非同步等待
async
和 await
是兩個運算子,旨在通過釋放執行緒並等待操作在向前移動之前完成來提高效能。
這是一個在返回字串長度之前獲取字串的示例:
//This method is async because:
//1. It has async and Task or Task<T> as modifiers
//2. It ends in "Async"
async Task<int> GetStringLengthAsync(string URL){
HttpClient client = new HttpClient();
//Sends a GET request and returns the response body as a string
Task<string> getString = client.GetStringAsync(URL);
//Waits for getString to complete before returning its length
string contents = await getString;
return contents.Length;
}
private async void doProcess(){
int length = await GetStringLengthAsync("http://example.com/");
//Waits for all the above to finish before printing the number
Console.WriteLine(length);
}
下面是另一個下載檔案並處理其進度發生變化以及下載完成時的情況的示例(有兩種方法可以執行此操作):
方法 1:
//This one using async event handlers, but not async coupled with await
private void DownloadAndUpdateAsync(string uri, string DownloadLocation){
WebClient web = new WebClient();
//Assign the event handler
web.DownloadProgressChanged += new DownloadProgressChangedEventHandler(ProgressChanged);
web.DownloadFileCompleted += new AsyncCompletedEventHandler(FileCompleted);
//Download the file asynchronously
web.DownloadFileAsync(new Uri(uri), DownloadLocation);
}
//event called for when download progress has changed
private void ProgressChanged(object sender, DownloadProgressChangedEventArgs e){
//example code
int i = 0;
i++;
doSomething();
}
//event called for when download has finished
private void FileCompleted(object sender, AsyncCompletedEventArgs e){
Console.WriteLine("Completed!")
}
方法 2:
//however, this one does
//Refer to first example on why this method is async
private void DownloadAndUpdateAsync(string uri, string DownloadLocation){
WebClient web = new WebClient();
//Assign the event handler
web.DownloadProgressChanged += new DownloadProgressChangedEventHandler(ProgressChanged);
//Download the file async
web.DownloadFileAsync(new Uri(uri), DownloadLocation);
//Notice how there is no complete event, instead we're using techniques from the first example
}
private void ProgressChanged(object sender, DownloadProgressChangedEventArgs e){
int i = 0;
i++;
doSomething();
}
private void doProcess(){
//Wait for the download to finish
await DownloadAndUpdateAsync(new Uri("http://example.com/file"))
doSomething();
}