在并行 ForEach 循环中异步从网络下载图像

Question

您好，我想在并行循环 foreach 中异步从网络下载图像。

我有带有签名 IDictionary 的字典。 用户类有两个属性：

Uri ProfilePhoto
BitmapImage ProfilePhotoAsBitmapImage

我的目标是，如果 ProfilePhoto 为 null，则循环遍历字典我得到默认头像，而不是我想从网络异步下载图像。

    private void GetProfilePhotosFromServer(IEnumerable<UserInfo> friends)
    {
        Parallel.ForEach(friends, f =>
        {
            //get defualt
            if (f.ProfilePhoto == null) 
                f.ProfilePhotoAsBitmap = CreateDefaultAvatar(f.Sex);

            //start downloading from web server asynchronly
            //problem is that I don’t know how retrieve BitmapImage object from 
            //StartDownloading method or method ProcessImage, which is on the bottom
            //of my question
            var image = StartDownloadingImage(f.MediumProfilePhoto);
            image.Freeze();

            f.ProfilePhotoAsBitmap = image;
        });
    }

问题是我不知道如何从 StartDownloading 方法或 ProcessImage 方法检索 BitmapImage 对象，这是我问题的底部。

启动 Web 请求：

private void StartDownloadingImage(Uri imageUri)
{
    _webRequest = WebRequest.Create(imageUri);
    _webRequest.BeginGetResponse(this.ProcessImage, null);

    //how retrieve result of ProcessImage method
}

Web 请求完成后，我调用此方法：

    private void ProcessImage(IAsyncResult asyncResult)
    {
        var response = _webRequest.EndGetResponse(asyncResult);

        using (var stream = response.GetResponseStream())
        {
            var buffer = new Byte[response.ContentLength];
            int offset = 0, actuallyRead = 0;
            do
            {
                actuallyRead = stream.Read(buffer, offset, buffer.Length - offset);
                offset += actuallyRead;
            }
            while (actuallyRead > 0);


            var image = new BitmapImage
            {
                CreateOptions = BitmapCreateOptions.None,
                CacheOption = BitmapCacheOption.OnLoad
            };
            image.BeginInit();

            image.StreamSource = new MemoryStream(buffer);

            image.EndInit();

            //problem
            return image;
        }

    }

在 ProcessImage 方法中创建 BitmapImage 对象，如何将此对象传递给 GetProfilePhotosFromServer 方法中使用的属性 od User 对象？

上面的方法从 MemoryStream BitampImage 对象创建。

Answer 1

您需要传入附加操作和 UserInfo 对象作为异步方法的回调。最简单的方法是创建一个包含它们的类并将其作为方法的异步状态传递。

private class ImageCallbackState
{
     public UserInfo Friend { get; set; }
     public Action<UserInfo,BitmapImage> Callback { get; set; }
}

private void GetProfilePhotosFromServer(IEnumerable<UserInfo> friends)
{
    Parallel.ForEach(friends, f =>
    {
        //get defualt
        if (f.ProfilePhoto == null) 
            f.ProfilePhotoAsBitmap = CreateDefaultAvatar(f.Sex);

        Action<UserInfo,BitmapImage> action = (u,i) => {
              i.Freeze();
              u.ProfilePhotoAsBitMap = i;
        };
        var state = new ImageCallbackState { Friend = f, Callback = action };
        StartDownloadingImage(f.MediumProfilePhoto,state);

    });
}

private void StartDownloadingImage(Uri imageUri, ImageCallbackState state)
{
    _webRequest = WebRequest.Create(imageUri);
    _webRequest.BeginGetResponse(this.ProcessImage, state); // pass callback state
}

private void ProcessImage(IAsyncResult asyncResult)
{
    var response = _webRequest.EndGetResponse(asyncResult);

    using (var stream = response.GetResponseStream())
    {
        var buffer = new Byte[response.ContentLength];
        int offset = 0, actuallyRead = 0;
        do
        {
            actuallyRead = stream.Read(buffer, offset, buffer.Length - offset);
            offset += actuallyRead;
        }
        while (actuallyRead > 0);


        var image = new BitmapImage
        {
            CreateOptions = BitmapCreateOptions.None,
            CacheOption = BitmapCacheOption.OnLoad
        };
        image.BeginInit();

        image.StreamSource = new MemoryStream(buffer);

        image.EndInit();

        var state = asyncResult.AsyncState as ImageCallbackState

        state.Callback(state.Friend,image);
    }

}