Windows Phone App基于文件系统的图片缓存方案

最近在做一个Windows Phone 8.1的应用，因为应用里面使用了很多图片，所以打算将图片文件缓存到文件系统里面，以便节省流量和提升体验。

一般显示图片的做法大多只需要将对应的Uri地址绑定到对应控件的ImageSource属性上即可，或者将Uri传入BitmapImage对象，其会自动将资源异步下载好然后加载。

为了不将缓存的逻辑侵入实体模型层，所以打算在Uri->BitmapImage绑定上做文章，也就是利用IValueConverter接口构造值转换器。

这里有一个问题就是，因为绑定的原因，IValueConverter接口只能是同步的方法，而图片的下载的过程是一个异步的过程，所以我们需要一种解决方案实现一个异步的ValueConverter。

这个解决方案我思考了很久，最后在这里受到了启发：http://stackoverflow.com/questions/15003827/async-implementation-of-ivalueconverter。

Update: 顺藤摸瓜找到了这个问题的答主的一个开源库，主要是对async和await的拓展，推荐：https://github.com/StephenCleary/AsyncEx

具体而言，就是在同步的ValueConverter中构造一个Task-like的对象，然后将对应的属性绑定到这个Task-like对象的一个属性中去，然后Task-like待Task完成后更新对应的状态。

这个解决方案实现很巧妙，同时也没有对Model和ViewModel做任何的侵入，下面是我修改后用于图片缓存的相关代码：

首先是界面绑定所做的改动，大概如下，就是通过构造一个新的DataContext来实现绑定：

<Image DataContext="{Binding Image, Converter={StaticResource ImageConverter}}"
             Stretch="UniformToFill" Source="{Binding Result}" />

对应的ValueConverter类：

public class CacheImageValueConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, string language)
        {
            var image = value as Image;
            if (image == null || string.IsNullOrEmpty(image.Url))
            {
                var bitmap = new BitmapImage(new Uri("ms-appx:///assets/default.png"));
                var notifier = new TaskCompletionNotifier<BitmapImage>();
                notifier.SetTask(Task.FromResult(bitmap));
                return notifier;
            }
            else
            {
                var task = Task.Run(async () =>
                {
                    var cache = HiwedoContainer.Current.Resolve<ImageCache>();
                    var uri = await cache.GetImageSourceFromUrlAsync(image.Url);
                    return uri;
                });
                var notifier = new TaskCompletionNotifier<BitmapImage>();
                notifier.SetTask(task, c => new BitmapImage(c));
                return notifier;
            }
        }


        public object ConvertBack(object value, Type targetType, object parameter, string language)
        {
            throw new NotImplementedException();
        }
    }

其次是Task-like的类。

public sealed class TaskCompletionNotifier<TResult> : INotifyPropertyChanged
    {
        private TResult _result;
        private IAsyncResult _task;

        public TaskCompletionNotifier()
        {
            this._result = default(TResult);
        }

        public void SetTask<T>(Task<T> task, Func<T, TResult> factoryFunc)
        {
            this._task = task;
            if (!task.IsCompleted)
            {
                var scheduler = (SynchronizationContext.Current == null)
                    ? TaskScheduler.Current
                    : TaskScheduler.FromCurrentSynchronizationContext();
                task.ContinueWith(t =>
                {
                    var propertyChanged = PropertyChanged;
                    if (propertyChanged != null)
                    {
                        this.OnPropertyChanged("IsCompleted");
                        if (t.IsFaulted)
                        {
                            InnerException = t.Exception;
                            this.OnPropertyChanged("ErrorMessage");
                        }
                        else
                        {
                            try
                            {
                                this._result = factoryFunc(task.Result);
                            }
                            catch (Exception ex)
                            {
                                Debug.WriteLine("Factory error: " + ex.Message);
                                this.InnerException = ex;
                                this.OnPropertyChanged("ErrorMessage");
                            }
                            this.OnPropertyChanged("Result");
                        }
                    }
                },
                    CancellationToken.None,
                    TaskContinuationOptions.ExecuteSynchronously,
                    scheduler);
            }
            else
            {
                this._result = factoryFunc(task.Result);
            }
        }

        public void SetTask(Task<TResult> task)
        {
            this._task = task;
            if (!task.IsCompleted)
            {
                var scheduler = (SynchronizationContext.Current == null)
                    ? TaskScheduler.Current
                    : TaskScheduler.FromCurrentSynchronizationContext();
                task.ContinueWith(t =>
                {
                    var propertyChanged = PropertyChanged;
                    if (propertyChanged != null)
                    {
                        this.OnPropertyChanged("IsCompleted");
                        if (t.IsFaulted)
                        {
                            InnerException = t.Exception;
                            this.OnPropertyChanged("ErrorMessage");
                        }
                        else
                        {
                            this._result = task.Result;
                            this.OnPropertyChanged("Result");
                        }
                    }
                },
                    CancellationToken.None,
                    TaskContinuationOptions.ExecuteSynchronously,
                    scheduler);
            }
            else
            {
                this._result = task.Result;
            }
        }

        public TResult Result
        {
            get { return this._result; }
        }

        public bool IsCompleted
        {
            get { return _task.IsCompleted; }
        }

        public Exception InnerException { get; set; }

        public string ErrorMessage
        {
            get { return (InnerException == null) ? null : InnerException.Message; }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        [NotifyPropertyChangedInvocator]
        private void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
        }
    }

提供一个Task和Factory的函数重载是必须的，因为对于像BitmapImage这样的类，其初始化需要在对应的UI线程中，不能直接将BitmapImage作为结果返回，所以在下面也可以看到，实际上Task返回的只是Uri，然后构造BitmapImage是在ContinueWith的方法中完成的。

最后就是用于图片缓存的类，这个类比较简单，就是如果判断如果图片已经存在，就直接从文件系统返回，如果不存在就先下载再返回对应的Uri。然后会有一个变量缓存所有已经缓存的文件名。

public class ImageCache
    {
        private const string ImageCacheFolder = "ImageCaches";
        private StorageFolder _cacheFolder;
        private IList<string> _cachedFileNames;

        public async Task<Uri> GetImageSourceFromUrlAsync(string url)
        {
            string fileName = url.Substring(url.LastIndexOf('/') + 1);
            if (this._cachedFileNames.Contains(fileName))
            {
                return new Uri("ms-appdata:///local/ImageCaches/" + fileName);
            }
            if (await DownloadAndSaveAsync(url, fileName))
            {
                _cachedFileNames.Add(fileName);
                return new Uri("ms-appdata:///local/ImageCaches/" + fileName);
            }
            Debug.WriteLine("Download image failed. " + url);
            return new Uri(url);
        }

        private async Task<bool> DownloadAndSaveAsync(string url, string filename)
        {
            try
            {
                var request = WebRequest.CreateHttp(url);
                request.Method = "GET";
                using (var response = await request.GetResponseAsync())
                {
                    using (var responseStream = response.GetResponseStream())
                    {
                        var file =
                            await this._cacheFolder.CreateFileAsync(filename, CreationCollisionOption.ReplaceExisting);
                        using (var fs = await file.OpenStreamForWriteAsync())
                        {
                            await responseStream.CopyToAsync(fs);
                            Debug.WriteLine("Downloaded: " + url);
                            return true;
                        }
                    }
                }
            }
            catch (Exception ex)
            {
                Debug.WriteLine("Error: " + ex.Message);
                return false;
            }
        }

        public async Task LoadCache()
        {
            var folders = await ApplicationData.Current.LocalFolder.GetFoldersAsync();
            foreach (var folder in folders)
            {
                if (folder.Name == ImageCacheFolder)
                {
                    this._cacheFolder = folder;
                    break;
                }
            }
            if (this._cacheFolder == null)
            {
                this._cacheFolder = await ApplicationData.Current.LocalFolder.CreateFolderAsync(ImageCacheFolder);
            }
            this._cachedFileNames = (await this._cacheFolder.GetFilesAsync()).Select(c => c.Name).ToList();
        }

        public IList<string> CachedFileNames
        {
            get { return _cachedFileNames; }
            set { _cachedFileNames = value; }
        }
    }