Ios-method + Initialize and +load (reprint)

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 ---main函数中的代码---

  #import <UIKit/UIKit.h> #import "AppDelegate.h" int main (int argc, char * argv[]) {  NSLog (@ "%s", __func__);  @autoreleasepool {      return Uiapplicationmain (argc, argv, Nil, Nsstringfromclass ([Appdelegate class]);  }}

   #pragram ---基于NSObject的Person类--- 
  

   #import "Person.h" @implementation person + (void) load{NSLog (@ "%s", __func__);}     + (void) initialize{[super Initialize]; NSLog (@ "%s%@", __func__,[self class]); }-(Instancetype) init{if (self = [super init]) {NSLog (@ "%s", __func__);} return to self;} @end 

   #pragram--- Son class based on person---
                

  #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 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:

  2015-10-27 15:33:56.195 initialize[11711:342410] +[person load]2015-10-27 15:33:56.196 initialize[11711:342410] +[ Girl Load] 2015-10-27 15:33:56.197 initialize[11711:342410] main2015-10-27 15:33:56.259 initialize[11711:342410] +[ Person Initialize] person2015-10-27 15:33:56.259 initialize[11711:342410]-[person init]2015-10-27 15:33:56.259 INITIALIZE[11711:342410]-[person init]2015-10-27 15:33:56.259 initialize[11711:342410] +[Girl Initialize] 2015-10-27 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:

  2015-10-27 15:44:55.762 initialize[12024:351576] +[person load]2015-10-27 15:44:55.764 initialize[12024:351576] +[Son Load]2015-10-27 15:44:55.764 initialize[12024:351576] +[girl load] 2015-10-27 15:44:55.764 initialize[12024:351576] Main2015-10-27 15:44:55.825 initialize[12024:351576] +[person Initialize] person2015-10-27 15:44:55.825 initialize[ 12024:351576]-[person init]2015-10-27 15:44:55.825 initialize[12024:351576]-[person init]2015-10-27 15:44:55.826 INITIALIZE[12024:351576] +[person Initialize] son2015-10-27 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-method + Initialize and +load (reprint)