Xamarin Forms - 图像传入/传出 IRandomAccessStreamReference

为了个人的需要，为了Xamarin.Forms.Map控制，我需要创建一个CustomPin扩大。UWP部分（PCL项目）

我创建一个MapIcon喜欢它：

nativeMap.MapElements.Add(new MapIcon()
{
    Title = pin.Name,
    Image = RandomAccessStreamReference.CreateFromUri(new Uri("ms-appx:///Assets/Pin/customicon.png")),
    Location = new Geopoint(new BasicGeoposition() { Latitude = pin.Position.Latitude, Longitude = pin.Position.Longitude }),
    NormalizedAnchorPoint = new Windows.Foundation.Point(0.5, 1.0)
});

但是，通过这种方式，我无法设置Image的尺寸。

然后我想用一个Image从我的 PCL 部分，调整其大小并将其转换为IRandomAccessStreamReference。为了实现这一点，我需要转换我的Image进入流，但我找不到让它工作的方法>

所需功能示例：

private IRandomAccessStreamReference ImageToIRandomAccessStreamReference(Image image)
{
    //Here I can set the size of my Image

    //I convert it into a stream
    IRandomAccessStreamReference irasr = RandomAccessStreamReference.CreateFromStream(/* img? */);

    //irasr is then created from img

    //I return the IRandomAccessStreamReference needed by the MapIcon element
    return irasr;
}

Note: The Image范围img is a Xamarin.Forms.Image

那么首先，这可能吗？如果是，那么感谢任何可以帮助我的帮助。我已经搜索了如何调整 MapIcon 的大小，但不可能直接从类 [MapIcon] 中直接进行操作。（https://msdn.microsoft.com/library/windows/apps/windows.ui.xaml.controls.maps.mapicon.aspx https://msdn.microsoft.com/library/windows/apps/windows.ui.xaml.controls.maps.mapicon.aspx)

感谢您的帮助！

你是对的。我们无法调整大小MapIcon https://msdn.microsoft.com/library/windows/apps/windows.ui.xaml.controls.maps.mapicon.aspx直接，因为它不提供此类属性或方法。 MapIcon 的大小主要由图像的大小控制，图像的大小由地图图标.Image https://msdn.microsoft.com/en-us/library/windows/apps/windows.ui.xaml.controls.maps.mapicon.image.aspx财产。我们可以设置该图像的大小而不使用Xamarin.Forms.Image.

要设置该图像的大小，我们可以利用位图解码器类 https://msdn.microsoft.com/en-us/library/windows/apps/xaml/windows.graphics.imaging.bitmapdecoder.aspx, 位图编码器类 https://msdn.microsoft.com/en-us/library/windows/apps/xaml/windows.graphics.imaging.bitmapencoder.aspx and 位图变换类 https://msdn.microsoft.com/en-us/library/windows/apps/windows.graphics.imaging.bitmaptransform.aspx像下面这样：

private async System.Threading.Tasks.Task<RandomAccessStreamReference> ResizeImage(StorageFile imageFile, uint scaledWidth, uint scaledHeight)
{
    using (IRandomAccessStream fileStream = await imageFile.OpenAsync(FileAccessMode.Read))
    {
        var decoder = await BitmapDecoder.CreateAsync(fileStream);

        //create a RandomAccessStream as output stream
        var memStream = new InMemoryRandomAccessStream();

        //creates a new BitmapEncoder and initializes it using data from an existing BitmapDecoder
        BitmapEncoder encoder = await BitmapEncoder.CreateForTranscodingAsync(memStream, decoder);

        //resize the image
        encoder.BitmapTransform.ScaledWidth = scaledWidth;
        encoder.BitmapTransform.ScaledHeight = scaledHeight;

        //commits and flushes all of the image data
        await encoder.FlushAsync();

        //return the output stream as RandomAccessStreamReference
        return RandomAccessStreamReference.CreateFromStream(memStream);
    }
}

然后我们可以使用这个方法先创建一个调整大小的图像流引用，然后将其设置为MapIcon's Image like:

var file = await StorageFile.GetFileFromApplicationUriAsync(new Uri("ms-appx:///Assets/Pin/customicon.png"));
var imageReference = await ResizeImage(file, 64, 64);

nativeMap.MapElements.Add(new MapIcon()
{
    Title = pin.Name,
    Image = imageReference,
    Location = new Geopoint(new BasicGeoposition() { Latitude = pin.Position.Latitude, Longitude = pin.Position.Longitude }),
    NormalizedAnchorPoint = new Windows.Foundation.Point(0.5, 1.0)
});