Assigning a variable value from an Objective-C Block

问题

In Swift I can give a variable a value using an anonymous closure:

let thumbnailImageView: UIImageView = {
   let imageView = UIImageView()
   imageView.backGroundColor = UIColor.blueColor()
   return imageView;
}

addSubView(thumbnailImageView)
thumbnailImageView.frame = CGRectMake(0,0,100,100)

I am trying to do the same in Obj-C, but this results in an error when adding the subview and setting its frame:

UIImageView* (^thumbnailImageView)(void) = ^(void){
    UIImageView *imageView = [[UIImageView alloc] init];
    imageView.backgroundColor = [UIColor blueColor];
    return imageView;
};

[self addSubview:thumbnailImageView];

thumbnailImageView.frame = CGRectMake(0, 0, 100, 100);

回答1:

You're trying to write in Objective-C with Swift syntax. The Swift example describes a lazily initialized variable, while Objective-C one declares a simple block that returns UIImageView. You'd need to call the block with

[self addSubview:thumbnailImageView()];

However, in this case using the block to initialize a variable makes little sense. If you're looking for lazily initialized properties, it would look like this in Objective-C

@interface YourClass : Superclass

@property (nonatomic, strong) UIImageView* imageView;

@end

@synthesize imageView = _imageView;

- (UIImageView*)imageView
{
    if (!_imageView) {
        // init _imageView here
    }
    return _imageView;
}

来源：https://stackoverflow.com/questions/45941619/assigning-a-variable-value-from-an-objective-c-block