1.懒加载基本
懒加载——也称为延迟加载，即在需要的时候才加载（效率低，占用内存小）。所谓懒加载，写的是其get方法.
注意：如果是懒加载的话则一定要注意先判断是否已经有了，如果没有那么再去进行实例化
2.使用懒加载的好处：
（1）不必将创建对象的代码全部写在viewDidLoad方法中，代码的可读性更强
（2）每个控件的getter方法中分别负责各自的实例化处理，代码彼此之间的独立性强，松耦合
3.代码示例
1 // 2 // SLViewController.m 3 // 03-图片浏览器初步 4 // 5 // Created by apple on 14-5-21. 6 // Copyright (c) 2014年 itcase. All rights reserved. 7 // 8 9 #import "SLViewController.h" 10 11 #define PHOTOIMGW 200 12 #define PHOTOIMGH 300 13 #define PHOTOIMGX 60 14 #define PHOTOIMGY 50 15 16 @interface SLViewController () 17 18 // 变量声明 19 @property(nonatomic, strong) UILabel *firstLab; 20 @property(nonatomic, strong) UILabel *lastLab; 21 @property(nonatomic, strong) UIImageView *icon; 22 @property(nonatomic, strong) UIButton *leftBtn; 23 @property(nonatomic, strong) UIButton *rightBtn; 24 @property(nonatomic, strong) NSArray *array; 25 @property(nonatomic, assign) int i; 26 27 -(void)change; 28 @end 29 30 @implementation SLViewController 31 32 - (void)viewDidLoad 33 { 34 [super viewDidLoad]; 35 [self change]; 36 } 37 38 -(void)change 39 { 40 //先get再set 41 [self.firstLab setText:[NSString stringWithFormat:@"%d / 5",self.i + 1]]; 42 43 self.icon.image = [UIImage imageNamed:self.array[self.i][@"name"]]; 44 self.lastlab.text = self.array[self.i][@"desc"]; 45 46 self.leftBtn.enabled = (self.i!=0); 47 self.rightBtn.enabled = (self.i!=4); 48 } 49 50 // 延迟加载 51 /** 1.图片的序号标签 */ 52 -(UILabel *)firstLab 53 { 54 // 判断是否已经有了，若没有，则进行实例化 55 if (!_firstLab) { 56 _firstLab=[[UILabel alloc]initWithFrame:CGRectMake(20, 10, 300, 30)]; 57 [_firstLab setTextAlignment:NSTextAlignmentCenter]; 58 [self.view addSubview:_firstLab]; 59 } 60 return _firstLab; 61 } 62 63 /** 2.图片控件的延迟加载 */ 64 -(UIImageView *)icon 65 { 66 // 判断是否已经有了，若没有，则进行实例化 67 if (!_icon) { 68 _icon=[[UIImageView alloc]initWithFrame:CGRectMake(PHOTOIMGX, PHOTOIMGY, PHOTOIMGW, PHOTOIMGH)]; 69 UIImage *image=[UIImage imageNamed:@"biaoqingdi"]; 70 _icon.image=image; 71 [self.view addSubview:_icon]; 72 } 73 return _icon; 74 } 75 76 /** 3.描述控件的延迟加载 */ 77 -(UILabel *)lastlab 78 { 79 // 判断是否已经有了，若没有，则进行实例化 80 if (!_lastLab) { 81 _lastLab=[[UILabel alloc]initWithFrame:CGRectMake(20, 400, 300, 30)]; 82 [_lastLab setTextAlignment:NSTextAlignmentCenter]; 83 [self.view addSubview:_lastLab]; 84 } 85 return _lastLab; 86 } 87 88 /** 4.左键按钮的延迟加载 */ 89 -(UIButton *)leftbtn 90 { 91 // 判断是否已经有了，若没有，则进行实例化 92 if (!_leftBtn) { 93 _leftBtn=[UIButton buttonWithType:UIButtonTypeCustom]; 94 _leftBtn.frame=CGRectMake(0, self.view.center.y, 40, 40); 95 [_leftBtn setBackgroundImage:[UIImage imageNamed:@"left_normal"] forState:UIControlStateNormal]; 96 [_leftBtn setBackgroundImage:[UIImage imageNamed:@"left_highlighted"] forState:UIControlStateHighlighted]; 97 [self.view addSubview:_leftBtn]; 98 [_leftBtn addTarget:self action:@selector(leftclick:) forControlEvents:UIControlEventTouchUpInside]; 99 } 100 return _leftBtn; 101 102 } 103 104 /** 5.右键按钮的延迟加载 */ 105 -(UIButton *)rightbtn 106 { 107 if (!_rightBtn) { 108 _rightBtn=[UIButton buttonWithType:UIButtonTypeCustom]; 109 _rightBtn.frame=CGRectMake(PHOTOIMGX+PHOTOIMGW+10, self.view.center.y, 40, 40); 110 [_rightBtn setBackgroundImage:[UIImage imageNamed:@"right_normal"] forState:UIControlStateNormal]; 111 [_rightBtn setBackgroundImage:[UIImage imageNamed:@"right_highlighted"] forState:UIControlStateHighlighted]; 112 [self.view addSubview:_rightBtn]; 113 [_rightBtn addTarget:self action:@selector(rightclick:) forControlEvents:UIControlEventTouchUpInside]; 114 } 115 return _rightBtn; 116 } 117 118 // array的get方法 119 -(NSArray *)array 120 { 121 if (_array == nil) { 122 NSString *path = [[NSBundle mainBundle] pathForResource:@"data" ofType:@"plist"]; 123 _array=[[NSArray alloc]initWithContentsOfFile:path]; 124 } 125 return _array; 126 } 127 128 -(void)rightclick:(UIButton *)btn 129 { 130 self.i++; 131 [self change]; 132 } 133 134 -(void)leftclick:(UIButton *)btn 135 { 136 self.i--; 137 [self change]; 138 } 139 140 @end