ios - 关于在 Swift 4 中返回 CGFloat 值

Question

下载图像后，我创建了一个返回图像垂直值的函数。

但是，图像的垂直值将失败并返回 CGFloat 值。

此源码下载返回

extension HomeViewController : PinterestLayoutDelegate
{
    func collectionView(_ collectionView: UICollectionView, heightForPhotoAtIndexPath indexPath:IndexPath) -> CGFloat
    {
        let arts = self.artList

        let model = arts![indexPath.row]

        let urlString = model.url!
        let urll = NSURL(string: urlString)

        return downloadImage(from: urll! as URL)
    }
}

extension HomeViewController
{
    func getData(from url: URL, completion: @escaping (Data?, URLResponse?, Error?) -> ())
    {
        URLSession.shared.dataTask(with: url, completionHandler: completion).resume()
    }

    func downloadImage(from url: URL) -> CGFloat
    {
        getData(from: url)
        { data, response, error in
        guard let data = data, error == nil else { return }
            {
                let image = UIImage(data: data)
                let heightInPoints = image!.size.height
                return heightInPoints
            }
        }
    }
}

但是我在

return heightInPoints

错误状态

Unexpected non-void return value in void function

请帮帮我。

Answer 1

问题是你想在完成 block 中返回一个值，你需要

func downloadImage(from url: URL,completion(_ value:CGFloat) -> Void )
{
    getData(from: url)
    { data, response, error in
    guard let data = data, error == nil else { return }
        {
            let image = UIImage(data: data)
            let heightInPoints = image!.size.height
            completion(heightInPoints)
        }
    }
}

顺便说一句，你不能这样做

return downloadImage(from: urll! as URL)

在 heightForPhotoAtIndexPath 中，因为 downloadImage 是异步方法，它的响应取决于网络状态，而不是在代码中串行，它的调用看起来像

// array of heights
var arr = [CGFloat]()
let urll = URL(string: urlString)
downloadImage(from: urll!) { (value) in 
  print(value)
  // append the value to it's corresponding index in an array of floats
  DispatchQueue.main.async {
    self.collectionview.reloadData()
  }
}

---

如果可能，您需要为图像预定义高度列表，如果没有，则获取所有高度然后刷新集合