weak vs assign

@interface ViewController ()
@property (weak, nonatomic) UILabel *label;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    UILabel *lb = [UILabel new];
    _label = lb;
    lb = nil;
    //lb和_label都是nil
    _label.text = @"";
}

@interface ViewController ()
@property (assign, nonatomic) UILabel *label;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    UILabel *lb = [UILabel new];
    _label = lb;
    lb = nil;
    //lb是nil，_label不是nil，执行下面的方法会报错
    _label.text = @"";
}