从 mono 上的本地存储读取图像文件，用于 android

Question

In mono for android I have an app that saves images to local storage for caching purposes. When the app launches it tries to load images from the cache before trying to load them from the web.

I'm currently having a hard time finding a good way to read and load them from local storage.

I'm currently using something equivilant to this:

        List<byte> byteList = new List<byte>();
        using (System.IO.BinaryReader binaryReader = new System.IO.BinaryReader(context.OpenFileInput("filename.jpg")))
        {
            while (binaryReader.BaseStream.IsDataAvailable())
            {
                byteList.Add(binaryReader.ReadByte());
            }
        }
        return byteList.toArray();

OpenFileInput() returns a stream that does not give me a length so I have to read one byte at a time. It also can't seek. This seems to be causing images to load much slower than they aughto. Loading images from Resrouce.Drawable is almost instantanious by comparison, but with my method there a very noticable pause, maybe 300ms, for loading a 8kb file. This seems like a really obvious task to be able to do, but I've tried many solutions and searched a lot for advise but to no avail.

I've also noticed this code seems to crash with an EndOfStream exception when not run on the UI thread.

Any help would be hugely appreciated

Answer 1

What do you intend on doing with the List<byte> ? You want to "load images from the cache," but you don't specify what you want to load them into .

If you want to load them into aAndroid.Graphics.Bitmap , you could use BitmapFactory.DecodeStream(Stream) :

Bitmap bitmap = BitmapFactory.DecodeStream(context.OpenFileInput("filename.jpg"));

This would remove the List<byte> intermediary.

If you really need all the bytes (for whatever reason), you can rely on the fact that System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal) is the same as Context.FilesDir , which is what context.OpenFileInput() will use, permitting:

byte[] bytes = System.IO.File.ReadAllBytes(
        Path.Combine (
            System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal),
            "filename.jpg"));

However, if this is truly a cache, you should be using Context.CacheDir instead of Context.FilesDir , which is Path.GetTempPath returns:

byte[] cachedBytes = System.IO.File.ReadAllBytes(
        Path.Combine(System.IO.Path.GetTempPath(), "filename.jpg"));