IOS-+ Initialize and +load

Objective-c has two magical methods: +load and +initialize, which are called automatically when the class is used. However, the differences in the two methods can lead to significant differences in performance at the application level.

First, + Initialize method and +load call timing

• Let's start with the + Initialize method: Apple officially has a description of this method: This method is called before the class is initialized for the first time , and we use it to initialize the static variable.
• • The Load method is called when the class is loaded, that is, when the iOS app starts, all classes are loaded, and the + Load method for each class is called.
• Then we combine the code to explore the Initialize and + load two method call timing, first of all is+ Load：

  #pragram---The code in the main function---#import<UIKit/UIKit.h>#import"AppDelegate.h"int main (int argc,char * argv[]) {NSLog (@ "%s", __func__);@autoreleasepool {ReturnUiapplicationmain (argc, argv,NilNsstringfromclass ([Appdelegate class])); }}#pragram---The person class based on nsobject---#import"Person.h"@implementationperson+ (void) load{NSLog (@ "%s", __func__);} + (void) initialize{[Super Initialize];NSLog (@ "%s%@", __func__,[Self class]);} -(Instancetype) init{if (Self = [Super Init]) {NSLog (@ "%s", __func__); }ReturnSelf;}@end #pragram---person-based son class--- #import " Girl.h " @implementation girl+ (void) load{NSLog (@ "%s", __func__);} + (void) initialize{[super initialize]; nslog (@"%s ", __func__);} -(instancetype) init{if (self = [ Super Init]) {nslog (@ "%s", __func__);} return self;}  @end             

  To run the program, let's look at the output log:

  2015-10-27 Span class= "Hljs-number" >15:21:07.545 Initialize[11637:334237] +[person load]2015-10-27 15:21:07.546 initialize[11637:334237] +[Girl Load] 2015-10-27 15:21:07.546 Initialize[11637:334237] main        

  This means that when I do not do anything with the class, the +load method is executed by default and is executed before the main function.
• Let's take a look at+ Initializemethod, first create the person and Girl objects in Viewcontroller:

  #Import"ViewController.h" #Import"Person.h" #import  "Son.h" #import  "Girl.h"  @interface viewcontroller ()  @end  @implementation  viewcontroller-(void) viewdidload {[super viewdidload]; person * a = [person new]; person * b = [person new]; girl *c = [girl new]; girl *d = [girl new];}  @end             

  Let's take a look at the output log:

  2015-10-2715:33:56.195 initialize[11711:342410] +[person Load]2015-10-2715:33:56.196 initialize[11711:342410] +[girl Load]2015-10-2715:33:56.197 initialize[11711:342410] Main2015-10-2715:33:56.259 initialize[11711:342410] +[person Initialize] Person2015-10-2715:33:56.259 initialize[11711:342410]-[person Init]2015-10-2715:33:56.259 initialize[11711:342410]-[person Init]2015-10-27 15:33:56.259 Initialize[11711:342410] +[girl Initialize] 2015-< Span class= "Hljs-number" >10-27 15: 33:56.260 initialize[11711:342410 ]-[girl init]2015-10-27 15:33:56.260 initialize[ 11711:342410]-[girl init]        

  With this experiment we can determine two points:
  • + Initialize method is similar to a lazy load, if you do not use this class, then the system will not call this method by default, and the default is only loaded once;
  • + Initialize call occurs before the +init method.
• Let's look at the next+ InitializeIn the relationship between the parent class and the child class, create a son class that inherits from the person class:

  #pragram ---ViewController 中的代码---#import "ViewController.h"#import "Person.h"#import "Son.h"#import "Girl.h"@interface ViewController ()@end@implementation ViewController- (void)viewDidLoad { [super viewDidLoad]; Person * a = [Person new]; Person * b = [Person new]; Son*z = [Son new];}@end

  Check out the output log:

  2015-10-2715:44:55.762 initialize[12024:351576] +[person Load]2015-10-2715:44:55.764 initialize[12024:351576] +[son Load]2015-10-2715:44:55.764 initialize[12024:351576] +[girl Load]2015-10-2715:44:55.764 initialize[12024:351576] Main2015-10-2715:44:55.825 initialize[12024:351576] +[person Initialize] Person2015-10-2715:44:55.825 initialize[12024:351576]-[person Init] 2015-10-27 15:44:55.825 Initialize[12024:351576]-[person init]2015-10-27 15:44:< Span class= "Hljs-number" >55.826 initialize[12024:351576] +[ Person Initialize] Son2015-10-27 Span class= "Hljs-number" >15:44:55.826 Initialize[12024:351576]-[person init]        

  We will find that the person class+ Initializemethod is called, but it is called by the subclass son, that is, when the subclass is created, the subclass calls the parent class's+ InitializeMethod.

Second, summary

• If you implement the + load method, it will be called automatically when the class is loaded. This call is very early. If you implement a + loadfor an app or frame, and your app is linked to this frame, then + load is called before the main () function. If you implement + loadin a loadable bundle, it will be called during bundle loading.
• The call to the + initialize method looks more reasonable, and usually it is better to write code in it than in + load . + Initialize is interesting because it is lazy to call, or it may not be called at all. Class is loaded for the first time,
• + Initialize will not be called. When a class receives a message, the runtime checks whether + Initialize has been called. If not, it is called before the message is processed.

IOS-+ Initialize and +load