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IOS 消息转发
最近在看消息转发的资料，发现大部分都是理论知识，很少有完整的代码。现在以代码的形式形象的解释一下：
用Xcode创建一个工程
1.正常方法调用
创建一个类Person 代码如下
Person.h代码如下：
#import <Foundation/Foundation.h>@interface Person : NSObject- (instancetype)init NS_UNAVAILABLE;- (instancetype)initWithUserName:(NSString *)userName;- (void)logUserName;@end
Person.m代码如下：
#import "Person.h"@interface Person()@property (nonatomic, copy) NSString *userName;@end@implementation Person- (instancetype)initWithUserName:(NSString *)userName{ self = [super init]; if (self) { _userName = [userName copy]; } return self;}- (void)logUserName{ NSLog(@"userName = %@", self.userName);}@end
ViewController.m代码如下：
#import "ViewController.h"#import "Person.h"@interface ViewController ()@end@implementation ViewController- (void)viewDidLoad { [super viewDidLoad]; // Do any additional setup after loading the view, typically from a nib. Person *person = [[Person alloc] initWithUserName:@"小王"]; [person logUserName];}- (void)didReceiveMemoryWarning { [super didReceiveMemoryWarning]; // Dispose of any resources that can be recreated.}@end
运行工程结果为：
2017-03-01 22:47:07.296 RunTimeDemo[24364:1776826] userName = 小王
结果正常
2. unrecognized selector 情况
把Person.m 的logUserName方法删除，此时Person.m 的代码如下：
#import "Person.h"@interface Person()@property (nonatomic, copy) NSString *userName;@end@implementation Person- (instancetype)initWithUserName:(NSString *)userName{ self = [super init]; if (self) { _userName = [userName copy]; } return self;}@end
运行工程会出现 如下结果
2017-03-01 23:06:25.788 RunTimeDemo[24729:1788071] -[Person logUserName]: unrecognized selector sent to instance 0x608000018e002017-03-01 23:06:25.796 RunTimeDemo[24729:1788071] *** Terminating app due to uncaught exception ‘NSInvalidArgumentException‘, reason: ‘-[Person logUserName]: unrecognized selector sent to instance 0x608000018e00‘
假如不在Person类中实现logUsrName这个方法，也可以让工程正常运行，此时就涉及到消息转发，Objective-C运行时会给出三次拯救程序崩溃的机会。
如下：
(1).Method resolution
objc运行时会调用+resolveInstanceMethod:或者 +resolveClassMethod:，让你有机会提供一个函数实现。如果你添加了函数，那运行时系统就会重新启动一次消息发送的过程，否则 ，运行时就会移到下一步，消息转发（Message Forwarding）。
(2).Fast forwarding
如果目标对象实现了-forwardingTargetForSelector:，Runtime 这时就会调用这个方法，给你把这个消息转发给其他对象的机会。 只要这个方法返回的不是nil和self，整个消息发送的过程就会被重启，当然发送的对象会变成你返回的那个对象。否则，就会继续Normal Fowarding。 这里叫Fast，只是为了区别下一步的转发机制。因为这一步不会创建任何新的对象，但下一步转发会创建一个NSInvocation对象，所以相对更快点。
(3).Normal forwarding
这一步是Runtime最后一次给你挽救的机会。首先它会发送-methodSignatureForSelector:消息获得函数的参数和返回值类型。如果-methodSignatureForSelector:返回nil，Runtime则会发出-doesNotRecognizeSelector:消息，程序这时也就挂掉了。如果返回了一个函数签名，Runtime就会创建一个NSInvocation对象并发送-forwardInvocation:消息给目标对象。
下面就验证一下上面所对应的函数的执行顺序：
Person.m的代码如下：
#import "Person.h"#define LOG_FunctionName NSLog(@"%s", __func__)@interface Person()@property (nonatomic, copy) NSString *userName;@end@implementation Person- (instancetype)initWithUserName:(NSString *)userName{ self = [super init]; if (self) { _userName = [userName copy]; } return self;}+ (BOOL)resolveInstanceMethod:(SEL)sel{ LOG_FunctionName; return YES;}- (id)forwardingTargetForSelector:(SEL)aSelector{ LOG_FunctionName; return [super forwardingTargetForSelector:aSelector];}- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector{ LOG_FunctionName; return [super methodSignatureForSelector:aSelector];}- (void)forwardInvocation:(NSInvocation *)anInvocation{ LOG_FunctionName;}@end
运行结果为：
2017-03-01 23:45:21.723 RunTimeDemo[25672:1817388] +[Person resolveInstanceMethod:]2017-03-01 23:45:21.724 RunTimeDemo[25672:1817388] -[Person forwardingTargetForSelector:]2017-03-01 23:45:21.724 RunTimeDemo[25672:1817388] -[Person methodSignatureForSelector:]2017-03-01 23:45:21.724 RunTimeDemo[25672:1817388] +[Person resolveInstanceMethod:]2017-03-01 23:45:21.725 RunTimeDemo[25672:1817388] -[Person logUserName]: unrecognized selector sent to instance 0x60000001cef02017-03-01 23:45:21.729 RunTimeDemo[25672:1817388] *** Terminating app due to uncaught exception ‘NSInvalidArgumentException‘, reason: ‘-[Person logUserName]: unrecognized selector sent to instance 0x60000001cef0‘
运行结果正好验证了上述说明。
IOS 消息转发