Runtime 是 iOS編程人員的核心基礎(chǔ)知識(shí)，Objc Runtime使得C具有了面向?qū)ο竽芰?，在程序運(yùn)行時(shí)創(chuàng)建，檢查，修改類(lèi)、對(duì)象和它們的方法。
runtime是開(kāi)源的，GitHub 有可調(diào)式的源碼 objc4_debug(待工程配置過(guò)程??)，也可以去官網(wǎng)找 objc4.
學(xué)習(xí)參考 蘋(píng)果官方的Runtime編程指南
isa & id
第一印象指針，在<objc.h>中可以看到：
/// An opaque type that represents an Objective-C class.
typedef struct objc_class *Class;
/// Represents an instance of a class.
struct objc_object {
Class _Nonnull isa OBJC_ISA_AVAILABILITY;
};
/// A pointer to an instance of a class.
typedef struct objc_object *id;
};
-
isa是objc_object一個(gè)屬性，類(lèi)型是Class -
Class是指向objc_class的一個(gè)指針，存放著objc_class的地址 -
id是一個(gè)objc_object結(jié)構(gòu)類(lèi)型的指針，這個(gè)類(lèi)型的對(duì)象能夠轉(zhuǎn)換成任何一種對(duì)象
Class
objc_class 是什么呢？讓我們看到<objc/runtime.h>
struct objc_class {
Class _Nonnull isa OBJC_ISA_AVAILABILITY;
#if !__OBJC2__
Class _Nullable super_class OBJC2_UNAVAILABLE; // 父類(lèi)
const char * _Nonnull name OBJC2_UNAVAILABLE; // 類(lèi)名
long version OBJC2_UNAVAILABLE; // 類(lèi)的版本信息，默認(rèn)為0
long info OBJC2_UNAVAILABLE; // 類(lèi)信息，供運(yùn)行期間使用的一些位標(biāo)識(shí)
long instance_size OBJC2_UNAVAILABLE; // 該類(lèi)的實(shí)例變量大小
struct objc_ivar_list * _Nullable ivars OBJC2_UNAVAILABLE; // 該類(lèi)的成員變量鏈表
struct objc_method_list * _Nullable * _Nullable methodLists OBJC2_UNAVAILABLE; // 方法定義的鏈表
struct objc_cache * _Nonnull cache OBJC2_UNAVAILABLE; // 方法緩存
struct objc_protocol_list * _Nullable protocols OBJC2_UNAVAILABLE;
#endif
} OBJC2_UNAVAILABLE;
/* Use `Class` instead of `struct objc_class *` */
-
objc_class中也有一個(gè)isa指針，指向Meta Class。作用：因?yàn)?Objc 的類(lèi)本身也是一個(gè)對(duì)象，為了處理這個(gè)關(guān)系，runtime就創(chuàng)造了Meta Class，當(dāng)給類(lèi)發(fā)送 [NSObject alloc] 的消息時(shí)，實(shí)際上是把這個(gè)消息發(fā)給 NSObject Class Object。 -
cache作用：對(duì)象接受到一個(gè)消息，首次會(huì)根據(jù)isa指針查找消息對(duì)象，然后在methodLists遍歷，調(diào)用后加入cache，下次直接從緩存獲取，提高調(diào)用效率。
objc_ivar_list
objc_ivar_list 結(jié)構(gòu)體存儲(chǔ)objc_ivar(成員變量)數(shù)組列表
struct objc_ivar {
char * _Nullable ivar_name OBJC2_UNAVAILABLE; // 命名
char * _Nullable ivar_type OBJC2_UNAVAILABLE; // 類(lèi)型
int ivar_offset OBJC2_UNAVAILABLE;
#ifdef __LP64__
int space OBJC2_UNAVAILABLE;
#endif
}
struct objc_ivar_list {
int ivar_count OBJC2_UNAVAILABLE;
#ifdef __LP64__
int space OBJC2_UNAVAILABLE;
#endif
/* variable length structure */
struct objc_ivar ivar_list[1] OBJC2_UNAVAILABLE;
} OBJC2_UNAVAILABLE;
objc_method_list
objc_method_list結(jié)構(gòu)體存儲(chǔ)objc_method(方法)數(shù)組列表
struct objc_method {
SEL _Nonnull method_name OBJC2_UNAVAILABLE;
char * _Nullable method_types OBJC2_UNAVAILABLE;
IMP _Nonnull method_imp OBJC2_UNAVAILABLE;
} OBJC2_UNAVAILABLE;
struct objc_method_list {
struct objc_method_list * _Nullable obsolete OBJC2_UNAVAILABLE;
int method_count OBJC2_UNAVAILABLE;
#ifdef __LP64__
int space OBJC2_UNAVAILABLE;
#endif
/* variable length structure */
struct objc_method method_list[1] OBJC2_UNAVAILABLE;
}
objc_cache
struct objc_cache {
unsigned int mask /* total = mask + 1 */ OBJC2_UNAVAILABLE;
unsigned int occupied OBJC2_UNAVAILABLE;
Method _Nullable buckets[1] OBJC2_UNAVAILABLE;
};
-
mask: 指定分配緩存bucket的總數(shù)。runtime 使用這個(gè)字段確定線(xiàn)性查找的索引位置 -
occupied: 實(shí)際占用緩存bucket的總數(shù) - 指向
Method數(shù)據(jù)結(jié)構(gòu)指針的數(shù)組，總數(shù)不能超過(guò)mask+1；指針可能為空，這標(biāo)識(shí)緩存bucket沒(méi)有被占用，數(shù)組隨著時(shí)間增長(zhǎng)
Meta Class & Super Class
提供一個(gè)實(shí)例，來(lái)體感metaclass和superclass
#import <Foundation/Foundation.h>
#import <objc/runtime.h>
typedef struct mtm_objc_class *mClass;
struct mtm_objc_class {
mClass ISA;
mClass superclass;
};
void testMetaClass(id self, SEL _cmd) {
NSLog(@"This object is %p", self);
NSLog(@"Class is %@, super class is %@", [self class], [self superclass]);
Class currentClass = [self class];
for (int i = 0; i < 4; i++) {
NSLog(@"Following the [isa] pointer %d timer gives %p, class name is %@", i, currentClass, NSStringFromClass(currentClass));
currentClass = object_getClass(currentClass);
}
mClass testClass = (__bridge mClass)[self superclass];
for (int i = 0; i < 4; i++) {
NSLog(@"Following the [super] pointer %d timer gives %p", i, testClass);
testClass = testClass ? testClass->superclass : nil;
}
NSLog(@"NSObject's class is %p", [NSObject class]);
Class meta_NSObject = object_getClass([NSObject class]);
NSLog(@"NSobject's meta class is %p, meta super class is %p", meta_NSObject, ((__bridge mClass)meta_NSObject)->superclass);
}
int main(int argc, const char * argv[]) {
@autoreleasepool {
// insert code here...
Class newClass = objc_allocateClassPair([NSURL class], "TestClass", 0);
class_addMethod(newClass, @selector(testHandler), (IMP)testMetaClass, "v@:");
objc_registerClassPair(newClass);
id instance = [[newClass alloc] initWithString:@"http://www.baidu.com"];
[instance performSelector:@selector(testHandler)];
}
return 0;
}
結(jié)果如下
ObjcTest[74791:1048449] This object is 0x100784c00
ObjcTest[74791:1048449] Class is TestClass, super class is NSURL
ObjcTest[74791:1048449] Following the [isa] pointer 0 timer gives 0x100784420, class name is TestClass
ObjcTest[74791:1048449] Following the [isa] pointer 1 timer gives 0x100784450, class name is TestClass
ObjcTest[74791:1048449] Following the [isa] pointer 2 timer gives 0x1003310f0, class name is NSObject
ObjcTest[74791:1048449] Following the [isa] pointer 3 timer gives 0x1003310f0, class name is NSObject
ObjcTest[74791:1048449] Following the [super] pointer 0 timer gives 0x7fff859d7230
ObjcTest[74791:1048449] Following the [super] pointer 1 timer gives 0x100331140
ObjcTest[74791:1048449] Following the [super] pointer 2 timer gives 0x0
ObjcTest[74791:1048449] Following the [super] pointer 3 timer gives 0x0
ObjcTest[74791:1048449] NSObject's class is 0x100331140
ObjcTest[74791:1048449] NSobject's meta class is 0x1003310f0, meta super class is 0x100331140
把日志中的地址放到圖里，應(yīng)該就更好理解 isa 鏈表 和 super 鏈表
class_isa_super_class.png
小測(cè)試
來(lái)看看能答對(duì)幾個(gè)？
BOOL res1 = [(id)[NSObject class] isKindOfClass:[NSObject class]];
BOOL res2 = [(id)[NSObject class] isMemberOfClass:[NSObject class]];
BOOL res3 = [(id)[NSURL class] isKindOfClass:[NSURL class]];
BOOL res4 = [(id)[NSURL class] isMemberOfClass:[NSURL class]];
解密開(kāi)始
- 誤區(qū)：
isKindOfClass有類(lèi)方法和對(duì)象方法，實(shí)現(xiàn)有區(qū)別 - res1：第一次，Class 和 MetaClass(Class->ISA)對(duì)比，失敗；再跟NSObject's meta class 的 superclass 即 NSObject class，結(jié)果為true
- res2：Class->ISA == Class 為false，下面依此類(lèi)推
- res3 = false
- res4 = false
+ (BOOL)isKindOfClass:(Class)cls {
for (Class tcls = self->ISA(); tcls; tcls = tcls->superclass) {
if (tcls == cls) return YES;
}
return NO;
}
- (BOOL)isKindOfClass:(Class)cls {
for (Class tcls = [self class]; tcls; tcls = tcls->superclass) {
if (tcls == cls) return YES;
}
return NO;
}
+ (BOOL)isMemberOfClass:(Class)cls {
return self->ISA() == cls;
}
- (BOOL)isMemberOfClass:(Class)cls {
return [self class] == cls;
}
