最近在项目开发中，想实现一个图片下载并展示进度百分比的功能。很自然的，用到了URLSessionDataTask，通过这个类下载图片，并通过回调方法来观察下载的进度。代码如下：

session = URLSession(configuration: URLSessionConfiguration.default, delegate: self, delegateQueue: OperationQueue.main)    

session?.dataTask(with: url) { (data, response, error) in
    print("receive reponse:\(response)")
}.resume()

//回调函数
public func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive response: URLResponse, completionHandler: @escaping (URLSession.ResponseDisposition) -> Swift.Void) {
    print("didReceive response")
}
    
public func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) {
    print("didReceive data")
}

然而，发现这两个回调函数根本就没有被调用。发现导致这个原因的问题是上面的代码设置了completionHandler: { (<#Data?#>, <#URLResponse?#>, <#Error?#>)。只要设置了completionHandler，回调函数就不执行了。所以上面的代码要改为:

session = URLSession(configuration: URLSessionConfiguration.default, delegate: self, delegateQueue: OperationQueue.main)
                
session?.dataTask(with: url).resume()

然而，运行后发现只执行了public func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive response: URLResponse, completionHandler: @escaping (URLSession.ResponseDisposition)，public func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data)还是没有执行。网上搜索后发现需要在didReceive response回调里设置下面这一行，这样didReceive data方法才会被调用。

func URLSession(session: NSURLSession, dataTask: NSURLSessionDataTask, didReceiveResponse response: NSURLResponse, completionHandler: (NSURLSessionResponseDisposition) -> Void) {
    completionHandler(URLSession.ResponseDisposition.allow) 
}

NSURLSessionResponseDisposition有三个属性，含义如下

NSURLSessionResponseCancel
Cancel the load. Equivalent to calling cancel on the task.
NSURLSessionResponseAllow
Allow the load operation to continue.
NSURLSessionResponseBecomeDownload
Convert this request into a download task.

然而，如果我们不实现didReceive response，那么didReceive data方法是会被调用的。