如何从 url 下载照片并保存在 Xamarin

Question

I need Xamarin to call a GET API that downloads a photo from a url and save it in the phone. When xamarin calls the API, it will download a image from a url and then return it as response to xamarin, after this I will save the photo in the phone.

The API is a C# 4.0 MVC. I created a Get API that downloads the image successfully and returns it as FileStreamResult.

The API downloads the image from a url and return it

public FileStreamResult DownloadFoto()
        {

            Stream rtn = null;
            string aURL = "https://image.shutterstock.com/image-vector/sample-stamp-grunge-texture-vector-260nw-1389188336.jpg";
            HttpWebRequest aRequest = (HttpWebRequest)WebRequest.Create(aURL);
            HttpWebResponse aResponse = (HttpWebResponse)aRequest.GetResponse();
            rtn = aResponse.GetResponseStream();
            return File(rtn, "image/jpeg", foto);
        }

Xamarin Forms HttpConnectionis an instance of HttpClient.

var Response = await Application.HttpConnection.GetAsync("DownloadFoto");
                        DependencyService.Get<IFileService>().SavePicture("ImageName.jpg", Response.Content.ReadAsStreamAsync().Result, "imagesFolder");

Xamarin Android

[assembly: Dependency(typeof(FileService))]
namespace Diario.Droid
{
    public class FileService : IFileService
    {
        public void SavePicture(string name, Stream data, string location = "temp")
        {
            var documentsPath = System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal);
            string filePath = Path.Combine(documentsPath, name);
            byte[] bArray = new byte[data.Length];
            using (FileStream fs = new FileStream(filePath, FileMode.OpenOrCreate))
            {
                using (data)
                {
                    data.Read(bArray, 0, (int)data.Length);
                }
                int length = bArray.Length;
                fs.Write(bArray, 0, length);
            }
        }
    }
}

I don't get any error, it looks to be saving the image, but when I look for it in File Management I don't find the image there.

Answer 1

Yeah the directory path your are getting with:

var documentsPath = System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal);

It is pointing at somewhere in /data/user/ which will be private to the App. If you want to make it browse-able using a File Explorer App, you will probably need to save it somewhere in External Storage (public storage).

You can get a path for that with:

var externalDir = Android.OS.Environment.GetExternalStoragePublicDirectory(Android.OS.Environment.DirectoryDocuments);

If you are targeting Android API 18 and lower, remember to add permissions for that in your AndroidManifest:

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" android:maxSdkVersion="18" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" android:maxSdkVersion="18" />

Since some devices on Android can have their External Storage on microSD cards, you will need to check whether you can write to it first:

var stateOk = Android.OS.Environment.ExternalStorageState == Android.OS.Environment.MediaMounted;
if (stateOk)
{
    // I can write!
}

From Android API 23 and up, you will need to request permission before being able to read and write to external storage. You could use this plugin to easily ask for permissions: https://github.com/jamesmontemagno/PermissionsPlugin

Another couple of things to notice. You are using .Result on your ReadAsStreamAsync() call. Just keep in mind that you will be blocking the thread you are on. I highly suggest you design your methods to be async all the way.