ios - 如何快速循环遍历一组对象

Question

我正在尝试访问存储在数组中的对象的 url，但无论我使用什么方法，我都会遇到错误。

let userPhotos = currentUser?.photos

    for var i = 0; i < userPhotos!.count ; ++i {
        let url = userPhotos[i].url
    }

我来了

Could not find member 'url'

还有一个foreach:

for photo in userPhotos {

        Utils.getImageAsync(photo.url , completion: { (img: UIImage?) -> () in

        })
    }

我得到:

'[ModelAttachment]?' does not have a member named 'Generator'

我的数组是 var photos: Array<ModelAttachment>?我的 ModelAttachment 看起来像这样:

class ModelAttachment :Model {
var id: String?
var url: String?
var thumb: String?
}

任何指向我做错了什么的指针都会很棒 :)

Answer 1

在使用简单的 for in 循环进行迭代之前，使用 if let 安全地将对象解包并向下转型为正确的类型。

if let currentUser = currentUser, 
    let photos = currentUser.photos as? [ModelAttachment] 
{
    for object in photos {
        let url = object.url
    }
}

如果您希望结果在范围内可用，还有 guard let else 而不是 if let:

guard let currentUser = currentUser, 
    let photos = currentUser.photos as? [ModelAttachment] else 
{
    // break or return
}
// now 'photos' is available outside the guard
for object in photos {
    let url = object.url
}