约束动画

-(void)modifyConstraint:(UIButton *)btn{
    btn.translatesAutoresizingMaskIntoConstraints = NO;
    NSArray* constrains = btn.constraints;
    NSLayoutConstraint *widthConstraint = [NSLayoutConstraint constraintWithItem:btn attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1 constant:0];
    for (NSLayoutConstraint* constraint in constrains) {
        if (constraint.firstAttribute == NSLayoutAttributeWidth) {
            [UIView animateWithDuration:1 animations:^{
                
                [btn addConstraint:widthConstraint];
                [btn removeConstraint:constraint];
                [btn layoutIfNeeded];
            } completion:^(BOOL finished) {
               
                dispatch_time_t timer =
                dispatch_time(DISPATCH_TIME_NOW,
                              0.5 *
                              NSEC_PER_SEC);
                dispatch_after(timer, dispatch_get_main_queue(), ^{
                    [UIView animateWithDuration:.3 animations:^{
                        [btn removeConstraint:widthConstraint];
                        [btn addConstraint:[NSLayoutConstraint constraintWithItem:btn attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1 constant:40]];
                        [btn layoutIfNeeded];
                    } completion:^(BOOL finished) {

                    }];
                });
            }];
            break;
        }
    }
}