我正在基于一组字典为图像创建异步 NSURLConnections，每个字典都有自己的图像 URL:

var posts = [
    ["url": "url0", "calledIndex": 0],
    ["url": "url1", "calledIndex": 1],
    ["url": "url2", "calledIndex": 2],
    ["url": "url3", "calledIndex": 3]
]

考虑到连接的异步性质(这是我想要的，首先加载最快的图像)，图像可能会以不同的顺序加载，例如:

url0
url2
url3
url1

但是，如果图像加载顺序不正确，则需要根据图像加载时间重新组织原始 posts 数组。因此，根据上面的示例，posts 现在应该如下所示:

var posts = [
    ["url": "url0", "calledIndex": 0],
    ["url": "url2", "calledIndex": 2],
    ["url": "url3", "calledIndex": 3],
    ["url": "url1", "calledIndex": 1]
]

在 Swift 中有什么方法可以将特定索引处的数组值与不同索引处的同一数组的值交换？我首先尝试使用 swap 函数:

// Index the images load
var loadedIndex = 0

func connectionDidFinishLoading(connection: NSURLConnection) {

    // Index of the called image in posts
    let calledIndex = posts["calledIndex"] as! Int

    // Index that the image actually loaded
    let loadedIndex = loadedIndex

    // If the indicies are the same, the image is already in the correct position
    if loadedIndex != calledIndex {

        // If they're not the same, swap them
        swap(&posts[calledIndex], &posts[loadedIndex])
    }
}

然后我在没有 swap 函数的情况下尝试了类似的事情:

// The post that was actually loaded
let loadedPost = posts[calledIndex]

// The post at the correct index
let postAtCorrectIndex = posts[loadedIndex]

posts[calledIndex] = postAtCorrectIndex
posts[loadedIndex] = loadedPost 

然而，在这两种情况下，数组值都没有正确交换。我意识到这是一个逻辑错误，但我看不出错误到底在哪里。

据我所知，它第一次正确交换，但后来新字典有一个不正确的 calledIndex 值，导致它交换回原来的位置。

这个假设可能是完全错误的，我意识到我很难描述这种情况，但我会尝试提供尽可能多的澄清。

我做了一个测试用例，你可以download the source code here .它的代码是:

var allPosts:Array<Dictionary<String, AnyObject>> = [
    ["imageURL": "http://i.imgur.com/aLsnGqn.jpg", "postTitle":"0"],
    ["imageURL": "http://i.imgur.com/vgTXEYY.png", "postTitle":"1"],
    ["imageURL": "http://i.imgur.com/OXzDEA6.jpg", "postTitle":"2"],
    ["imageURL": "http://i.imgur.com/ilOKOx5.jpg", "postTitle":"3"],
]

var lastIndex = 0
var threshold = 4
var activeConnections = Dictionary<NSURLConnection, Dictionary<String, AnyObject?>>()

func loadBatchInForwardDirection(){
    func createConnection(i: Int){
        allPosts[i]["calledIndex"] = i
        var post = allPosts[i]
        let imageURL = NSURL(string: post["imageURL"] as! String)
        if imageURL != nil {
            let request = NSMutableURLRequest(URL: imageURL!, cachePolicy: .ReloadIgnoringLocalCacheData, timeoutInterval: 60)
            let connection = NSURLConnection(request: request, delegate: self, startImmediately: true)
            if connection != nil {
                activeConnections[connection!] = post
            }
        }
    }
    let startingIndex = lastIndex;
    for (var i = startingIndex; i < startingIndex + threshold; i++){
        createConnection(i)
        lastIndex++
    }
}

func connection(connection: NSURLConnection, didReceiveData data: NSData) {
    if activeConnections[connection] != nil {
        let dataDict = activeConnections[connection]!["data"]
        if dataDict == nil {
            activeConnections[connection]!["data"] = NSMutableData(data: data)
        } else {
            (activeConnections[connection]!["data"] as! NSMutableData).appendData(data)
        }
    }
}

var loadedIndex = 0
func connectionDidFinishLoading(connection: NSURLConnection) {
    let loadedPost = activeConnections[connection]!
    activeConnections.removeValueForKey(connection)
    let data = loadedPost["data"] as? NSData
    let calledIndex = loadedPost["calledIndex"] as! Int
    println(calledIndex)

    swap(&allPosts[calledIndex], &allPosts[loadedIndex])
    //(allPosts[calledIndex], allPosts[loadedIndex]) = (allPosts[loadedIndex], allPosts[calledIndex])

    loadedIndex++
    done(loadedIndex)
}

func done(index: Int){
    if index == 4 {
        println()
        println("Actual: ")
        println(allPosts[0]["postTitle"] as! String)
        println(allPosts[1]["postTitle"] as! String)
        println(allPosts[2]["postTitle"] as! String)
        println(allPosts[3]["postTitle"] as! String)
    }
}

func applicationDidFinishLaunching(aNotification: NSNotification) {
    loadBatchInForwardDirection()
    println("Loaded: ")
}

func applicationWillTerminate(aNotification: NSNotification) {
    // Insert code here to tear down your application
}

输出是:

Loaded: 1 0 2 3

Actual: 0 1 2 3

然而，预期的“实际”输出应该是:

1 0 2 3

值得注意的是，使用元组代码会产生稍微不稳定的结果，但不会与实际顺序相匹配。您可以通过取消注释该行来明白我的意思。

最佳答案

你可以通过元组赋值:

var xs = [1,2,3]   
(xs[1], xs[2]) = (xs[2], xs[1])

但是您在使用 swap 时遇到了什么问题？以下应该可以正常工作:

swap(&xs[1], &xs[2])

关于arrays - 交换特定索引处的数组值？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/31596077/