UI基础第五弹:懒加载

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 //  YYViewController.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 "YYViewController.h" 10  11 #define POTOIMGW    200 12 #define POTOIMGH    300 13 #define POTOIMGX    60 14 #define  POTOIMGY    50 15  16 @interface YYViewController () 17  18 @property(nonatomic,strong)UILabel *firstlab; 19 @property(nonatomic,strong)UILabel *lastlab; 20 @property(nonatomic,strong)UIImageView *icon; 21 @property(nonatomic,strong)UIButton *leftbtn; 22 @property(nonatomic,strong)UIButton *rightbtn; 23 @property(nonatomic,strong)NSArray *array; 24 @property(nonatomic ,assign)int i; 25 -(void)change; 26 @end 27  28  29  30 @implementation YYViewController 31  32 - (void)viewDidLoad 33 { 34     [super viewDidLoad]; 35     [self change]; 36 } 37  38 -(void)change 39 { 40     [self.firstlab setText:[NSString stringWithFormat:@"%d/5",self.i+1]]; 41     //先get再set 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(POTOIMGX, POTOIMGY, POTOIMGW, POTOIMGH)]; 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 *)rightbtn106 {107     if (!_rightbtn) {108         _rightbtn=[UIButton buttonWithType:UIButtonTypeCustom];109         _rightbtn.frame=CGRectMake(POTOIMGX+POTOIMGW+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 *)array120 {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 *)btn129 {130     self.i++;131     [self change];132 }133 134 -(void)leftclick:(UIButton *)btn135 {136     self.i--;137     [self change];138 }139 
140 @end
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