runtime这个东西,项目是很少用到的,但面试又避不可少,了解其内部的机制对底层的理解还是很有必要的。
1.动态添加属性
拓展类别属性的简单实现
a.定义字面量指针 static char dynamicAttributes;
b.设置属性 objc_setAssociatedObject(self,&dynamicAttributes,(id)value,OBJC_ASSOCIATION_COPY_NONATOMIC|OBJC_ASSCOCIATION_RETAIN)
c.获取属性值 objc_getAsscociatedObject(self,&dynamicAttributes)
2.model转字典
每个类中都有一个属性列表,我们要做的就是遍历该列表,利用KVC取值,装入字典
unsigned int count = 0; objc_property_t *ptyList = class_copyPropertyList(self.class,&count); NSMutableDictonary *dic = [NSMutableDictionary dictionary]; for (int i=0;i<count;i++) { objc_property pty = ptyList[i]; const char *cname = property_getName(pty); NSString *name = [NSString stringWithUTF8String:cname]; if (name.length > 0) { id value = [self valueForKey:name]; [dic setObject:value forKey:name]; } }
3.方法交换
C中方法是编译时就得实现。OC中方法是编译时无需实现,可在运行是动态插入方法及实现。利用这一点,可以轻松实现运行时方法的动态交换。
因为load方法早于main方法,并且不会覆盖父类实现,为了提高代码的可读性,一半是在分类中实现相关方法的交换。
+ load { SEL original = @selector(willMoveToSuperView:); SEL exchange = @selector(gl_willMoveToSuperView:); Method originalMethod = class_getInstanceMethod(self.class,original); Method exchangeMethod = class_getInstanceMethod(self.class,exchange); // 动态添加方法,若选择器存在方法实现,会失败 BOOL isAdd = class_addMethod(self,original,method_getImplementation(exchangeMethod),method_getTypeEncoding(exchangeMethod)); if (isAdd) { // 添加成功,将新的选择器替换为旧实现 class_replaceMethod(self,exchange,method_getImplementation(originalMethod),method_getTypeEncoding(originalMethod)); }else { // 直接交换两方法实现 method_changeImplementations(originalMethod,exchangeMethod); } } - (void)gl_villMoveToSuperView:(UIView *)superView { // do something ... }
isa指针
是一个指向所属类的指针。OC中消息机制是依靠objc_msgSend(receiver,selector)这个函数发送消息的。
objc_msgSend会根据实例对象的isa指针查找对象的类,然后查找该类的dispatch_table中的selector,找不到就依次查找父类,直到NSObject类。实在找不到方法就抛出异常。找到selector后,会根据dispatch_table中的内存地址该selector。为了提高转发效率,系统会将所有的selector地址和已使用的selector地址缓存起来,通过类的形式划分不同的缓冲区域。obj_msgSend去查找dispatch_table前,会先检查该类的缓存,如果缓存命中,就直接调用selector。
消息转发机制(前提是dispatch_table找不到对应的selector)
1.对象收到无法解读的消息后,首先会调用resolveInstanceMethod:询问是否有动态添加方法来处理
void speak(id self, SEL _cmd){ NSLog(@"Now I can speak."); } + (BOOL)resolveInstanceMethod:(SEL)sel { NSLog(@"resolveInstanceMethod: %@", NSStringFromSelector(sel)); if (sel == @selector(speak)) { class_addMethod([self class], sel, (IMP)speak, "V@:"); return YES; } return [super resolveInstanceMethod:sel]; }
2.第一步若没有添加新方法,那就询问有没别人帮忙处理,调用的是forwardingTargetForSelector:
- (id)forwardingTargetForSelector:(SEL)aSelector { NSLog(@"forwardingTargetForSelector: %@", NSStringFromSelector(aSelector)); Bird *bird = [[Bird alloc] init]; if ([bird respondsToSelector: aSelector]) { return bird; } return [super forwardingTargetForSelector: aSelector]; } // Bird.m - (void)fly { NSLog(@"I am a bird, I can fly."); }
3.当前两步均没有响应时,走到第三部,调用forwardInvocation:,该方法会首先调用methodSignatureForSelector:方法获取选择子的方法签名
- (void)forwardInvocation:(NSInvocation *)anInvocation { NSLog(@"forwardInvocation: %@", NSStringFromSelector([anInvocation selector])); if ([anInvocation selector] == @selector(speak)) { Monkey *monkey = [[Monkey alloc] init]; [anInvocation invokeWithTarget:monkey]; } } - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector { NSLog(@"method signature for selector: %@", NSStringFromSelector(aSelector)); if (aSelector == @selector(code)) { return [NSMethodSignature signatureWithObjCTypes:"V@:@"]; } return [super methodSignatureForSelector:aSelector]; }
4.若前三步均没有处理,则抛出异常doesNotRecognizeSelector:
NSInvocation
对方法的另一种封装。相对于performSelector:withObject:只能传2参数的弊端,invocation类可以设置封装多参数
a.对选择子签名
NSMethodSignature *instanceSignature = [self instanceMethodSignatureWithSelector:@selector(run:)];//实例方法签名
NSMethodSignature *classSignature = [self methodSignatureWithSelector:@selector(run:)];//类方法签名
b.实例化invocation,并设置参数
NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature]; //设置方法调用者 invocation.target = self; //注意:这里的方法名一定要与方法签名类中的方法一致 invocation.selector = @selector(run:); NSString *way = @"byCar"; //这里的Index要从2开始,以为0跟1已经被占据了,分别是self(target),selector(_cmd) [invocation setArgument:&way atIndex:2]; //3、调用invoke方法 [invocation invoke];
c.获取invocation返回值
id res = nil; if (signature.methodReturnLength != 0) {//有返回值 //将返回值赋值给res [invocation getReturnValue:&res]; } return res;
IMP指针
指向选择子方法实现的指针,直接调用它相对于调用方法,会提高程序运行效率。
相对于Method的方法交换,method_exchangeImplementations(method1,method2),使用IMP指针可以简化其对方法的额外实现,显得更加优雅。
如果想IMP指针带参数或者返回值,需要将proprecessing:enable strict checking of objc_msgSend calls 配置为NO,默认是YES,表示IMP无参无返回值。
typedef id (*_IMP)(id,SEL,...); typedef void (*_VIMP)(id,SEL,...); + (void)load { static dispatch_once onceToken; dispatch_once(&onceToken,^{ Method viewDidLoad = class_getInstanceMethod(self,@selector(viewDidLoad)); _IMP viewDidLoad_IMP = (_IMP)method_getImplementation(viewDidLoad); method_setImplementation(viewDidLoad,imp_implementationWithBlock(^(id target,SEL action){ viewDidLoad_IMP(target,@selector(viewDidLoad)); // 新增代码 do extra things })); }); }
最后贴一张class的内部结构图,容以后细细研究