在 Windows Phone 8 中下载图像 [英] Download Image in Windows Phone 8

问题描述

我必须下载大量图像并将其保存到本地文件夹.此外，我必须在下载图像时更新 UI.现在我使用下面的代码来一张一张的下载图片.但问题是 UI 在每次下载请求时都被阻止.我应该如何处理下载方法?我对线程了解不多.谁能帮我一个好的方法?

I have to download and save lot of images to local folder. Also I have to update UI while downloading images. Now I am using the following code to download images one by one. But the problem is UI getting blocked on each download request. How should I handle download method? I don't know much about threading. Can anyone help me with a good method?

public async Task<T> ServiceRequest<T>(string serviceurl, object request)
{
    string response = "";
    httpwebrequest = WebRequest.Create(new Uri(serviceurl)) as HttpWebRequest;
    httpwebrequest.Method = "POST";

    httpwebrequest.ContentType = "application/json";
    byte[] data = Serialization.SerializeData(request);

    using (var requestStream = await Task<Stream>.Factory.FromAsync(httpwebrequest.BeginGetRequestStream, httpwebrequest.EndGetRequestStream, null))
    {
        await requestStream.WriteAsync(data, 0, data.Length);
    }

    response = await httpRequest(httpwebrequest);

    var result = Serialization.Deserialize<T>(response);
    return result;
}


public async Task<string> httpRequest(HttpWebRequest request)
{
    try
    {
        string received;

        using (var response = (HttpWebResponse)(await Task<WebResponse>.Factory.FromAsync(request.BeginGetResponse, request.EndGetResponse, null)))
        {
            using (var responseStream = response.GetResponseStream())
            {
                using (var sr = new StreamReader(responseStream))
                {
                    received = await sr.ReadToEndAsync();
                }
            }            
            response.Close();
        }

        return received;
    }
    catch(Exception ex)
    {
        return "";
    }
}

推荐答案

我建议您使用 System.Net.Http.HttpClient.您可以从 Nuget 获取它，只需选择搜索选项以包含预发布频道.不过有一个问题:它还没有正式发布，所以你现在不能在生产代码中使用它.并且没有关于它何时最终发布的消息.但如果你只是在学习，你可以自由地使用它.它使您描述的事情变得非常简单.这是我的代码中的一个示例，巧合的是:)，它完全符合您的要求:

I would recommend you to use the System.Net.Http.HttpClient. You could get it from the Nuget, just select the search option to include the pre-release channel as well. There's one catch though: it's not officially released yet, so you cannot use it in production code right now. And there's no word on when it will be finally released. But if you're just learning, you could use it freely. And it make things like you describe very straightforward. Here's a sample from my code, which, by a happy coincidence :), does exactly what you want:

private IEnumerable<string> CountPictures(int from, int to, string folder)
{
    for (int i = from; i < to; i++)
        yield return string.Format("{0}/image{1}.jpg", folder, i.ToString("D2"));
}

private async Task ImportImages()
{
    HttpClient c = new HttpClient();
    int count = 0;
    c.BaseAddress = new Uri("http://www.cs.washington.edu/research/imagedatabase/groundtruth/", UriKind.Absolute);
    foreach (var pic in CountPictures(1, 48, "leaflesstrees"))
    {
        var pic_response = await c.GetAsync(pic, HttpCompletionOption.ResponseContentRead);
        if (pic_response.IsSuccessStatusCode)
        {
            await SaveImageAsync(pic.Replace('/', '_'), await pic_response.Content.ReadAsStreamAsync());
            Debug.WriteLine(pic + " imported");
            count++;
        }
    }           
    Debug.WriteLine(string.Format("{0} images imported", count));
}

private Task SaveImageAsync(string filename, Stream stream)
{
    var task = Task.Factory.StartNew(() =>
    {
        if (stream == null || filename == null)
        {
            throw new ArgumentException("one of parameters is null");
        }
        try
        {
            using (IsolatedStorageFile isoStore = IsolatedStorageFile.GetUserStoreForApplication())
            {
                using (IsolatedStorageFileStream targetStream = isoStore.OpenFile(filename, FileMode.Create, FileAccess.Write))
                {
                    byte[] readBuffer = new byte[4096];
                    int bytesRead = -1;
                    stream.Position = 0;
                    targetStream.Position = 0;

                    while ((bytesRead = stream.Read(readBuffer, 0, readBuffer.Length)) > 0)
                    {
                        targetStream.Write(readBuffer, 0, bytesRead);
                    }
                }
            }
        }
        catch (Exception e)
        {
            System.Diagnostics.Debug.WriteLine("DocumentStorageService::LoadImage FAILED " + e.Message);
        }
    });
    return task;
}

为了显示来自隔离存储的图像，您可以参考我的回答中描述的方法之一此处.

In order to display an image from Isolated Storage then, you could refer to one of the approaches described in my answer here.

这篇关于在 Windows Phone 8 中下载图像的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！