UWP关于图片缓存的那些破事儿

看似简单的功能，实施起来却是有着一堆大坑。

按着基本功能来写吧

1.选择图片并显示到Image控件中

2.图片序列化为byte数组以及反序列化

3.本地存储与读取

1.选择图片：

逻辑就是使用FileOpenPicker选择一个图片文件后，将该文件从StorageFile对象转化成可以赋值给Image.Source的ImageSource类的对象。

（这里需要注意的是WriteableBitmap，BitmapImage，SoftwareBitmapSource都可以赋值给Image.Source，你赋值给Image.Source的是什么对象，你就只能用as符把Image.Source转化成什么对象。 比如：你使用WriteableBitmap赋值给Image.Source，那么你后面只能用 var t = imageSource as WriteableBitmap,  你如果使用var t = imageSource as BitmapImage的话编译器不会报错，但是t的值最后会是null）

1.1 选取图片文件并实例化为WriteableBitmap对象

   private async void ChoosePhoto()
        {
            FileOpenPicker fileOpenPicker = new FileOpenPicker();
            fileOpenPicker.SuggestedStartLocation = PickerLocationId.PicturesLibrary;
            fileOpenPicker.FileTypeFilter.Add(".jpg");
            fileOpenPicker.FileTypeFilter.Add(".png");
            fileOpenPicker.ViewMode = PickerViewMode.Thumbnail;
            StorageFile imgFile_tmp = await fileOpenPicker.PickSingleFileAsync();
            if (imgFile_tmp == null)
            {
                return;
            }
            else
            {
                 this.imgFile = imgFile_tmp;
                 using (IRandomAccessStream stream = await imgFile.OpenAsync(FileAccessMode.Read))
                {
                    var srcImage = new WriteableBitmap(,);
                    await srcImage.SetSourceAsync(stream);
                    Image_Thumbnail.Source = srcImage;
                }
            }
        } 

1.2 选取图片文件并实例化为BitmapImage对象

 private async void ChoosePhoto()
        {
            FileOpenPicker fileOpenPicker = new FileOpenPicker();
            fileOpenPicker.SuggestedStartLocation = PickerLocationId.PicturesLibrary;
            fileOpenPicker.FileTypeFilter.Add(".jpg");
            fileOpenPicker.FileTypeFilter.Add(".png");
            fileOpenPicker.ViewMode = PickerViewMode.Thumbnail;
            StorageFile imgFile_tmp = await fileOpenPicker.PickSingleFileAsync();
            if (imgFile_tmp == null)
            {
                return;
            }
            else
            {
                 this.imgFile = imgFile_tmp;
                 using (IRandomAccessStream stream = await imgFile.OpenAsync(FileAccessMode.Read))
                {
                    var srcImage = new BitmapImage();
                    await srcImage.SetSourceAsync(stream);
                    Image_Thumbnail.Source = srcImage;
                }
            }
        } 

1.3 选取图片实例化为SoftwareBitmapSource对象

 private async void ChoosePhoto()
        {
            FileOpenPicker fileOpenPicker = new FileOpenPicker();
            fileOpenPicker.SuggestedStartLocation = PickerLocationId.PicturesLibrary;
            fileOpenPicker.FileTypeFilter.Add(".jpg");
            fileOpenPicker.FileTypeFilter.Add(".png");
            fileOpenPicker.ViewMode = PickerViewMode.Thumbnail;
            StorageFile imgFile_tmp = await fileOpenPicker.PickSingleFileAsync();
            if (imgFile_tmp == null)
            {
                return;
            }
            else
            {
                SoftwareBitmap softwareBitmap;
                this.imgFile = imgFile_tmp;
                using (IRandomAccessStream stream = await imgFile.OpenAsync(FileAccessMode.Read))
                {
                    // Create the decoder from the stream
                    BitmapDecoder decoder = await BitmapDecoder.CreateAsync(stream);
                    // Get the SoftwareBitmap representation of the file
                    softwareBitmap = await decoder.GetSoftwareBitmapAsync();
                }
                if (softwareBitmap.BitmapPixelFormat != BitmapPixelFormat.Bgra8 || softwareBitmap.BitmapAlphaMode == BitmapAlphaMode.Straight)
                {
                    softwareBitmap = SoftwareBitmap.Convert(softwareBitmap, BitmapPixelFormat.Bgra8, BitmapAlphaMode.Premultiplied);
                }
                var source = new SoftwareBitmapSource();
                await source.SetBitmapAsync(softwareBitmap);
                // Set the source of the Image control
                Image_Thumbnail.Source = source;
                }
        } 

2.图片序列化为byte数组以及反序列化

2.1 WriteableBitmap对象（Image.Source赋值的时候使用的是WriteableBitmap）序列化为byte数组

 public static async Task<byte[]> SaveToBytesAsync(ImageSource imageSource)
        {
            byte[] imageBuffer;
            var localFolder = ApplicationData.Current.TemporaryFolder;
            var file = await localFolder.CreateFileAsync("temp.jpg", CreationCollisionOption.GenerateUniqueName);
            using (var ras = await file.OpenAsync(FileAccessMode.ReadWrite, StorageOpenOptions.None))
            {
                WriteableBitmap bitmap = imageSource as WriteableBitmap;
                var stream = bitmap.PixelBuffer.AsStream();
                byte[] buffer = new byte[stream.Length];
                await stream.ReadAsync(buffer, , buffer.Length);
                BitmapEncoder encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.JpegEncoderId, ras);
                encoder.SetPixelData(BitmapPixelFormat.Bgra8, BitmapAlphaMode.Ignore, (uint)bitmap.PixelWidth, (uint)bitmap.PixelHeight, 96.0, 96.0, buffer);
                await encoder.FlushAsync();

                var imageStream = ras.AsStream();
                imageStream.Seek(, SeekOrigin.Begin);
                imageBuffer = new byte[imageStream.Length];
                var re = await imageStream.ReadAsync(imageBuffer, , imageBuffer.Length);
            }
            await file.DeleteAsync(StorageDeleteOption.Default);
            return imageBuffer;
        }

2.2Byte数组反序列化为ImageSource（WriteableBitmap）

 public static async Task<ImageSource> SaveToImageSource(byte[] imageBuffer)
        {
            ImageSource imageSource = null;
            try
            {
                using (MemoryStream stream = new MemoryStream(imageBuffer))
                {
                    var ras = stream.AsRandomAccessStream();
                    BitmapDecoder decoder = await BitmapDecoder.CreateAsync(BitmapDecoder.JpegDecoderId, ras);
                    var provider = await decoder.GetPixelDataAsync();
                    byte[] buffer = provider.DetachPixelData();
                    WriteableBitmap bitmap = new WriteableBitmap((int)decoder.PixelWidth, (int)decoder.PixelHeight);
                    await bitmap.PixelBuffer.AsStream().WriteAsync(buffer, , buffer.Length);
                    imageSource = bitmap;
                }
            }
            catch (Exception ex)
            {

            }
            return imageSource;
        }

2.3 BitmapImage对象序列化为byte数组

需要注意的是这里所调用的BitmspImage对象，必须是使用BitmapImage(uri)方法实例化出来的BitmapImage对象，因此下列方法主要用于实现对网络图片的序列化与反序列化。

如果使用从FileOpenPicker选取出来图片文件实例化出来的BitmapImage对象的话，该对象的UriSource属性为空，故下列方法无法实现。

                RandomAccessStreamReference random = RandomAccessStreamReference.CreateFromUri(himage.UriSource);
                IRandomAccessStreamWithContentType streamWithContent = await random.OpenReadAsync();
                byte[] buffer = new byte[streamWithContent.Size];
                await streamWithContent.ReadAsync(buffer.AsBuffer(), (uint)streamWithContent.Size, InputStreamOptions.None); 

2.4 byte数组反序列化为BitmapImage对象

                BitmapImage image = new BitmapImage();
                using (InMemoryRandomAccessStream stream = new InMemoryRandomAccessStream())
                {
                    await stream.WriteAsync(obj.ImgArray.AsBuffer());
                    stream.Seek();
                    await image.SetSourceAsync(stream);
                } 

3.本地存储与读取

（需要注意的是：StorageFile这类的存储操作貌似只允许存一些简单的数据类型，所以我的解决方法就是把自定义的类的对象里的图片转化为byte数组，然后再将自定义类的对象的List转化成json字符串存进文件，读取文件的时候再读出这些Json字符串并反序列化为我需要的类型）

3.1 存储数据

 public static async Task SaveData(string filename, T data)
        {
            try
            {
                StorageFile file = await ApplicationData.Current.LocalFolder.CreateFileAsync(filename, CreationCollisionOption.ReplaceExisting);
                using (IRandomAccessStream raStream = await file.OpenAsync(FileAccessMode.ReadWrite))
                {
                    using (IOutputStream outStream = raStream.GetOutputStreamAt())
                    {

                        var json = JsonConvert.SerializeObject(data);
                        DataContractSerializer serializer = new DataContractSerializer(typeof(string));
                        serializer.WriteObject(outStream.AsStreamForWrite(), json);
                        var t =JsonConvert.DeserializeObject<T>(json);
                        await outStream.FlushAsync();
                    }
                }
            }
            catch (Exception exc)
            {
                throw exc;
            }
        }

3.2 加载数据

public static async System.Threading.Tasks.Task<List<ShowInfo>> LoadData(string filename)
        {
            try
            {
                if (ApplicationData.Current.LocalFolder.TryGetItemAsync(filename) != null)
                {
                    StorageFile file = await ApplicationData.Current.LocalFolder.GetFileAsync(filename);
                    using (IInputStream inStream = await file.OpenSequentialReadAsync())
                    {
                        DataContractSerializer serializer = new DataContractSerializer(typeof(string));
                        var json = (string)serializer.ReadObject(inStream.AsStreamForRead());
                        var data = JsonConvert.DeserializeObject<List<ShowInfo>>(json);
                        return data;
                    }
                }
                else
                {
                    return new List<ShowInfo>();
                }     

            }
            catch (FileNotFoundException ex)
            {
                throw ex;
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }