• 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 / filterlet data = 1...3let 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 resultsi is 1result is 2i is 2result is 4i is 3result is 6end*/