重写UILabler的sizeThatFits方法，需要触发两次才会有效果

#import "ViewController.h"
@interface SpecialLabel:UILabel
@end

@implementation SpecialLabel

- (CGSize)sizeThatFits:(CGSize)size {

    CGSize s = [super sizeThatFits:size];

    return CGSizeMake(s.width +50, s.height +50);
}
@end

@interface ViewController ()

@property (weak, nonatomic) IBOutlet SpecialLabel *label;

@end

@implementation ViewController

- (IBAction)changeText {
    
    self.label.text = @"fsdgdssssssssssss ";
#if 1
    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
        [self.label sizeToFit];
    }];
#else
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.001 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
         [self.label sizeToFit];
    });
#endif
    
}

self.label.text = @"fsdgdssssssssssss ";  
[self.label sizeToFit];