iOS-延时加载，延时初始化

• OC版

• // classA.h
  @property (nonatomic, copy) NSString *testString;
  
  //classA.m
  - (NSString*)testString {
      if (!_testString) {
          _testString = @"Hello";
          NSLog(@"Print at first time");
      }
      return _testString
  }

• Swift版

• //在变量属性前加 lazy 指定延时加载
  class ClassA {
      lazy var str: String = {
          let str = "Hello"
          print("print at first time")
          return str
      }()
      
      // or 
      // lazy var str: String = "Hello"
  
  }
  
  // lazy + array.map / filter
  let data = 1...3
  let result = data.lazy.map {
      (i: Int) -> Int in
      print("i is (i)")
      return i * 2
  }
  
  print("print results")
  
  for i in result {
      print("result is (i)")
  }
  
  print("end")
  
  /*
  print results
  i is 1
  result is 2
  i is 2
  result is 4
  i is 3
  result is 6
  end
  */