Runtime things (message mechanism)

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1. About runtime

I have used runtime to solve the problem of changing the global font in a project before, so I once again felt the power of runtime black magic. Now I have the opportunity to share some of my understanding of runtime. Object calling method is a frequently used function in Objective-C, that is, message passing. Objective-C is a superset of C, so unlike C, Objective-C uses dynamic binding, that is, runtime. I won't say much about Objective-C's message passing and message mechanism. Today I will mainly talk about dynamic methods, that is, function calling.

2. Several related functions

The following figure summarizes in detail the order of each function call and the prerequisites for execution

Message passing function calls

1. When an object receives a message that it cannot interpret, it first calls the

 + (BOOL)resolveInstanceMethod:(SEL)sel

This method will be executed at runtime if the IML of SEL is not found. This function gives the class the opportunity to add functions using class_addMethod. According to the documentation, if the added function code is implemented, it returns YES, and if it is not implemented, it returns NO. For example, I created a new project. First, I executed the doSomething1 method in the ViewController class. The code is as follows

 //  
 // ViewController.m  
 // RuntimeTest1  
 //  
 // Created by HenryCheng on 15/12/24.  
 // Copyright ? (Copyright symbol) 2015 www.igancao.com All rights reserved.  
 //   
  
 # import   "ViewController.h"   
  
 @interface ViewController () 
  
 @end   
  
 @implementation ViewController 
  
 - ( void )viewDidLoad {
 [ super viewDidLoad];
    [self performSelector: @selector (doSomething)];
 } 
  
 - ( void )didReceiveMemoryWarning {
    [ super didReceiveMemoryWarning];
 // Dispose of any resources that can be recreated.  
 } 
  
 @end  

Operation Results

 ** 2015-12-24​​​​   10 : 35 : 37.726 RuntimeTest1[ 1877 : 337842 ]-[ViewController doSomething]: unrecognized selector sent to instance 0x7fe9f3736680 **
 ** 2015-12-24​​​   10 : 35 : 37.729 RuntimeTest1[ 1877 : 337842 ] *** Terminating app due to uncaught exception 'NSInvalidArgumentException' , reason: '-[ViewController doSomething]: unrecognized selector sent to instance 0x7fe9f3736680' **
 ***** First throw call stack:**

As expected, the program crashes because the doSomething method is not found. Next, we implement the + (BOOL)resolveInstanceMethod:(SEL)sel method in it and determine if SEL is doSomething and output add method here

 //  
 // ViewController.m  
 // RuntimeTest1  
 //  
 // Created by HenryCheng on 15/12/24.  
 // Copyright ? (Copyright symbol) 2015 www.igancao.com All rights reserved.  
 //   
  
 # import   "ViewController.h"   
  
 @interface ViewController () 
  
 @end   
  
 @implementation ViewController 
  
 - ( void )viewDidLoad {
 [ super viewDidLoad];
    [self performSelector: @selector (doSomething)];
 } 
  
 + (BOOL)resolveInstanceMethod:(SEL)sel {
 if (sel == @selector (doSomething)) {
        NSLog(@ "add method here" );
 return YES;
 }
 return [ super resolveInstanceMethod:sel];
 } 
  
 - ( void )didReceiveMemoryWarning {
    [ super didReceiveMemoryWarning];
 // Dispose of any resources that can be recreated.  
 } 
  
 @end  

Continue running and see the log

 ** 2015-12-24​​​​   10 : 47 : 24.687 RuntimeTest1[ 2007 : 382077 ] add method here**
 ** 2015-12-24​​​   10 : 47 : 24.687 RuntimeTest1[ 2007 : 382077 ]-[ViewController doSomething]: unrecognized selector sent to instance 0x7f9568c331f0 **
 ** 2015-12-24​​​   10 : 47 : 24.690 RuntimeTest1[ 2007 : 382077 ] *** Terminating app due to uncaught exception 'NSInvalidArgumentException' , reason: '-[ViewController doSomething]: unrecognized selector sent to instance 0x7f9568c331f0' **
 ***** First throw call stack:**

You can see that the program still crashes, but we can see that add method here is output, which means that our + (BOOL)resolveInstanceMethod:(SEL)sel method has been executed and entered the judgment. Therefore, here, we can do something to make this method respond so that it will not crash in the ***- (void)doesNotRecognizeSelector:(SEL)aSelector method. Next, we continue to operate as follows

 //  
 // ViewController.m  
 // RuntimeTest1  
 //  
 // Created by HenryCheng on 15/12/24.  
 // Copyright ? (Copyright symbol) 2015 www.igancao.com All rights reserved.  
 //   
  
 # import   "ViewController.h"   
  
 # import [objc/runtime.h] (Due to recognition issues, the angle brackets are changed to square brackets here) 
  
 @interface ViewController () 
  
 @end   
  
 @implementation ViewController 
  
 - ( void )viewDidLoad {
 [ super viewDidLoad];
    [self performSelector: @selector (doSomething)];
 } 
  
 + (BOOL)resolveInstanceMethod:(SEL)sel {
 if (sel == @selector (doSomething)) {
        NSLog(@ "add method here" );
        class_addMethod([self class ], sel, (IMP)dynamicMethodIMP, "v@:" );
 return YES;
 }
 return [ super resolveInstanceMethod:sel];
 }
 void dynamicMethodIMP (id self, SEL _cmd) {
    NSLog(@ "doSomething SEL" );
 } 
  
 - ( void )didReceiveMemoryWarning {
    [ super didReceiveMemoryWarning];
 // Dispose of any resources that can be recreated.  
 } 
  
 @end  

Imported and executed the class_addMethod method in + (BOOL)resolveInstanceMethod:(SEL)sel, then defined a void dynamicMethodIMP (id self, SEL _cmd) function, ran the project, and looked at the log

 ** 2015-12-24​​​​   11 : 45 : 11.934 RuntimeTest1[ 2284 : 478571 ] add method here**
 ** 2015-12-24​​​   11 : 45 : 11.934 RuntimeTest1[ 2284 : 478571 ] doSomething SEL**

At this time, we found that the program did not crash, and it also output doSomething SEL, which means that we have successfully added a method to our class through the runtime. The class_addMethod method is defined as follows

• cls adds the method to this class, that is, the class to which the method is added

• name method name, you can choose anything you want

• imp is the function that implements this method

• types is a string that defines the return value type and parameter type of the number. For example, "v@:", where v is void, the return type is empty, @ represents the parameter, here it refers to id (self), here: refers to the method SEL (_cmd), for example, I define a function

 int newMethod (id self, SEL _cmd, NSString *str) { 
  
 return   100 ;
 }

Then the method to add this function should be ass_addMethod([self class], @selector(newMethod), (IMP)newMethod, "i@:@");

2. If the method is not found or added in + (BOOL)resolveInstanceMethod:(SEL)sel

The message continues to be passed down to - (id)forwardingTargetForSelector:(SEL)aSelector to see if there is an object that can execute this method. Let's rebuild a project and create a new class called SecondViewController, which has a - (void)secondVCMethod method as follows

 //  
 // SecondViewController.m  
 // RuntimeTest2  
 //  
 // Created by HenryCheng on 15/12/24.  
 // Copyright ? (Copyright symbol) 2015 www.igancao.com All rights reserved.  
 //   
  
 # import   "SecondViewController.h"   
  
 @interface SecondViewController () 
  
 @end   
  
 @implementation SecondViewController 
  
 - ( void )viewDidLoad {
 [ super viewDidLoad];
 // Do any additional setup after loading the view.  
 } 
  
 - ( void )secondVCMethod {
    NSLog(@ "This is secondVC method !" );
 } 
  
 - ( void )didReceiveMemoryWarning {
    [ super didReceiveMemoryWarning];
 // Dispose of any resources that can be recreated.  
 } 
  
 /*
 #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  

The project structure should be like this

Project Catalog

Now I want to call the - (void)secondVCMethod method in ViewController. We know that ViewController and SecondViewController have no inheritance relationship. If we follow the normal steps, the program will definitely crash directly because it cannot find the - (void)secondVCMethod method in ViewController.

 //  
 // ViewController.m  
 // RuntimeTest2  
 //  
 // Created by HenryCheng on 15/12/24.  
 // Copyright ? (Copyright symbol) 2015 www.igancao.com All rights reserved.  
 //   
  
 # import   "ViewController.h"  
 # import   @interface ViewController () 
  
 @end   
  
 @implementation ViewController 
  
 - ( void )viewDidLoad {
 [ super viewDidLoad];
     [self performSelector: @selector (secondVCMethod)];
 } 
  
 - ( void )didReceiveMemoryWarning {
    [ super didReceiveMemoryWarning];
 // Dispose of any resources that can be recreated.  
 } 
  
 @end  

Operation Results

 ** 2015-12-24​​​​   13 : 54 : 44.314 RuntimeTest2[ 3164 : 835814 ]-[ViewController secondVCMethod]: unrecognized selector sent to instance 0x7fc3a8535c10 **
 ** 2015-12-24​​​   13 : 54 : 44.317 RuntimeTest2[ 3164 : 835814 ] *** Terminating app due to uncaught exception 'NSInvalidArgumentException' , reason: '-[ViewController secondVCMethod]: unrecognized selector sent to instance 0x7fc3a8535c10' **
 ***** First throw call stack:**

Now let's process this message as follows

 //  
 // ViewController.m  
 // RuntimeTest2  
 //  
 // Created by HenryCheng on 15/12/24.  
 // Copyright ? (Copyright symbol) 2015 www.igancao.com All rights reserved.  
 //   
  
 # import   "ViewController.h"  
 # import   @interface ViewController () 
  
 @end   
  
 @implementation ViewController 
  
 - ( void )viewDidLoad {
 [ super viewDidLoad];
     [self performSelector: @selector (secondVCMethod)];
 } 
  
 - (id)forwardingTargetForSelector:(SEL)aSelector {
    Class class = NSClassFromString(@ "SecondViewController" );
    UIViewController *vc = class . new ;
 if (aSelector == NSSelectorFromString(@ "secondVCMethod" )) {
        NSLog(@ "secondVC do this!" );
 return vc;
 } 
      
 return nil;
 } 
  
 + (BOOL)resolveInstanceMethod:(SEL)sel { 
  
 return [ super resolveInstanceMethod:sel];
 } 
  
 - ( void )didReceiveMemoryWarning {
    [ super didReceiveMemoryWarning];
 // Dispose of any resources that can be recreated.  
 }
 @end  

Operation Results

 ** 2015-12-24​​​​   14 : 00 : 34.168 RuntimeTest2[ 3284 : 870957 ]secondVC do   this !**
 ** 2015-12-24​​​   14 : 00 : 34.169 RuntimeTest2[ 3284 : 870957 ] This is secondVC method!**

We will find that - (void)secondVCMethod is executed and the program does not crash. The reason is that at this step

 - (id)forwardingTargetForSelector:(SEL)aSelector {
    Class class = NSClassFromString(@ "SecondViewController" );
    UIViewController *vc = class . new ;
 if (aSelector == NSSelectorFromString(@ "secondVCMethod" )) {
        NSLog(@ "secondVC do this!" );
 return vc;
 } 
      
 return nil;
 }

When the - (void)secondVCMethod method is not found, the message continues to be passed until - (id)forwardingTargetForSelector:(SEL)aSelector, and then I create a SecondViewController object in it, and if this method exists, I return the SecondViewController object. This function is the message forwarding. Here we successfully pass the message to SecondViewController and let it execute, so the method is executed. At the same time, it is equivalent to completing a multiple inheritance!

3. ***

Of course, there are several more functions, which are clearly expressed in the above picture. If you are interested, you can try it yourself to see what the order of message delivery is. The above-mentioned knowledge is just the tip of the iceberg of runtime. The power of runtime black magic is far more than this, such as method swizzling, which is still very useful in actual project combat. I will introduce it later when I have time.

refer to

• Objective-C Runtime Reference

• Objective-C Runtime Programming Guide