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

//
// YYViewController.m
// 03-图片浏览器初步
//
// Created by apple on 14-5-21.
// Copyright (c) 2014年 itcase. All rights reserved.
//

#import "YYViewController.h"

#define POTOIMGW 200
#define POTOIMGH 300
#define POTOIMGX 60
#define POTOIMGY 50

@interface YYViewController ()

@property(nonatomic,strong)UILabel *firstlab;
@property(nonatomic,strong)UILabel *lastlab;
@property(nonatomic,strong)UIImageView *icon;
@property(nonatomic,strong)UIButton *leftbtn;
@property(nonatomic,strong)UIButton *rightbtn;
@property(nonatomic,strong)NSArray *array;
@property(nonatomic ,assign)int i;
-(void)change;
@end

@implementation YYViewController

- (void)viewDidLoad
{
[super viewDidLoad];
[self change];
}

-(void)change
{
[self.firstlab setText:[NSString stringWithFormat:@"%d/5",self.i+1]];
//先get再set

self.icon.image=[UIImage imageNamed:self.array[self.i][@"name"]];
self.lastlab.text=self.array[self.i][@"desc"];

self.leftbtn.enabled=(self.i!=0);
self.rightbtn.enabled=(self.i!=4);
}

//延迟加载
/**1.图片的序号标签*/
-(UILabel *)firstlab
{
//判断是否已经有了，若没有，则进行实例化
if (!_firstlab) {
_firstlab=[[UILabel alloc]initWithFrame:CGRectMake(20, 10, 300, 30)];
[_firstlab setTextAlignment:NSTextAlignmentCenter];
[self.view addSubview:_firstlab];
}
return _firstlab;
}

/**2.图片控件的延迟加载*/
-(UIImageView *)icon
{
//判断是否已经有了，若没有，则进行实例化
if (!_icon) {
_icon=[[UIImageView alloc]initWithFrame:CGRectMake(POTOIMGX, POTOIMGY, POTOIMGW, POTOIMGH)];
UIImage *image=[UIImage imageNamed:@"biaoqingdi"];
_icon.image=image;
[self.view addSubview:_icon];
}
return _icon;
}

/**3.描述控件的延迟加载*/
-(UILabel *)lastlab
{
//判断是否已经有了，若没有，则进行实例化
if (!_lastlab) {
_lastlab=[[UILabel alloc]initWithFrame:CGRectMake(20, 400, 300, 30)];
[_lastlab setTextAlignment:NSTextAlignmentCenter];
[self.view addSubview:_lastlab];
}
return _lastlab;
}

/**4.左键按钮的延迟加载*/
-(UIButton *)leftbtn
{
//判断是否已经有了，若没有，则进行实例化
if (!_leftbtn) {
_leftbtn=[UIButton buttonWithType:UIButtonTypeCustom];
_leftbtn.frame=CGRectMake(0, self.view.center.y, 40, 40);
[_leftbtn setBackgroundImage:[UIImage imageNamed:@"left_normal"] forState:UIControlStateNormal];
[_leftbtn setBackgroundImage:[UIImage imageNamed:@"left_highlighted"] forState:UIControlStateHighlighted];
[self.view addSubview:_leftbtn];
[_leftbtn addTarget:self action:@selector(leftclick:) forControlEvents:UIControlEventTouchUpInside];
}
return _leftbtn;

}

/**5.右键按钮的延迟加载*/
-(UIButton *)rightbtn
{
if (!_rightbtn) {
_rightbtn=[UIButton buttonWithType:UIButtonTypeCustom];
_rightbtn.frame=CGRectMake(POTOIMGX+POTOIMGW+10, self.view.center.y, 40, 40);
[_rightbtn setBackgroundImage:[UIImage imageNamed:@"right_normal"] forState:UIControlStateNormal];
[_rightbtn setBackgroundImage:[UIImage imageNamed:@"right_highlighted"] forState:UIControlStateHighlighted];
[self.view addSubview:_rightbtn];
[_rightbtn addTarget:self action:@selector(rightclick:) forControlEvents:UIControlEventTouchUpInside];
}
return _rightbtn;
}

//array的get方法
-(NSArray *)array
{
if (_array==nil) {
NSString *path=[[NSBundle mainBundle] pathForResource:@"data" ofType:@"plist"];
_array=[[NSArray alloc]initWithContentsOfFile:path];
}
return _array;
}

-(void)rightclick:(UIButton *)btn
{
self.i++;
[self change];
}

-(void)leftclick:(UIButton *)btn
{
self.i--;
[self change];
}

@end