第三篇 Runtime消息转发

秒速五厘米 2022-05-21 12:10 233阅读 0赞

发送消息时会在对应的类对象中搜索方法列表，如果找不到则会沿着继承树向上一直搜索知道继承树根部（通常为NSObject），如果还是找不到并且消息转发都失败了就回执行`doesNotRecognizeSelector:`方法报`unrecognized selector`错。

消息转发到底是什么呢？接下来将会逐一介绍最后的三次机会。  
1、动态方法解析  
2、备用接收者  
3、完整消息转发

动态方法解析  
首先，Objective-C运行时会调用`+resolveInstanceMethod:`或者`+resolveClassMethod:`，让你有机会提供一个函数实现。  
如果你添加了函数并返回`YES`， 那运行时系统就会重新启动一次消息发送的过程。

备用接收者  
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。

完整消息转发  
如果在上一步还不能处理未知消息，则唯一能做的就是启用完整的消息转发机制了。  
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。

签名参数`"v@:"`

示例：执行某个没有实现的方法时，会抛出异常并造成crash，但使用runtime运行时特性进行动态方法解析，或使用备用接收者，或进行完整的消息转发后，则可以很好的避免crash问题。

crash时的源码

执行结果，抛出异常，造成crash

2018-07-05 18:41:18.160643+0800 DemoRuntime[9374:632785] -[ViewController work:]: unrecognized selector sent to instance 0x7f917a610f50
    2018-07-05 18:41:18.209071+0800 DemoRuntime[9374:632785] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[ViewController work:]: unrecognized selector sent to instance 0x7f917a610f50'
    *** First throw call stack:
    (
     0   CoreFoundation                      0x000000010a1d21e6 __exceptionPreprocess + 294
     1   libobjc.A.dylib                     0x0000000109867031 objc_exception_throw + 48
     2   CoreFoundation                      0x000000010a253784 -[NSObject(NSObject) doesNotRecognizeSelector:] + 132
     3   UIKit                               0x000000010a87d6db -[UIResponder doesNotRecognizeSelector:] + 295
     4   CoreFoundation                      0x000000010a154898 ___forwarding___ + 1432
     5   CoreFoundation                      0x000000010a154278 _CF_forwarding_prep_0 + 120
     6   DemoRuntime                         0x0000000108f63745 -[ViewController viewDidLoad] + 277
     7   UIKit                               0x000000010a7f3131 -[UIViewController loadViewIfRequired] + 1215
     8   UIKit                               0x000000010a83a20c -[UINavigationController _updateScrollViewFromViewController:toViewController:] + 68
     9   UIKit                               0x000000010a83a4ea -[UINavigationController _startTransition:fromViewController:toViewController:] + 136
     10  UIKit                               0x000000010a83b61e -[UINavigationController _startDeferredTransitionIfNeeded:] + 870
     11  UIKit                               0x000000010a83c86c -[UINavigationController __viewWillLayoutSubviews] + 150
     12  UIKit                               0x000000010aa94d0b -[UILayoutContainerView layoutSubviews] + 231
     13  UIKit                               0x000000010a71e7a8 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 1515
     14  QuartzCore                          0x00000001101bc456 -[CALayer layoutSublayers] + 177
     15  QuartzCore                          0x00000001101c0667 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 395
     16  QuartzCore                          0x00000001101470fb _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 343
     17  QuartzCore                          0x000000011017479c _ZN2CA11Transaction6commitEv + 568
     18  UIKit                               0x000000010a669269 __34-[UIApplication _firstCommitBlock]_block_invoke_2 + 141
     19  CoreFoundation                      0x000000010a174b0c __CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__ + 12
     20  CoreFoundation                      0x000000010a1592db __CFRunLoopDoBlocks + 331
     21  CoreFoundation                      0x000000010a158a84 __CFRunLoopRun + 1284
     22  CoreFoundation                      0x000000010a15830b CFRunLoopRunSpecific + 635
     23  GraphicsServices                    0x000000010f345a73 GSEventRunModal + 62
     24  UIKit                               0x000000010a64f057 UIApplicationMain + 159
     25  DemoRuntime                         0x0000000108f645ff main + 111
     26  libdyld.dylib                       0x000000010dc2f955 start + 1
     27  ???                                 0x0000000000000001 0x0 + 1
     )
    libc++abi.dylib: terminating with uncaught exception of type NSException
    (lldb)

方案一：动态解析进行方法转发

#import "ViewController.h"
    #import <objc/runtime.h>
    #import <objc/message.h>
    
    @interface ViewController ()
    
    @end
    
    @implementation ViewController
    
    - (void)viewDidLoad {
        [super viewDidLoad];
        // Do any additional setup after loading the view, typically from a nib.
    
        self.title = @"runtime";
    
        // 执行work函数
        [self performSelector:@selector(work:)];
    }
    
    
    
    #pragma mark - 消息转发（方法添加）
    
    + (BOOL)resolveInstanceMethod:(SEL)sel
    {
        if (sel == @selector(work:))
        {
            // 如果是执行work函数，就动态解析，指定新的IMP
            class_addMethod([self class], sel, (IMP)workMethod, "v@:");
            return YES;
        }
        return [super resolveInstanceMethod:sel];
    }
    
    void workMethod(id obj, SEL _cmd)
    {
        // 新的work函数
        NSLog(@"Doing work");
    }
    
    - (void)didReceiveMemoryWarning {
        [super didReceiveMemoryWarning];
        // Dispose of any resources that can be recreated.
    }
    
    @end

方法二 使用备用接收者

#import <Foundation/Foundation.h>
    
    @interface Person : NSObject
    
    @end
    
    
    #import "Person.h"
    #import <objc/runtime.h>
    
    @implementation Person
    
    - (void)work
    {
        NSLog(@"[%@]%@ is working now~", self.class, self.name);
    }
    
    @end

方案三 消息完整转发

#import "ViewController.h"
    #import "Person.h"
    #import <objc/runtime.h>
    #import <objc/message.h>
    
    @interface ViewController ()
    
    @end
    
    @implementation ViewController
    
    - (void)viewDidLoad {
        [super viewDidLoad];
        // Do any additional setup after loading the view, typically from a nib.
    
        self.title = @"runtime";
    
        // 执行work函数
        [self performSelector:@selector(work)];
    }
    
    #pragma mark - 消息完整转发
    
    + (BOOL)resolveInstanceMethod:(SEL)sel
    {
        // 返回YES，进入下一步转发
        return YES;
    }
    
    - (id)forwardingTargetForSelector:(SEL)aSelector
    {
        // 返回nil，进入下一步转发
        return nil;
    }
    
    - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
    {
        if ([NSStringFromSelector(aSelector) isEqualToString:@"work"])
        {
            // 签名，进入forwardInvocation
            return [NSMethodSignature signatureWithObjCTypes:"v@:"];
        }
    
        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];
        }
    }
    
    - (void)didReceiveMemoryWarning {
        [super didReceiveMemoryWarning];
        // Dispose of any resources that can be recreated.
    }
    
    @end