iOS 懒加载

1.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getter方法。说的通俗一点，就是在开发中，当程序中需要利用的资源时。在程序启动的时候不加载资源，只有在运行当需要一些资源时，再去加载这些资源。

我们知道iOS设备的内存有限，如果在程序在启动后就一次性加载将来会用到的所有资源，那么就有可能会耗尽iOS设备的内存。这些资源例如大量数据，图片，音频等等，所以我们在使用懒加载的时候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）只有当真正需要资源时，再去加载，节省了内存资源。

<span style="font-size:14px;">//
#import "ViewController.h"

@interface ViewController ()
@property (nonatomic, strong) UILabel *noLabel;
@property (nonatomic, strong) UIImageView *iconImage;
@property (nonatomic, strong) UILabel *descLabel;
@property (nonatomic, strong) UIButton *leftButton;
@property (nonatomic, strong) UIButton *rightButton;
@property (nonatomic, strong) UILabel *name;
/**当前显示的照片索引*/
@property (nonatomic, assign) NSInteger index;
@property (nonatomic, strong) NSArray *imageList;
@end

@implementation ViewController
/**
懒加载（延迟加载），通过getter方法实现
效果：让对象在最需要的时候才创建！
*/

- (NSArray *)imageList
{
if (_imageList == nil) {
//"包" Bundle  NSBundle mainBundle编译安装之后对应的程序包
NSString *path = [[NSBundle mainBundle] pathForResource:@"ImageList" ofType:@"plist"];
//在oc中看到了ContentsOfFile,通常需要完整的路径
_imageList = [NSArray arrayWithContentsOfFile:path];
}
return _imageList;
}
#pragma mark - 控件的懒加载
//在getter方法中，不要再使用self
-(UILabel *)noLabel
{
if (_noLabel == nil) {
_noLabel = [[UILabel alloc] initWithFrame:CGRectMake(0, 20, self.view.bounds.size.width, 40)];
_noLabel.textAlignment = NSTextAlignmentCenter;//调整居中
[self.view addSubview:_noLabel];
}
return _noLabel;
}
- (UIImageView *)iconImage
{
if (_iconImage == nil) {
CGFloat imageW = 250;
CGFloat imageH = 400;
CGFloat imageX = (self.view.bounds.size.width - imageW)*0.5;
CGFloat imageY = CGRectGetMaxY(self.noLabel.frame) + 20;
_iconImage = [[UIImageView alloc] initWithFrame: CGRectMake(imageX, imageY, imageW, imageH)];
[self.view addSubview:_iconImage];
}
return _iconImage;
}

- (UILabel *)descLabel
{
if (_descLabel == nil) {
CGFloat descY = CGRectGetMaxY(self.iconImage.frame);
_descLabel = [[UILabel alloc] initWithFrame:CGRectMake(0, descY, self.view.bounds.size.width, 100)];
//_descLabel.text = @"高圆圆";
_descLabel.textAlignment = NSTextAlignmentCenter;
//需要label具有足够的高度，不限制显示的行数
_descLabel.numberOfLines = 0;
[self.view addSubview:_descLabel];
}
return _descLabel;
}
-(UIButton *)leftButton
{
if (_leftButton == nil) {
_leftButton = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 40, 40)];
CGFloat centerY = self.iconImage.center.y;
CGFloat centerX = self.iconImage.frame.origin.x*0.5;
_leftButton.center = CGPointMake(centerX, centerY);
[_leftButton setBackgroundImage:[UIImage imageNamed:@"left_normal"] forState:UIControlStateNormal];
[_leftButton setBackgroundImage:[UIImage imageNamed:@"left_highlighted"] forState:UIControlStateHighlighted];
_leftButton.tag = -1;
[self.view addSubview:_leftButton];
[_leftButton addTarget:self action:@selector(clickButton:) forControlEvents:UIControlEventTouchUpInside];
}
return _leftButton;
}

-(UIButton *)rightButton
{
if (_rightButton == nil) {
CGFloat centerY = self.iconImage.center.y;
CGFloat centerX = self.iconImage.frame.origin.x*0.5;
_rightButton = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 40, 40)];
_rightButton.center = CGPointMake(self.view.bounds.size.width -centerX, centerY);
[_rightButton setBackgroundImage:[UIImage imageNamed:@"right_normal"] forState:UIControlStateNormal];
[_rightButton setBackgroundImage:[UIImage imageNamed:@"right_highlighted"] forState:UIControlStateHighlighted];
_rightButton.tag = 1;
[self.view addSubview:_rightButton];

[_rightButton addTarget:self action:@selector(clickButton:) forControlEvents:UIControlEventTouchUpInside];
}
return _rightButton;
}

- (UILabel *)name
{
if (_name == nil) {
_name = [[UILabel alloc] initWithFrame:CGRectMake(0, 600, self.view.bounds.size.width, 40)];
_name.text = @"现代范儿";
_name.textAlignment = NSTextAlignmentCenter;//调整居中
[self.view addSubview:_name];
}
return _name;
}
- (void)viewDidLoad {
// Do any additional setup after loading the view, typically from a nib.
[super viewDidLoad];

//显示照片信息
[self showPhoto];
}
-(void) showPhoto
{
//设置序号
self.noLabel.text = [NSString stringWithFormat:@"%lu/%d", self.index + 1, 5];

//设置图像和描述
self.iconImage.image = [UIImage imageNamed:self.imageList[self.index][@"name"]];
self.descLabel.text = self.imageList[self.index][@"desc"];

self.rightButton.enabled = (self.index != 4);
self.leftButton.enabled = (self.index != 0);
}
/**在oc中，很多方法的第一个参数，都是触发该方法的对象*/
-(void) clickButton: (UIButton *) button
{
//根据按钮调整当前显示图片的索引
self.index += button.tag;
[self showPhoto];
}
@end