确定 UIView 何时完成布局

Question

我有 UIView 子类，并且为了布局 UI 元素，我不使用自动布局。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，看起来 self.frame.size.height 或宽度为零。对于临时解决方案，我做了以下操作：

    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];
    });

但我当然需要知道，视图何时完成加载以及它的实际 self.frame.size.width 和高度在哪里。

Answer 1

如果您想在自定义视图子类中执行此操作，请覆盖该类中的 layoutSubviews。由于 iOS 可能会多次发送layoutSubviews，请注意只在第一次创建_arrowImgView：

- (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);
}

Answer 2

使用这个。它处理方向变化和渐变背景的重绘。

import UIKit

@IBDesignable
final class GradientButton: UIButton {

    @IBInspectable var startColor: UIColor = UIColor.clear
    @IBInspectable var endColor: UIColor = UIColor.clear

    override func draw(_ rect: CGRect) {

        if let gradient = layer.sublayers?.first as? CAGradientLayer {
            gradient.removeFromSuperlayer()
        }

        let gradient: CAGradientLayer = CAGradientLayer()
        gradient.frame = CGRect(origin: CGPoint.zero, size: self.frame.size)
        gradient.colors = [startColor.cgColor, endColor.cgColor]
        layer.insertSublayer(gradient, at: 0)

        super.draw(rect)
    }

   @IBInspectable var imageContentMode: Int {
        get {
            return imageView?.contentMode.rawValue ?? 0
        }
        set {
            imageView?.contentMode = UIViewContentMode(rawValue: newValue) ?? .scaleAspectFit

        }
    }
}

Answer 3

当'图层操作'例如添加边框需要视图的最终大小时，您可以使用（在swift 4中）：

override func layoutSublayers(of layer: CALayer) {
    super.layoutSublayers(of: layer)
    // 'layer operations'
}

我正在努力通过一个函数只在底部添加一个边框， 我遇到了边框比按钮的最终宽度短的问题。

我把这个留给那些遇到同样问题并尝试过layoutSubviews但没有解决问题的人。

Answer 4

viewWillLayoutSubviews 和最后一个 viewDidLayoutSubviews

- (void)viewDidLayoutSubviews {
   [super viewDidLayoutSubviews];
   // your code for positioning any subview here
}