IOS 消息转发

Posted 2020-09-02 Stephen Li--blog

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 0x608000018e00
2017-03-01 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 0x60000001cef0
2017-03-01 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。