Summary of the use of runtime in iOS

   // Create a person object  Person *p = [[Person alloc] init];

  // Call object method  [p eat];
  // SEL: Method number, you can find the corresponding method implementation according to the method number  [p performSelector:@selector(eat)];

  // Essence: Let the object send a message  objc_msgSend(p, @selector(eat));

  // Call class methods: two ways  // The first type converts the essential class name into a class object by calling the class name  [Person eat];
  // The second type is called through class object  [[Person class] eat];

  [personClass performSelector:@selector(eat)];
  // Call the class method with a class name, the underlying layer will automatically convert the class name into a class object call  // Essence: Let the class object send a message  objc_msgSend([Person class], @selector(eat));

@implementation ViewController


- (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.
  // Requirements: Provide functions for the imageNamed method, and determine whether the image is loaded successfully every time the image is loaded.  // Step 1: First create a classification and define a method that can load images and print them + (UIImage *)xmg_imageNamed:(NSString *)imageName;  // Step 2: Exchange the implementation of imageNamed and xmg_imageNamed, and you can call xmg_imageNamed and indirectly call the implementation of xmg_imageNamed.  UIImage *image = [UIImage imageNamed:@"123"];

     imageNamed:
   Implementation method:Underlying callPH_imageNamed

   nature:Implementation of two methods of exchangeimageNamedandPH_imageNamedmethod
   CallimageNamed其实就是CallPH_imageNamed


   imageNamedLoading pictures,I don't know if the image is loading successfully
   以后CallimageNamedWhen,You will know if the picture is loading


}

@end


@implementation UIImage (Image)
// Called when loading the classification into memory+ (void)load
{
  // Implementation of exchange methods, methods are defined in the class  // class_getMethodImplementation: Get method implementation  // class_getInstanceMethod: Get the object  // class_getClassMethod: Get class method  // IMP: Method implementation
  // imageNamed
  // Class: Which class method to get  // SEL: Get the method number, and you can find the method according to the SEL
  Method imageNameMethod = class_getClassMethod([UIImage class], @selector(imageNamed:));

  Method PH_imageNameMethod = class_getClassMethod([UIImage class], @selector(PH_imageNamed:));

  // Implementation of exchange method  method_exchangeImplementations(imageNameMethod, PH_imageNameMethod);
}

 

}

// You cannot rewrite the system method imageNamed in the classification because it will overwrite the system's functions, and super cannot be called in the classification.
// Can load pictures and print+ (UIImage *)PH_imageNamed:(NSString *)imageName
{
  // Load the picture  UIImage *image = [UIImage PH_imageNamed:imageName];
  // 2. Judgment function  if (image == nil) {
    NSLog(@"Loading empty");
  }

  return image;
}


@end

@implementation ViewController

- (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.

  Person *p = [[Person alloc] init];

  // The default person does not implement the eat method. You can call it through performSelector, but an error will be reported.  // Dynamically add methods will not report errors  [p performSelector:@selector(eat)];

}


@end

@implementation Person
// void(*)()
// The default method has two implicit parameters.By default, one method has two parameters,self,_cmd,Implicit parameters
 self:Method caller
 _cmd:The number of the call method
void eat(id self,SEL sel)
{
  NSLog(@"%@ %@",self,NSStringFromSelector(sel));
}

// When an object calls an unimplemented method, this method will be called to process and the corresponding method list will be passed over.// It just happens to be used to determine whether the unimplemented method is the method we want to add dynamically
<!--Dynamic addition method,Implement this firstresolveInstanceMethod-->
<!-- resolveInstanceMethodCall:当Call了没有实现的方法没有实现就会CallresolveInstanceMethod-->
<!-- resolveInstanceMethodeffect:You know which methods are not implemented,从而Dynamic addition method-->
<!-- sel:No implementation method-->
+ (BOOL)resolveInstanceMethod:(SEL)sel
{

  if (sel == @selector(eat)) {
    // Dynamically add eat method
    // First parameter: Which class to add a method    // The second parameter: method number of the method added    // The third parameter: function implementation of the method (function address)    // The fourth parameter: the type of the function, (return value + parameter type) v:void @: object->self: means SEL->_cmd    class_addMethod(self, @selector(eat), eat, "v@:");

  }

  return [super resolveInstanceMethod:sel];
}
@end

@implementation ViewController

- (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.

  // Dynamically add attribute name to the system NSObject class
  NSObject *objc = [[NSObject alloc] init];
   = @"abc";
  NSLog(@"%@",);
}
@end


// Define the associated keystatic const char *key = "name";

- (void)setName:(NSString *)name
{
  // Add properties and objects  // Create an association to an object and add attributes  // object: Which object to add attributes  // key: attribute name, obtain the associated object according to key, void * == id  // value: the associated value  // policy: policy
  objc_setAssociatedObject(self, @"name", name, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
- (NSString *)name
{

  return objc_getAssociatedObject(self, @"name");
}