Objective-C与Runtime

什么是Runtime

Runtime的普遍定义

引用Wiki的定义：

In computer science, runtime, run time, or execution time is the final phase of a computer program’s life cycle, in which the code is being executed on the computer’s central processing unit (CPU) as machine code.

也就是说，Runtime就是程序经过编译之后在CPU上运行的状态。Runtime Library就是程序处于Runtime的时候所能引用的库，Runtime Error就是程序在运行时所得到的错误，常见的有除数为零错误、作用域错误、数组溢出错误等。

Objective-C中的Runtime

C语言中，在编译过程中，编译器就会确定所要调用的函数的位置；

而OC中的函数是动态的，在编译过程中并不能确定要调用函数的位置，因此就需要通过Runtime来动态地创建类和对象以及传递和转发消息。

OC&runtime.png

OC可以通过三种方式和Runtime进行交互：

OC源码
FoundationKit中NSObject协议和类中的如下方法

// @protocol NSObject
- (Class)class OBJC_SWIFT_UNAVAILABLE("use 'type(of: anObject)' instead");
- (BOOL)isKindOfClass:(Class)aClass;
- (BOOL)isMemberOfClass:(Class)aClass;
- (BOOL)conformsToProtocol:(Protocol *)aProtocol;
- (BOOL)respondsToSelector:(SEL)aSelector;
// @interface NSObject
// 返回制定方法的实现的地址
- (IMP)methodForSelector:(SEL)aSelector;

对Runtime库的直接调用，主要是<objc/Runtime.h>和<objc/message.h>两个头文件的引入

P.S. 如果需要打开Runtime库的代码提示，需要设置如下位置为NO

XCode

Runtime的用法

通过消息转发实现多继承

消息转发机制

p.s. 此处要通过转发的方式调用函数，就不能通过类似[myObject testFunc]或者[MyObject testFunc]的方式直接调用，这样在编译的时候就会报错，而是需要用如下方式调用：

1 2	[MyObject performSelector:@selector(testFunc1)]; // 类方法 [myObject performSelector:@selector(testFunc2)]; // 对象方法

类方法的转发

设当前类为MyObject，要调用的类方法为testFunc，即

1	[MyObject performSelector:@selector(testFunc)];

首先会去MyObject这个类中找testFunc这个方法，如果有则直接返回对应的SEL即可；
重写+ (BOOL)resolveClassMethod:(SEL)sel方法，该方法返回YES则说明可以为MyObject这个类动态添加一个类方法testFunc，例如：

+ (BOOL)resolveClassMethod:(SEL)sel
{
  NSLog(@"resolveClassMethod, %@", NSStringFromSelector(sel));
  if (sel == @selector(testFunc:Name:)) {
    // 如果要在这一步转发，则应当使用class_addMethod
    // return class_addMethod(objc_getMetaClass("MyObject"), sel, (IMP)testFunc, "v@:");
    return NO;
  }
  return [super resolveClassMethod:sel];
}

注意此处的函数class_addMethod的参数：

1	class_addMethod(Class _Nullable cls, SEL _Nonnull name, IMP _Nonnull imp, const char * _Nullable types)

第一个参数cls表示当前对象所属的类，由于这是类方法，因此cls应当为当前类的Meta Class；

第二个参数name为当前转发的选择器；

第三个参数imp为要转发到的方法的实现，该方法必须要有两个默认的参数(id self,SEL _cmd)（这两个参数对于每一个NSObject子类中的方法都是已经隐含了的）；

第四个参数types为返回类型和方法的参数类型的编码，具体可见Type Encodings

重写+ (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector方法，返回对应selector的signature，示例如下：

+ (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
  NSMethodSignature* signature = [super methodSignatureForSelector:aSelector];
  if (!signature) {
    signature = [NSMethodSignature signatureWithObjCTypes:"v@:"];
  }
  return signature;
}

重写+ (void)forwardInvocation:(NSInvocation *)anInvocation进行最后一步转发，此处可以随意转发到任意类的类方法，示例如下：

+ (void)forwardInvocation:(NSInvocation *)anInvocation
{
  if ([MyObject respondsToSelector:@selector(sing)]) {
    [anInvocation setSelector:@selector(sing)];
    // 注意此处target依然需要为class对象
    [anInvocation invokeWithTarget:[MyObject class]];
  }
  else {
    [super forwardInvocation:anInvocation];
  }
}

重写+ (void)doesNotRecognizeSelector:(SEL)aSelector方法，此方法会在找不到对应的selector的最后调用，其super方法会抛出异常，一般不建议覆盖该方法。

Code

#import <UIKit/UIKit.h>
#import "ViewController.h"
#import <objc/objc.h>
#import <objc/runtime.h>
#import <objc/message.h>
@interface OtherObject : NSObject
+ (void)speak;
@end
@implementation OtherObject
+ (void)speak {
  NSLog(@"Speak!!!");
}
@end
@interface MyObject : NSObject
@end
@implementation MyObject
+ (void) sing
{
  NSLog(@"Sing");
}
+ (BOOL)resolveClassMethod:(SEL)sel
{
  NSLog(@"resolveClassMethod, %@", NSStringFromSelector(sel));
  if (sel == @selector(testFunc:Name:)) {
    // 如果要在这一步转发，则应当使用class_addMethod
    // return class_addMethod(objc_getMetaClass("MyObject"), sel, (IMP)testFunc, "v@:");
    return NO;
  }
  return [super resolveClassMethod:sel];
}
+ (id)forwardingTargetForSelector:(SEL)aSelector
{
  NSLog(@"forwardingTargetForSelector, %@", NSStringFromSelector(aSelector));
  // 如果要在这一步使用OtherObject的类方法（同名）去相应
  // if([NSStringFromSelector(aSelector) isEqualToString:@"testFunc:Name:"]){
  //   return [OtherObject class];
  // }
  return [super forwardingTargetForSelector:aSelector];
}
+ (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
  NSLog(@"methodSignatureForSelector, %@", NSStringFromSelector(aSelector));
  NSMethodSignature* signature = [super methodSignatureForSelector:aSelector];
  if (!signature) {
    signature = [NSMethodSignature signatureWithObjCTypes:"v@:"];
  }
  return signature;
}
+ (void)forwardInvocation:(NSInvocation *)anInvocation
{
//   if ([OtherObject respondsToSelector:@selector(speak)]) {
//     NSLog(@"forwardInvocation YES, %@", anInvocation.target);
//     [anInvocation setSelector:@selector(speak)];
//     [anInvocation invokeWithTarget:[OtherObject class]];
//   }   
  if ([MyObject respondsToSelector:@selector(sing)]) {
    NSLog(@"forwardInvocation YES, %@", anInvocation.target);
    [anInvocation setSelector:@selector(sing)];
    [anInvocation invokeWithTarget:[MyObject class]];
  }
  else {
    NSLog(@"forwardInvocation NO");
    [super forwardInvocation:anInvocation];
  }
}
+ (void)doesNotRecognizeSelector:(SEL)aSelector
{
  NSLog(@"doesNotRecognizeSelector, %@", NSStringFromSelector(aSelector));
}
@end
@interface ViewController ()
@end
@implementation ViewController
- (void)viewDidLoad {
  [super viewDidLoad];
  self.title = @"RUNTIME";
  [MyObject performSelector:@selector(testFunc:Name:)];
}
@end

对象方法的转发

对象方法的转发所涉及的函数和类方法基本类似，只不过都从类方法变成了对象方法（除了resolveClassMethod变成resolveInstanceMethod）

Code

#import <UIKit/UIKit.h>
#import "ViewController.h"
#import <objc/objc.h>
#import <objc/runtime.h>
#import <objc/message.h>
@interface OtherObject : NSObject
- (void)speak;
@end
@implementation OtherObject
- (void)speak {
  NSLog(@"Speak!!!");
}
@end
@interface MyObject : NSObject
@end
@implementation MyObject
- (void) sing
{
  NSLog(@"Sing");
}
+ (BOOL)resolveInstanceMethod:(SEL)sel
{
  NSLog(@"resolveInstanceMethod, %@", NSStringFromSelector(sel));
  if (sel == @selector(testFunc:Name:)) {
    // 如果要在这一步转发，则应当使用class_addMethod
    // return class_addMethod([self class], sel, (IMP)testFunc, "v@:");
    return NO;
  }
  return [super resolveInstanceMethod:sel];
}
- (id)forwardingTargetForSelector:(SEL)aSelector
{
  NSLog(@"forwardingTargetForSelector, %@", NSStringFromSelector(aSelector));
  // 如果要在这一步使用OtherObject的对象方法（同名）去相应
  // if([NSStringFromSelector(aSelector) isEqualToString:@"testFunc:Name:"]){
  //   return [[OtherObject alloc] init];
  // }
  return [super forwardingTargetForSelector:aSelector];
}
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
  NSLog(@"methodSignatureForSelector, %@", NSStringFromSelector(aSelector));
  NSMethodSignature* signature = [super methodSignatureForSelector:aSelector];
  if (!signature) {
    signature = [NSMethodSignature signatureWithObjCTypes:"v@:"];
  }
  return signature;
}
- (void)forwardInvocation:(NSInvocation *)anInvocation
{
//   OtherObject *otherObject = [[OtherObject alloc] init];
//   if ([otherObject respondsToSelector:@selector(speak)]) {
//     NSLog(@"forwardInvocation YES, %@", anInvocation.target);
//     [anInvocation setSelector:@selector(speak)];
//     [anInvocation invokeWithTarget:otherObject];
//   }
  if ([self respondsToSelector:@selector(sing)]) {
    NSLog(@"forwardInvocation YES, %@", anInvocation.target);
    [anInvocation setSelector:@selector(sing)];
    [anInvocation invokeWithTarget:self];
  }
  else {
    NSLog(@"forwardInvocation NO");
    [super forwardInvocation:anInvocation];
  }
}
- (void)doesNotRecognizeSelector:(SEL)aSelector
{
  NSLog(@"doesNotRecognizeSelector, %@", NSStringFromSelector(aSelector));
}
@end
@interface ViewController ()
@end
@implementation ViewController
- (void)viewDidLoad {
  [super viewDidLoad];
  self.title = @"RUNTIME";
  MyObject *myObject = [[MyObject alloc] init];
  [myObject performSelector:@selector(testFunc:Name:)];
}
@end

通过消息转发实现的多继承

假设在forwardingTargetForSelector这一步就将MyObject的testFunc: Name: 转发给OtherObject，则:

1 2	MyObject *myObject = [[MyObject alloc] init]; [myObject performSelector:@selector(testFunc:Name:) withObject:@(12) withObject:@"Test"];

上述代码中myObject实际上执行的是OtherObject中的testFunc: Name: 方法。我们可以修改如下函数的代码：

- (id)forwardingTargetForSelector:(SEL)aSelector
{
  return [[OtherObject alloc] init];
}
+ (id)forwardingTargetForSelector:(SEL)aSelector
{
  return [OtherObject class];
}

这样在调用MyObject的类方法和对象方法的时候，如果在MyObject类中已经有该方法的实现，则会直接调用该方法的实现；如果没有，则会去OtherObject中寻找该方法的实现。这种模式和MyObject继承自OtherObject是一样的，因此相当于可以用消息转发来实现继承的操作，进而可以实现多继承。

但是这种方法和真正的继承还是不一样的，区别在于调用isKindOfClass函数的时候，真正的继承是可以判断出子类是父类的kindClass，但是消息转发实现的则不能。

Method Swizzling

这是一种OC hook机制的实现

关联对象

用法举例说明：

@interface NSObject (AssociateObject)
 
@property (nonatomic, strong) NSString *associatedObject;
@end
@implementation NSObject (AssociateObject)
@dynamic associatedObject;
- (void) setAssociatedObject:(id)associatedObject
{
  objc_setAssociatedObject(self, @selector(associatedObject), associatedObject, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
- (NSString *) associatedObject
{
  return objc_getAssociatedObject(self, @selector(associatedObject));
}
@end

涉及到的函数

设置关联对象

1	OBJC_EXPORT void objc_setAssociatedObject(id _Nonnull object, const void * _Nonnull key, id _Nullable value, objc_AssociationPolicy policy);

获取关联对象

1	OBJC_EXPORT id _Nullable objc_getAssociatedObject(id _Nonnull object, const void * _Nonnull key);

移除关联对象

1	OBJC_EXPORT void objc_removeAssociatedObjects(id _Nonnull object);

这三个函数中object对应于该关联对象的实例对象，key是用于区分不同的关联对象，可以使用字符串，或者@selector(associatedObject)作为key，value则是关联对象，policy对应的是关联对象的存取策略，与property的attributes一一对应，如下表：

Policy	@property
OBJC_ASSOCIATION_ASSIGN	@property(assign) / @property(unsafe_unretained)
OBJC_ASSOCIATION_RETAIN_NONATOMIC	@property(nonatomic, strong)
OBJC_ASSOCIATION_COPY_NONATOMIC	@property(nonatomic, copy)
OBJC_ASSOCIATION_RETAIN	@property(atomic, strong)
OBJC_ASSOCIATION_COPY	@property(atomic, copy)

P.S. 这里的remove函数是移除所有的关联对象，因此如果要单独删去某一个的话只要用set函数将其置为nil即可。

快速Encode和Decode

- (instancetype)initWithCoder:(NSCoder *)coder
{
  self = [super init];
  if (self) {
    unsigned int cnt = 0;
    Ivar *vars = class_copyIvarList([self class], &cnt);
    for(int i = 0; i < cnt; i++){
      NSString *key = [NSString stringWithUTF8String:ivar_getName(vars[i])];
      id value = [coder decodeObjectForKey:key];
      [self setValue:value forKey:key];
    }
  }
  return self;
}
- (void)encodeWithCoder:(nonnull NSCoder *)coder {
  unsigned int cnt = 0;
  Ivar *vars = class_copyIvarList([self class], &cnt);
  for(int i = 0; i < cnt; i++){
    NSString *key = [NSString stringWithUTF8String:ivar_getName(vars[i])];
    id value = [self valueForKey:key];
    [coder encodeObject:value forKey:key];
  }
}

Tips，现在的类在遵循NSCoding的同时还需要遵循NSSecureCoding，且使用这两个协议的方法如下：

NSData *data = [NSKeyedArchiver archivedDataWithRootObject:stu 
                requiringSecureCoding:YES 
                error:nil];
Student *stu2 = [NSKeyedUnarchiver unarchivedObjectOfClasses:
          [NSSet setWithObjects:[Student class], 
                      [NSMutableArray class], 
                      [NSMutableDictionary class], nil] 
                  fromData:data 
                  error:nil];
// 切记存在非常规类型的时候需要用unarchivedObjectofClasses这个方法并定义好NSSet类型的classes

Yiming

Coding for fun ~