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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_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 method_t&,
const method_t&, bool>
{
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<IMP, ptrauth_key_process_dependent_code>;
#else
using 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:
runtime.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.
编译后源码库
编译后的源码放在Github, 如果对你有帮助,请给一个star吧!
博客地址&相关文章
博客地址: https://waitwalker.github.io/
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