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; } } }