef 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.

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
<const nbsp="" method_t="" const="" bool="">
 
    {
        bool operator() (const method_t& lhs,
                         const method_t& rhs)
        { return lhs.name < rhs.name; }
    };
};
</const>

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
<imp nbsp="" ptrauth_key_process_dependent_code="">
 ;
    
#else

using MethodListIMP = IMP;
using MethodCacheIMP = IMP;

#endif
</imp>

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 _Nullable
class_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 _Nullable
class_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 _Nullable
class_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 BOOL
class_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 * _Nullable
class_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 _Nonnull
method_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 _Nonnull
method_getImplementation(Method _Nonnull m) 
    OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);
// MARK: - 获取一个方法的IMP
IMP 
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 * _Nullable
method_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 int
method_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 * _Nonnull
method_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 * _Nullable
method_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 void
method_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 _Nonnull
method_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IMP
IMP 
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 void
method_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IMP
void 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

链接：https://www.jianshu.com/p/d59056e49dfe