iOS Principle：Runtime（中）

Objc.h
/// An opaque type that represents a method selector.代表一个方法的不透明类型
typedef struct objc_selector *SEL;

@property SEL selector;

A method selector is a C string that has been registered (or “mapped“) with the Objective-C runtime. Selectors generated by the compiler are automatically mapped by the runtime when the class is loaded.

- (void)caculate(NSInteger)num;
- (void)caculate(CGFloat)num;

- (void)caculateWithInt(NSInteger)num;
- (void)caculateWithFloat(CGFloat)num;

/// A pointer to the function of a method implementation.  指向一个方法实现的指针
typedef id (*IMP)(id, SEL, ...);

struct category_t { 
    const char *name; 
    classref_t cls; 
    struct method_list_t *instanceMethods; 
    struct method_list_t *classMethods;
    struct protocol_list_t *protocols;
    struct property_list_t *instanceProperties;
};

+(BOOL)resolveInstanceMethod:(SEL)name

BOOL class_addMethod(Class cls, SEL name, IMP imp, const char *types);

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    //执行foo函数
    [self performSelector:@selector(foo:)];
}
+ (BOOL)resolveInstanceMethod:(SEL)sel {
    if (sel == @selector(foo:)) {//如果是执行foo函数，就动态解析，指定新的IMP
        class_addMethod([self class], sel, (IMP)fooMethod, "v@:");
        return YES;
    }
    return [super resolveInstanceMethod:sel];
}
void fooMethod(id obj, SEL _cmd) {
    NSLog(@"Doing foo");//新的foo函数
}

- (id)forwardingTargetForSelector:(SEL)aSelector;

#import "ViewController.h"
#import "objc/runtime.h"
@interface Person: NSObject
@end
@implementation Person
- (void)foo {
    NSLog(@"Doing foo");//Person的foo函数
}
@end
@interface ViewController ()
@end
@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    //执行foo函数
    [self performSelector:@selector(foo)];
}
+ (BOOL)resolveInstanceMethod:(SEL)sel {
    return YES;//返回YES，进入下一步转发
}
- (id)forwardingTargetForSelector:(SEL)aSelector {
    if (aSelector == @selector(foo)) {
        return [Person new];//返回Person对象，让Person对象接收这个消息
    }
    return [super forwardingTargetForSelector:aSelector];
}
@end

- (void)forwardInvocation: (NSInvocation*)invocation;

#import "ViewController.h"
#import "objc/runtime.h"
@interface Person: NSObject
@end
@implementation Person
- (void)foo {
    NSLog(@"Doing foo");//Person的foo函数
}
@end
@interface ViewController ()
@end
@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    //执行foo函数
    [self performSelector:@selector(foo)];
}
+ (BOOL)resolveInstanceMethod:(SEL)sel {
    return YES;//返回YES，进入下一步转发
}
- (id)forwardingTargetForSelector:(SEL)aSelector {
    return nil;//返回nil，进入下一步转发
}
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    if ([NSStringFromSelector(aSelector) isEqualToString:@"foo"]) {
        return [NSMethodSignature signatureWithObjCTypes:"v@:"];//签名，进入forwardInvocation
    }
    return [super methodSignatureForSelector:aSelector];
}
- (void)forwardInvocation:(NSInvocation *)anInvocation {
    SEL sel = anInvocation.selector;
    Person *p = [Person new];
    if([p respondsToSelector:sel]) {
        [anInvocation invokeWithTarget:p];
    } else {
        [self doesNotRecognizeSelector:sel];
    }
}
@end