iOS开发 懒加载

懒加载，顾名思义，在iOS程序中，当需要某些资源时才加载该资源，而不是在初始化时一次性加载所有的资源。下面以一个小例子说明懒加载的用法。

如果不使用懒加载，上图中的五个控件,需要在ViewController的- (void)viewDidLoad方法中创建，代码如下。

- (void)viewDidLoad
{
    [super viewDidLoad];

    // 实例化控件
    // 1. 序号标签
    UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(0, 20, 320, 40)];
    label.text = @"1/5";
    label.textAlignment = NSTextAlignmentCenter;
    [self.view addSubview:label];
    
    self.noLabel = label;
    
    // 2. 图片视图
    CGFloat imageW = 200;
    CGFloat imageX = (320 - imageW) / 2;
    CGFloat imageH = 200;
    CGFloat imageY = 80;
    // 实例化一个图像视图
    UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(imageX, imageY, imageW, imageH)];
    
    // 实例化一个图像
    UIImage *image = [UIImage imageNamed:@"biaoqingdi"];
    // 将图像显示在imageView中
    imageView.image = image;
    
    [self.view addSubview:imageView];
    
    self.icon = imageView;
    
    // 3. 描述标签
    UILabel *label1 = [[UILabel alloc] initWithFrame:CGRectMake(0, 300, 320, 80)];
    label1.text = @"神马表情都弱爆了";
    label1.textAlignment = NSTextAlignmentCenter;
    [self.view addSubview:label1];
    
    self.descLabel = label1;
    
    // 4. 左边的按钮
    UIButton *leftBtn = [[UIButton alloc] init];
    // 设置按钮的背景图像
    [leftBtn setBackgroundImage:[UIImage imageNamed:@"left_normal"] forState:UIControlStateNormal];
    [leftBtn setBackgroundImage:[UIImage imageNamed:@"left_highlighted"] forState:UIControlStateHighlighted];
    
    leftBtn.frame = CGRectMake(0, 0, 40, 40);
    leftBtn.center = CGPointMake(self.icon.frame.origin.x / 2, self.icon.center.y);
    
    [self.view addSubview:leftBtn];
    
    [leftBtn addTarget:self action:@selector(leftClick) forControlEvents:UIControlEventTouchUpInside];
    
    self.leftButton = leftBtn;
    
    // 5. 右边的按钮
    UIButton *rightBtn = [[UIButton alloc] init];
    [rightBtn setBackgroundImage:[UIImage imageNamed:@"right_normal"] forState:UIControlStateNormal];
    [rightBtn setBackgroundImage:[UIImage imageNamed:@"right_highlighted"] forState:UIControlStateHighlighted];
    
    rightBtn.frame = CGRectMake(0, 0, 40, 40);
    rightBtn.center = CGPointMake(self.view.frame.size.width - self.icon.frame.origin.x / 2, self.icon.center.y);
    
    [self.view addSubview:rightBtn];
    
    [rightBtn addTarget:self action:@selector(rightClick) forControlEvents:UIControlEventTouchUpInside];
    
    self.rightButton = rightBtn;
    
 
}

如果将每个控件的创建代码封装成一个方法，代码的可读性更强。懒加载就是将控件的创建代码放到getter方法中，当需要该控件时，如果该控件已经存在，直接返回该控件；如果该控件不存在，则创建控件。代码如下。

/** 序号标签 */
- (UILabel *)noLabel
{
    if (!_noLabel) {
        _noLabel = [[UILabel alloc] initWithFrame:CGRectMake(0, 20, 320, 40)];
        _noLabel.textAlignment = NSTextAlignmentCenter;
        
        [self.view addSubview:_noLabel];
    }
    return _noLabel;
}

- (UIImageView *)icon
{
    if (!_icon) {
        CGFloat imageW = 200;
        CGFloat imageX = (320 - imageW) / 2;
        CGFloat imageH = 200;
        CGFloat imageY = 80;

        _icon = [[UIImageView alloc] initWithFrame:CGRectMake(imageX, imageY, imageW, imageH)];
        
        [self.view addSubview:_icon];
    }
    return _icon;
}

- (UILabel *)descLabel
{
    if (!_descLabel) {
        _descLabel = [[UILabel alloc] initWithFrame:CGRectMake(0, 300, 320, 180)];
        _descLabel.numberOfLines = 0;
        _descLabel.textAlignment = NSTextAlignmentCenter;
        
        [self.view addSubview:_descLabel];
    }
    return _descLabel;
}

- (UIButton *)leftButton
{
    if (!_leftButton) {
        _leftButton = [[UIButton alloc] init];
        // 设置按钮的背景图像
        [_leftButton setBackgroundImage:[UIImage imageNamed:@"left_normal"] forState:UIControlStateNormal];
        [_leftButton setBackgroundImage:[UIImage imageNamed:@"left_highlighted"] forState:UIControlStateHighlighted];
        
        _leftButton.frame = CGRectMake(0, 0, 40, 40);
        _leftButton.center = CGPointMake(self.icon.frame.origin.x / 2, self.icon.center.y);
//        _leftButton.center = CGPointMake(self.icon.frame.origin.x / 2, _icon.center.y);
        
        [self.view addSubview:_leftButton];
        
        [_leftButton addTarget:self action:@selector(leftClick) forControlEvents:UIControlEventTouchUpInside];
    }
    return _leftButton;
}

- (UIButton *)rightButton
{
    if (!_rightButton) {
        _rightButton = [[UIButton alloc] init];
        [_rightButton setBackgroundImage:[UIImage imageNamed:@"right_normal"] forState:UIControlStateNormal];
        [_rightButton setBackgroundImage:[UIImage imageNamed:@"right_highlighted"] forState:UIControlStateHighlighted];
        
        _rightButton.frame = CGRectMake(0, 0, 40, 40);
        _rightButton.center = CGPointMake(self.view.frame.size.width - self.icon.frame.origin.x / 2, self.icon.center.y);
        
        [self.view addSubview:_rightButton];
        
        [_rightButton addTarget:self action:@selector(rightClick) forControlEvents:UIControlEventTouchUpInside];
    }
    return _rightButton;
}

使用懒加载，每个控件的getter方法负责自己的实例化，代码之间的独立性强，弱耦合。