1动态方法解析

如果一个对象调用了在这个类中方法列表中找不到选择子（SEL）,这个时候可以使用为这个选择子添加一个方法

#import "ViewController.h"
#import <objc/runtime.h>
#import <objc/message.h>
@interface ViewController ()

@end

@implementation ViewController
void noParameterMethod (id self , SEL sel);
- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    [self performSelector:@selector(method:) withObject:@"123"];
    [self performSelector:@selector(noParameterMethod)];
}

void method (id self ,SEL sel ,id objc){
    NSLog(@"%@",objc);
}
void noParameterMethod (id self , SEL sel){
    NSLog(@"无参数的函数");
}

+(BOOL)resolveInstanceMethod:(SEL)sel{
    NSLog(@"%@",NSStringFromSelector(sel));
    if ([NSStringFromSelector(sel) hasPrefix:@"method"]) {
        class_addMethod(self, sel, (IMP)method, "v@:@");
        return YES;
    }else if ([NSStringFromSelector(sel) hasPrefix:@"noParameterMethod"]){
        class_addMethod(self, sel, (IMP)noParameterMethod, "v@:");
        return YES;
    }else{
        return [super resolveInstanceMethod:sel];
    }
}

@end

打印信息为

备援接受者

可以把自己不能识别的选择子交给其他对象进行处理

 [self performSelector:@selector(forwardSelecter)];
-(id)forwardingTargetForSelector:(SEL)aSelector{
   NSString * selectorString =  NSStringFromSelector(aSelector);
    if ([selectorString isEqualToString:@"forwardSelecter"]) {
        SecondViewController * vc = [[SecondViewController alloc]init];
        return vc;
    }else{
        return [super forwardingTargetForSelector:aSelector];
    }
}


#import "SecondViewController.h"

@interface SecondViewController ()

@end

@implementation SecondViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
}

-(void)forwardSelecter{
    NSLog(@"备源消息转发");
}

完整的消息转发

  [self performSelector:@selector(normalForwardSelector)];
    [self performSelector:@selector(normalForwardPaSelector:) withObject:@"带有参数456"];

-(void)forwardInvocation:(NSInvocation *)anInvocation{
    SEL sel =  anInvocation.selector;
    
    SecondViewController * vc = [[SecondViewController alloc]init];
    if ([vc respondsToSelector:sel]) {
        [anInvocation invokeWithTarget:vc];
    }else{
        [self doesNotRecognizeSelector:sel];
    }
}

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


#import "SecondViewController.h"

@interface SecondViewController ()

@end

@implementation SecondViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
}

-(void)forwardSelecter{
    NSLog(@"备源消息转发");
}
-(void)normalForwardSelector{
    NSLog(@"完整的消息转发");
}
-(void)normalForwardPaSelector:(NSString *)objc{
    NSLog(@"%@",objc);
}
@end

Objective-C 中给一个对象发送消息会经过以下几个步骤：

在对象类的 dispatch table 中尝试找到该消息。如果找到了，跳到相应的函数IMP去执行实现代码；

如果没有找到，Runtime 会发送 +resolveInstanceMethod: 或者 +resolveClassMethod: 尝试去 resolve 这个消息；

如果 resolve 方法返回 NO，Runtime 就发送-forwardingTargetForSelector: 允许你把这个消息转发给另一个对象；

如果没有新的目标对象返回， Runtime 就会发送-methodSignatureForSelector: 和 -forwardInvocation:消息。你可以发送 -invokeWithTarget:消息来手动转发消息或者发送 -doesNotRecognizeSelector: 抛出异常。