load:

• +load是一個(gè)類方法榜配，當(dāng)程序載入時(shí)，在main函數(shù)執(zhí)行之前毒坛，帶有+load方法類（Class）或者種類（Category）就會(huì)執(zhí)行+load方法（每個(gè)+load方法只執(zhí)行一次），比如：

  @interface Person : NSObject
  @end
    
  @implementation Person
  +(void)load {
      NSLog(@"Person is loaded");
  }
  @end
    
  int main(int argc, const char *argv[])
  {
      @autoreleasepool {
          // insert code here...
          NSLog(@"main is executed");
      }
      return 0;
  }
  // Person is loaded
  // main is executed
  

  我們可以看到，Person類中的+load方法在main執(zhí)行前就被調(diào)用了

• 當(dāng)父類和子類都實(shí)現(xiàn)+load方法時(shí)辨泳，父類和子類的+load方法都會(huì)執(zhí)行（都只執(zhí)行一次），且父類的+load方法執(zhí)行順序要優(yōu)先于子類玖院，比如：

  @interface Person : NSObject
  @end
  
  @interface Father : Person
  @end
    
  @implementation Person
  +(void)load {
      NSLog(@"Person is loaded");
  }
  @end
  
  @implementation Father
  +(void)load {
      NSLog(@"Father is loaded");
  }
  @end
    
  //  Person is loaded
  //  Father is loaded
  

  我們可以看到Fahter是Person的子類菠红，Person執(zhí)行+load方法后Father才執(zhí)行+load方法

• 當(dāng)子類未實(shí)現(xiàn)+load方法時(shí)，不會(huì)調(diào)用父類+load方法难菌，比如：

  @implementation Person
  +(void)load {
      NSLog(@"Person is loaded");
  }
  @end
  
  @implementation Father
  //+(void)load {
  //    NSLog(@"Father is loaded");
  //}
  @end
  
  //  Person is loaded
  

• 當(dāng)子類實(shí)現(xiàn)+load方法试溯，而父類沒有實(shí)現(xiàn)+load方法時(shí)，只調(diào)用子類的+load方法：

  @implementation Person
  //+(void)load {
  //    NSLog(@"Person is loaded");
  //}
  @end
  
  @implementation Father
  +(void)load {
      NSLog(@"Father is loaded");
  }
  @end
    
  //  Father is loaded
  

• 類中的+load方法執(zhí)行順序要優(yōu)先于種類（Category）郊酒，種類中+load方法的執(zhí)行順序和Compile Sources中出現(xiàn)的順序一致:

  @implementation Person
  +(void)load {
      NSLog(@"Person is loaded");
  }
  @end
  
  @implementation Person (Category1)
  +(void)load {
      NSLog(@"Person Category1 is loaded");
  }
  @end
  
  @implementation Person (Category2)
  +(void)load {
      NSLog(@"Person Category2 is loaded");
  }
  @end
    
  @implementation Person (Category3)
  +(void)load {
      NSLog(@"Person Category3 is loaded");
  }
  @end
    
  // Person is loaded
  // Person Category3 is loaded
  // Person Category1 is loaded
  // Person Category2 is loaded
  

  可以看到Person類的+load先于所有種類的執(zhí)行順序遇绞，而Compile Sources中Person種類出現(xiàn)的順序是3，1燎窘，2摹闽，這對(duì)應(yīng)+load執(zhí)行的順序

• 當(dāng)有多個(gè)不同的類的時(shí)候,每個(gè)類+load 執(zhí)行順序與其在Compile Sources出現(xiàn)的順序一致

• +load是線程安全的，要避免在+load內(nèi)堵塞線程褐健。

initialize

• 在類收到第一條消息的時(shí)候調(diào)用付鹿，在main函數(shù)開始執(zhí)行后（因?yàn)轭惙椒ǖ恼{(diào)用在main中），可能某個(gè)類會(huì)一直不執(zhí)行+initialize蚜迅，因?yàn)檫@個(gè)類可能只是被加載到程序中舵匾，而沒有被調(diào)用（收到消息），每個(gè)類只執(zhí)行+initialize一次慢叨。

• 父類的+initialize方法會(huì)比子類先執(zhí)行：

  @implementation Person
  +(void)initialize {
      NSLog(@"Person is initialized");
  }
  @end
  
  @implementation Father
  +(void)initialize {
      NSLog(@"Father is initialized");
  }
  @end
    
  int main(int argc, const char *argv[])
  {
      @autoreleasepool {
          Father *father = [[Father alloc] init];
          Person *person = [[Person alloc] init];
      }
      return 0;
  }
  
  // Person is initialized
  // Father is initialized
  

• 當(dāng)子類未實(shí)現(xiàn)+initialize方法時(shí)纽匙，會(huì)調(diào)用父類+initialize方法（這樣使得同一種+initialize可能會(huì)被執(zhí)行多次，但這并不與每個(gè)類只執(zhí)行+initialize一次相矛盾）拍谐。

  @implementation Person
  +(void)initialize {
      NSLog(@"Person is initialized");
  }
  @end
  
  @implementation Father
  //+(void)initialize {
  //    NSLog(@"Father is initialized");
  //}
  @end
  
  int main(int argc, const char *argv[])
  {
      @autoreleasepool {
          Father *father = [[Father alloc] init];
      }
      return 0;
  }
  
  //  Person is initialized
  //  Person is initialized
  

• 當(dāng)有多個(gè)Category都實(shí)現(xiàn)了+initialize方法時(shí)烛缔，會(huì)覆蓋類中的方法，+initialize方法只執(zhí)行一次（會(huì)執(zhí)行Compile Sources 列表中最后一個(gè)Category 的+initialize方法)

  @implementation Person
  +(void)initialize {
      NSLog(@"Person is initialized");
  }
  @end
    
  @implementation Person (Category1)
  +(void)initialize {
      NSLog(@"Person Category1 is initialized");
  }
  @end
    
  @implementation Person (Category2)
  +(void)initialize {
      NSLog(@"Person Category2 is initialized");
  }
  @end
    
  @implementation Person (Category3)
  +(void)initialize {
      NSLog(@"Person Category3 is initialized");
  }
  @end
    
  // Person Category2 is initialized
  

  Compile Sources中Catergory出現(xiàn)的順序是3轩拨，2践瓷，1，+initialize執(zhí)行一次亡蓉，執(zhí)行的是最后一個(gè)（1）晕翠。

• +initiallize是線程安全的，要避免在+initiallize內(nèi)堵塞線程。

總結(jié)