如何更快地编写类似于目标C的getter方法，并提供更好的方法

Question

Here I have added code with getter method for one time allocation in obj c. How we will write same code in swift. I need one time allocation. I already seen the computed get and set method, but can't find a solution for this.

@property (nonatomic, strong) UIImageView *bgImageView;

- (UIImageView *)bgImageView{
    if (_bgImageView == nil) {
        _bgImageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, width, 277)];
        _bgImageView.image = [UIImage imageNamed:@"imageName"];
        _bgImageView.alpha = 0;
        _bgImageView.backgroundColor = [UIColor colorWithHexString:@"f4f4f4"];
    }
    return _bgImageView;
}

[self.view addSubview:self.bgImageView];

Answer 1

Swift introduced a new concept, lazy property. A lazy property is initialized only when you use it for the first time. Like a custom getter in objective-C, you can also add some logic after creating the object (for example, in your case, setting alpha value, image, and background color)

lazy var bgImageView:UIImageView  = {
    var imageView = UIImageView(frame: CGRectMake(0, 0, width, 27))
    imageView.image = UIImage(named: "imageName")
    imageView.alpha = 0
    imageView.backgroundColor = UIColor(red: 244/255, green:  244/255, blue:  244/255, alpha: 1)
    return imageView
}()

The code is called only once, but Apple warns about multithreading access :

If a property marked with the lazy modifier is accessed by multiple threads simultaneously and the property has not yet been initialized, there is no guarantee that the property will be initialized only once.