在C#中异步加载BitmapImage [英] async load BitmapImage in C#
问题描述
我正在尝试异步加载图像.
I'm trying to load a image asynchronously.
MainWindow代码
public partial class MainWindow : Window
{
private Data data = new Data();
public MainWindow()
{
InitializeComponent();
this.DataContext = data;
}
private async void button_Click(object sender, RoutedEventArgs e)
{
data.Image = await Data.GetNewImageAsync();
}
}
数据类
public class Data : INotifyPropertyChanged
{
private BitmapImage _Image = new BitmapImage();
public BitmapImage Image { get { return _Image; } set { _Image = value; OnPropertyChanged("Image"); } }
public static BitmapImage GetNewImage()
{
return new BitmapImage(new Uri("http://www.diseno-art.com/news_content/wp-content/uploads/2012/09/2013-Jaguar-F-Type-1.jpg"));
}
public async static Task<BitmapImage> GetNewImageAsync()
{
return await Task.Run(() => GetNewImage());
}
public event PropertyChangedEventHandler PropertyChanged;
public void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
WPF代码
<Button Name="button" Click="button_Click">Image</Button>
<Image Grid.Row="1" Source="{Binding Path=Image, UpdateSourceTrigger=PropertyChanged}"></Image>
问题
我得到了例外:
System.ArgumentException:必须在同一线程上创建DependencySource 作为DependencyObject."
System.ArgumentException: "Must create DependencySource on same Thread as the DependencyObject."
...在此行中:
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
... in this row:
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
但是如果我将BitmapImage更改为字符串,则此代码可以正常工作.
But if i change BitmapImage to string this code works fine.
我在做什么错了?
推荐答案
在后台线程中创建BitmapImage时,必须确保将其冻结后才能在UI线程中使用.
When a BitmapImage is created in a background thread, you have to make sure that it gets frozen before it is used in the UI thread.
您必须像这样自己加载它:
You would have to load it yourself like this:
public static async Task<BitmapImage> GetNewImageAsync(Uri uri)
{
BitmapImage bitmap = null;
var httpClient = new HttpClient();
using (var response = await httpClient.GetAsync(uri))
{
if (response.IsSuccessStatusCode)
{
using (var stream = new MemoryStream())
{
await response.Content.CopyToAsync(stream);
stream.Seek(0, SeekOrigin.Begin);
bitmap = new BitmapImage();
bitmap.BeginInit();
bitmap.CacheOption = BitmapCacheOption.OnLoad;
bitmap.StreamSource = stream;
bitmap.EndInit();
bitmap.Freeze();
}
}
}
return bitmap;
}
或更短的BitmapFrame.Create
,它将返回已冻结的BitmapSource:
Or shorter with BitmapFrame.Create
, which returns an already frozen BitmapSource:
public static async Task<BitmapSource> GetNewImageAsync(Uri uri)
{
BitmapSource bitmap = null;
var httpClient = new HttpClient();
using (var response = await httpClient.GetAsync(uri))
{
if (response.IsSuccessStatusCode)
{
using (var stream = new MemoryStream())
{
await response.Content.CopyToAsync(stream);
stream.Seek(0, SeekOrigin.Begin);
bitmap = BitmapFrame.Create(
stream, BitmapCreateOptions.None, BitmapCacheOption.OnLoad);
}
}
}
return bitmap;
}
请注意,第二种方法需要将Image
属性的类型更改为BitmapSource(或者更好的是ImageSource),无论如何这将提供更大的灵活性.
Note that the second method requires to change the type of your Image
property to BitmapSource (or even better, ImageSource), which would provide greater flexibility anyway.
另一种无需任何手动下载的方法可能如下所示.它也不需要冻结BitmatImage,因为它不是在Task线程中创建的.
An alternative method without any manual download might look like shown below. It also does not require to freeze the BitmatImage, because it is not created in a Task thread.
public static Task<BitmapSource> GetNewImageAsync(Uri uri)
{
var tcs = new TaskCompletionSource<BitmapSource>();
var bitmap = new BitmapImage(uri);
if (bitmap.IsDownloading)
{
bitmap.DownloadCompleted += (s, e) => tcs.SetResult(bitmap);
bitmap.DownloadFailed += (s, e) => tcs.SetException(e.ErrorException);
}
else
{
tcs.SetResult(bitmap);
}
return tcs.Task;
}
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