Message forwarding mechanism -- save your program from crashing

Preface ：

I have always known iOS Message forwarding mechanism in , I haven't summarized . This article is a summary of previous fragment knowledge . Mainly related to runtime Message passing and methods if no object is found , The system provides us with three rescue opportunities . I don't say much nonsense , Direct text .

 

Runtime Messaging for

We all know OC It's a dynamic language .OC Language is not directly recognized by computers . I need to convert to C Language , And then into assembly language , Finally, it turns into machine language recognized by computer .

When we call an object's method [objc foo] when , The compiler will convert the code into message sending objc_msgSend(objc foo). The specific implementation process is like this ：

1. First , The system goes through objc Of isa The pointer finds its corresponding class.
2. then , stay class Of method list Search method in the list of foo.
3. If no corresponding method is found , Just look in its parent class .
4. Find the corresponding method in this process , Just go through the method IMP The pointer （ Method address pointer ） Execution method .

Of course, sometimes we have no implementation method foo. For example, we call an immutable string modification method . Actually , The modification method is only implemented in the variable string class . Then the program will crash , Error message unrecognized selector. This method cannot be recognized . In fact, before this collapse , The system provides us with three opportunities to save , This is the message forwarding process .

forward

When the system cannot find the method you want to execute , Will provide three opportunities to save your program from crashing .

1、 Dynamic method analysis

oc At run time , Object method calls execute  +resolveInstanceMethod: Class method calls will execute  +resolveClassMethod: Method , In this way, we can , Perform a process . adopt runtime Dynamic adding method , And forward the message to the alternate receiving method .

You can see , I didn't realize foo: Method , But the running program did not crash , And implemented my new definition newFooMethod Method .

then , If we don't deal with this step accordingly , The program will proceed to the second step , Find the implementation object of the message .

 

2、 Define alternate recipients

If the first step , No rescue , Then the program will go to this step , The system will call - (id)forwardingTargetForSelector:(SEL)aSelector Method . Give you a chance to pass messages to other objects . therefore , If you want to save in this step . We need to define an alternate class , To process messages target. As long as we implement this method in our spare class , Then the system will execute this method .

#import <objc/runtime.h>

@interface OtherPerson : NSObject

@end

@implementation OtherPerson

// Definition foo Method , Handle the forwarded foo Method 
- (void)foo {
    NSLog(@"Person foo func");
}

@end

@implementation TestViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    [self performSelector:@selector(foo)];
    // Do any additional setup after loading the view.
}

void newFooMethod(){
    NSLog(@"Now I am new foo func.");
}
+ (BOOL)resolveInstanceMethod:(SEL)sel{

//    if(sel == @selector(foo)){
//        class_addMethod([self class], sel, (IMP)newFooMethod, "[email protected]:");
//        return YES;
//    }
    // Directly execute the system method , Don't take the above adding method , If you can't find it, you'll go down and forward 
    return [super resolveInstanceMethod:sel];
}

- (id)forwardingTargetForSelector:(SEL)aSelector{

    if(aSelector == @selector(foo)){
        return [OtherPerson new];// Set forwarded target processing method 
    }
    
    return [super forwardingTargetForSelector:aSelector];
}



/*
#pragma mark - Navigation

// In a storyboard-based application, you will often want to do a little preparation before navigation
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
    // Get the new view controller using [segue destinationViewController].
    // Pass the selected object to the new view controller.
}
*/

@end

Then run the code , I found it printed Person foo func. There was no collapse , explain foo Method has been forwarded to OtherPerson Class to execute .

3、 Full message forwarding

If the first two methods fail to save , Then the message forwarding goes to the last step . If the message cannot be processed at this step , that Runtime Will send out -doesNotRecognizeSelector: news , The program then hangs up .

Here we need a higher price to save , We need to get the method signature first . Pass the method signature to the system

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector Method . When this method receives the method signature ,runtime It creates a NSInvocation object , send out - (void)forwardInvocation:(NSInvocation *)anInvocation To the target .

#import <objc/runtime.h>

@interface OtherPerson : NSObject

@end

@implementation OtherPerson

- (void)foo {
    NSLog(@"Person foo func");
}

@end

@interface TestViewController ()

@end

@implementation TestViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.view.backgroundColor = [UIColor yellowColor];
    [self performSelector:@selector(foo)];
    // Do any additional setup after loading the view.
}

void newFooMethod(){
    NSLog(@"Now I am new foo func.");
}
+ (BOOL)resolveInstanceMethod:(SEL)sel{
//    if(sel == @selector(foo)){
        class_addMethod([self class], sel, (IMP)newFooMethod, "[email protected]:");
//        return NO;
//    }
    // Take a systematic approach , Method not found , Take the second step 
    return [super resolveInstanceMethod:sel];
}

- (id)forwardingTargetForSelector:(SEL)aSelector{

//    if(aSelector == @selector(foo)){
//        return [OtherPerson new];
//    }
    // The method of executing the system , Because there is no response class , So it will enter the third step , That's the method below 
    return [super forwardingTargetForSelector:aSelector];
}

// This is the last step to save the collapse 
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector{
    
    if([NSStringFromSelector(aSelector) isEqualToString:@"foo"]){

        // Method selector 
        SEL fooSEL = @selector(foo);
        // Method signature 
        NSMethodSignature *fooSignature = [NSString methodSignatureForSelector:fooSEL];
        // Returns a method signature 
        return fooSignature;
    }
    
    return [super methodSignatureForSelector:aSelector];
}

// Forward by method signature 
- (void)forwardInvocation:(NSInvocation *)anInvocation{
    
    SEL sel = anInvocation.selector;
    
    OtherPerson *person = [OtherPerson new];
    if([person respondsToSelector:sel]){
        [anInvocation invokeWithTarget:person];
    }else{
        // There is no way to respond 
        [self doesNotRecognizeSelector:sel];
    }
}

/*
#pragma mark - Navigation

// In a storyboard-based application, you will often want to do a little preparation before navigation
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
    // Get the new view controller using [segue destinationViewController].
    // Pass the selected object to the new view controller.
}
*/

@end

 

By running the program , Print the Person foo func, There was no program crash . thus , The message is forwarded three times and the rescue opportunity is over .

Next, let's talk about method signature

There are two ways to obtain method signatures , One is to manually generate method signatures , The other is to automatically generate method signatures .

Generate method signatures manually

Generate method signatures manually , You need to pass the return value of the method , Parameters and other information to generate signatures through methods . for example [NSMethodSignature signatureWithObjCTypes:"@@*"]

Here, the side parameter is an array of strings . This array contains the return value of the method 、 Parameter information . The first @ Indicates that the return value of the method is a id Object of type , And the second one. @ The representation method receives a id Parameters , final * The representation method receives a string （char*） Parameters of type . In fact, apple officially has a comparison table TypeEncoding

Automatic generation of method signature

In general , We generate method signatures automatically . That is, the form of the code above , Don't pass one c The string array of represents the return value parameter information of the method . It is also more accurate , There are two methods below , Can generate the signature of the method .

 //  Get the instance method signature from the class 
    fooSignature = [NSString instanceMethodSignatureForSelector:fooSEL];
    //  Get the class method signature from the class 
    fooSignature = [NSString methodSignatureForSelector:fooSEL];