如何从网址下载图像并将其保存到本地SQLite数据库 [英] How to Download Image From Url and Save It to a Local SQLite Database

问题描述

在Xamarin中，如何从URL下载图像?

In Xamarin, how do you download an image from a URL?

然后，如何将图像保存到设备上的本地SQLite数据库中?

And then, how do you save the image to a local SQLite database on the device?

我的Xamarin.Forms应用当前遵循 Xamarin文档将URL用作Image的ImageSource属性，这很好用.但是，我注意到，每次启动该应用程序时，它都会通过网络重新下载该映像.我希望从URL下载一次图像，然后将其本地保存到设备中.此方法将为用户节省电池和数据使用量.

My Xamarin.Forms app currently follows the Xamarin Documentation to use a URL for the ImageSource property of an Image, and this works fine. But, I've noticed that every time the app launches, it re-downloads this image over the network. I'd prefer to download the image from the URL once and save it locally to the device; this method will save battery and data usage for the user.

推荐答案

说明

要完成此操作，我们将使用HttpClient从网址中以byte[]的形式下载图像，然后将其保存到我们的本地SQLite数据库中.

Explanation

To accomplish this, we'll download the image from the Url as a byte[] using HttpClient, then save it to our local SQLite database.

此处是一个示例应用程序，它使用Xamarin.Forms完成了此任务.为了获得最佳理解，我建议从GitHub下载代码.

Here is a sample app that accomplishes this using Xamarin.Forms. For the best understanding, I recommend downloading the code from GitHub.

我们将使用HttpClient将图像下载为byte[].这样可以更轻松地将其存储在我们的SQLite数据库中.

We will download the image as a byte[] using HttpClient. This will make it easier to store it in our SQLite Database.

    const int _downloadImageTimeoutInSeconds = 15;
    readonly HttpClient _httpClient = new HttpClient { Timeout = TimeSpan.FromSeconds(_downloadImageTimeoutInSeconds) };

    async Task<byte[]> DownloadImageAsync(string imageUrl)
    {
        try
        {
            using (var httpResponse = await _httpClient.GetAsync(imageUrl))
            {
                if (httpResponse.StatusCode == HttpStatusCode.OK)
                {
                    return await httpResponse.Content.ReadAsByteArrayAsync();
                }
                else
                {
                    //Url is Invalid
                    return null;
                }
            }
         }
         catch (Exception e)
         {
             //Handle Exception
             return null;
         }
    }

将模型保存到数据库

将图像下载为byte[]后，我们会将其存储在SQLite数据库中.

Save Model to Database

Once you've downloaded the image as a byte[], we'll store it in the SQLite database.

我将在下一部分中添加有关该模型的更多信息.

I've added more information about the model in the next section.

public static async Task SaveDownloadedImage(DownloadedImageModel downloadedImage)
{
    var databaseConnection = await GetDatabaseConnectionAsync();
    await databaseConnection.InsertOrReplaceAsync(downloadedImage);
}

型号

在模型中，我创建了一个将图像存储为字符串的属性和一个将Image作为Xamarin.Forms.ImageSource返回的只读属性.

Model

In the model, I've created a property that stores the image as a string and a read-only property that returns the Image as a Xamarin.Forms.ImageSource.

public class DownloadedImageModel
{
    [PrimaryKey]
    public string ImageUrl { get; set;}

    public byte[] DownloadedImageBlob { get; set; }

    public ImageSource DownloadedImageAsImageStreamFromBase64String
    {
        get
        {
            try
            {
                if (DownloadedImageBlob == null)
                    return null;

                var imageByteArray = DownloadedImageBlob;

                return ImageSource.FromStream(() => new MemoryStream(imageByteArray));
            }
            catch (Exception e)
            {
                Debug.WriteLine(e);
                return null;
            }
        }
    }
}

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