确定UIView何时完成布局

Question

我有UIView子类,并且为了布局UI元素,我没有使用自动布局.我想设置框架取决于框架的宽度或高度.但是,当我在alloc]init]方法之后立即执行此操作时,它看起来像self.frame.size.height或width为零.对于临时解决方案我做了以下:

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 0.25 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
        UIImage *img = [UIImage imageNamed:@"sound_level_arrow"];
        _arrowImgView = [[UIImageView alloc] initWithFrame:CGRectMake(self.frame.size.width/2, 0, img.size.width, img.size.height)];
        _arrowImgView.center = CGPointMake(self.mainImgView.frame.size.width/2, self.mainImgView.frame.size.height-2);
        _arrowImgView.image = [UIImage imageNamed:@"sound_level_arrow"];
        _arrowImgView.contentMode   = UIViewContentModeCenter;
        [self.mainImgView addSubview:_arrowImgView];
    });

但是当然我需要知道,当视图完成加载并且它具有实际的self.frame.size.width和height.

Answer 1

如果要在自定义视图子类中执行此操作,请layoutSubviews在该类中重写.由于iOS可能会发送layoutSubviews多次,因此请注意仅_arrowImgView在第一次创建时:

- (void)layoutSubviews {
    [super layoutSubviews];

    if (_arrowImgView == nil) {
        UIImage *image = [UIImage imageNamed:@"sound_level_arrow"];
        _arrowImgView = [[UIImageView alloc] initWithImage:image];
        _arrowImgView.contentMode = UIViewContentModeCenter;
        [self addSubview:_arrowImgView];
    }

    CGRect myBounds = self.bounds;
    CGRect mainFrame = self.mainImageView.frame;
    _arrowImgView.bounds = CGRectMake(0, 0, myBounds.size.width, myBounds.size.height);
    _arrowImgView.center = CGPointMake(mainFrame.size.width / 2, mainFrame.size.height - 2);
}