我使用以下代码显示来自网络服务器的图片：
<Image Source="{Binding Url}" />
图像会自动下载，我假设还有一些基于Url的缓存。
我的问题是，当应用程序处于脱机状态时，无法显示缓存的图像。
有没有办法更改缓存行为，以便在没有网络可用时加载图像？有关缓存的文档指针也非常有用。
答案 0 :(得分：23)
BitmapImage默认自动缓存远程图像。最好与CreateOptions="BackgroundCreation"结合使用以获得最佳性能。
<Image Height="100" Width="100" Margin="12,0,9,0">
<Image.Source>
<BitmapImage UriSource="{Binding ImgURL}" CreateOptions="BackgroundCreation"/>
</Image.Source>
</Image>
This MSDN blog post，旧的但仍然相关，列出并解释了所有CreationOptions，并且在大多数模式下缓存都是自动的。
我使用这些选项显示带图像的许多新闻项目，效果很好。我可以加载文章列表，退出应用程序并将“飞行模式”设置为“打开”，然后启动应用程序的新实例，图像仍然会加载。
手动方法
如果你想自己控制缓存并缓存HTTPS资源，那么很少有很好的例子......
答案 1 :(得分：4)
我有一个解决方案给你。它是JetImageLoader，我为应用程序创建了它，我们需要加载，缓存并显示大量的徽标，图标等。
它可以用作绑定转换器，所以你甚至不应该改变你的代码！只需更新您的XAML！
请查看samples in repository，你会喜欢它;）
功能
以下是示例：
<Image Source="{Binding ImageUrl, Converter={StaticResource MyAppJetImageLoaderConverter}}"/>
答案 2 :(得分：1)
我认为没有可行的方法，但您可以将图像保存在IsolatedStorage中，并使用转换器检查互联网可用性，并返回在线或离线网址。
A quick search yielded this这可能正是您所寻找的（它与Windows Phone 7兼容，可能不是Windows Phone 8的最佳解决方案）
答案 3 :(得分：1)
您也可以使用FFImageLoading（https://github.com/molinch/FFImageLoading/）
它就像：
一样简单<ff:MvxCachedImage Name="image"
VerticalAlignment="Stretch"
HorizontalAlignment="Stretch"
LoadingPlaceholder="loading.png"
ErrorPlaceholder="error.png"
RetryCount="3"
RetryDelay="250"
DownsampleHeight="300"
ImagePath="http://lorempixel.com/output/city-q-c-600-600-5.jpg">
</ff: MvxCachedImage >
此处的示例项目：https://github.com/molinch/FFImageLoading/tree/master/samples/
答案 4 :(得分：1)
我的解决方案： （将图像从Web保存到本地存储并将保存的图像绑定到页面）
XAML
<ListView ItemsSource="{Binding Items}">
<ListView.ItemTemplate>
<DataTemplate>
<!--Some code removed-->
<Image Source="{Binding Img_Thumb.Result}" />
</DataTemplate>
</ListView.ItemTemplate>
的DataModel
public class DataModel_ListOfEvents
{
public DataModel_ListOfEvents(String img_thumb)
{
this.Img_Thumb = new NotifyTaskCompletion<string>(JsonCached.ImageFromCache2(img_thumb));
}
public NotifyTaskCompletion<string> Img_Thumb { get; private set; }
}
public sealed class SampleData_ListOfEvents
{
private static SampleData_ListOfEvents _sampleDataSource = new SampleData_ListOfEvents();
private ObservableCollection<DataModel_ListOfEvents> _items = new ObservableCollection<DataModel_ListOfEvents>();
public ObservableCollection<DataModel_ListOfEvents> Items { get { return this._items; } }
}
魔术
public class JsonCached
{
public static async Task<string> ImageFromCache2(string path)
{
int ru = path.IndexOf(".ru") + 4;// TODO: .com .net .org
string new_path = path.Substring(ru).Replace("/", "\\");
StorageFolder localFolder = ApplicationData.Current.LocalFolder;
try
{
Stream p = await localFolder.OpenStreamForReadAsync(new_path);
p.Dispose();
System.Diagnostics.Debug.WriteLine("From cache");
return localFolder.Path + "\\" + new_path;
}
catch (FileNotFoundException)
{
}
catch (Exception e)
{
System.Diagnostics.Debug.WriteLine("{0}", e.Message);
}
StorageFile storageFile = await localFolder.CreateFileAsync(new_path, CreationCollisionOption.OpenIfExists);
Uri Website = new Uri(path);
HttpClient http = new HttpClient();
// TODO: Check connection. Return message on fail.
System.Diagnostics.Debug.WriteLine("Downloading started");
byte[] image_from_web_as_bytes = await http.GetByteArrayAsync(Website);
MakeFolders(localFolder, path.Substring(ru));
Stream outputStream = await storageFile.OpenStreamForWriteAsync();
outputStream.Write(image_from_web_as_bytes, 0, image_from_web_as_bytes.Length);
outputStream.Position = 0;
System.Diagnostics.Debug.WriteLine("Write file done {0}", outputStream.Length);
outputStream.Dispose();
return localFolder.Path + "\\" + new_path;
}
private static async void MakeFolders(StorageFolder localFolder, string path)
{
//pics/thumbnail/050/197/50197442.jpg
int slash = path.IndexOf("/");
if (slash <= 0) // -1 Not found
return;
string new_path = path.Substring(0, slash);
StorageFolder opened_folder = await localFolder.CreateFolderAsync(new_path, CreationCollisionOption.OpenIfExists);
string very_new_path = path.Remove(0, new_path.Length + 1);
MakeFolders(opened_folder, very_new_path);
}
}
NotifyTaskCompletion
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.ComponentModel;
namespace App2.NotifyTask
{
public sealed class NotifyTaskCompletion<TResult> : INotifyPropertyChanged
{
public NotifyTaskCompletion(Task<TResult> task)
{
Task = task;
if (!task.IsCompleted)
{
var _ = WatchTaskAsync(task);
}
}
private async Task WatchTaskAsync(Task task)
{
try
{
await task;
}
catch
{
}
var propertyChanged = PropertyChanged;
if (propertyChanged == null)
return;
propertyChanged(this, new PropertyChangedEventArgs("Status"));
propertyChanged(this, new PropertyChangedEventArgs("IsCompleted"));
propertyChanged(this, new PropertyChangedEventArgs("IsNotCompleted"));
if (task.IsCanceled)
{
propertyChanged(this, new PropertyChangedEventArgs("IsCanceled"));
}
else if (task.IsFaulted)
{
propertyChanged(this, new PropertyChangedEventArgs("IsFaulted"));
propertyChanged(this, new PropertyChangedEventArgs("Exception"));
propertyChanged(this,
new PropertyChangedEventArgs("InnerException"));
propertyChanged(this, new PropertyChangedEventArgs("ErrorMessage"));
}
else
{
propertyChanged(this,
new PropertyChangedEventArgs("IsSuccessfullyCompleted"));
propertyChanged(this, new PropertyChangedEventArgs("Result"));
}
}
public Task<TResult> Task { get; private set; }
public TResult Result { get { return (Task.Status == TaskStatus.RanToCompletion) ? Task.Result : default(TResult); } }
public TaskStatus Status { get { return Task.Status; } }
public bool IsCompleted { get { return Task.IsCompleted; } }
public bool IsNotCompleted { get { return !Task.IsCompleted; } }
public bool IsSuccessfullyCompleted { get { return Task.Status == TaskStatus.RanToCompletion; } }
public bool IsCanceled { get { return Task.IsCanceled; } }
public bool IsFaulted { get { return Task.IsFaulted; } }
public AggregateException Exception { get { return Task.Exception; } }
public Exception InnerException { get { return (Exception == null) ? null : Exception.InnerException; } }
public string ErrorMessage { get { return (InnerException == null) ? null : InnerException.Message; } }
public event PropertyChangedEventHandler PropertyChanged;
}
}