Swift将PHAsset数组转换为URL数组

Question

我有一个PHAssets 的数组，所以我想将它转换为URL 的数组

func getURL(ofPhotoWith mPhasset: PHAsset, completionHandler : @escaping ((_ responseURL : URL?) -> Void)) {
    let options: PHContentEditingInputRequestOptions = PHContentEditingInputRequestOptions()
    options.canHandleAdjustmentData = {(adjustmeta: PHAdjustmentData) -> Bool in
        return true
    }
    mPhasset.requestContentEditingInput(with: options, completionHandler: { (contentEditingInput, info) in
        completionHandler(contentEditingInput!.fullSizeImageURL)
    })
}


func getUrlsFromPHAssets(assets:[PHAsset], completion: @escaping ((_ urls:[URL]) -> ())){
    var arrUrl:[URL] = []
    for asset in assets {
        self.getURL(ofPhotoWith: asset) { (url) in
            arrUrl.append(url!)
        }
    }
    completion(arrUrl)
}

我在 main 中调用下面的函数。

self.getUrlsFromPHAssets(assets: assets, completion: { (urls) in
            print(urls)
}) 

那么，在函数getUrlsFromPHAssets 中，如何等待将url 附加到url 数组的循环完成然后返回。

更新函数getUrlsFromPHAssets：

func getUrlsFromPHAssets(assets:[PHAsset], completion: @escaping ((_ urls:[URL]) -> ())){
    var arrUrl:[URL] = []
    var index:Int = 0
    for asset in assets {
        self.getURL(ofPhotoWith: asset, completionHandler: { (url) in
            arrUrl.append(url!)
            if index == assets.count - 1 {
                completion(arrUrl)
            }
            index = index + 1
        })
    }
}

更新功能后，结果似乎还可以，但我不确定它是否会随时更正。

Answer 1

在您的第二个函数中，您使用for 循环来执行一堆异步函数。每当您有多个完成都依赖于彼此的异步函数时，您可能需要DispatchGroup。该对象可以调用enter() 来指示异步块正在启动，并在异步块完成后调用leave()。

func getUrlsFromPHAssets(assets: [PHAsset], completion: @escaping ((_ urls:[URL]) -> ())) {
    var array: [URL] = []
    let group = DispatchGroup()
    for asset in assets {
        group.enter()
        self.getURL(ofPhotoWith: asset) { (url) in
            // I changed this from force unwrapping.
            // Seems like it's totally possible to get back a nil URL,
            // in which case, you don't want to crash
            if let url = url {
                array.append(url)
            }
            group.leave()
        }
    }
    // This closure will be called once group.leave() is called
    // for every asset in the above for loop
    group.notify(queue: .main) {
        completion(array)
    }
}