Changes to WriteableBitmap pixels don't update screen
I have a WriteableBitmap in a Windows Phone 8 application that is hooked up to an Image control. I'm looping through each row of the image and painting a row of pixels at a time asynchronously and then scheduling the next row for painting. However, it appears that changing the underlying pixel data does not fire a property changed so the control is not being updated. If I set the image source to a new WriteableBitmap created from the same pixels, the image updates fine but I'm doing a lot of excessive array copying.
void PaintImage(object state)
{
// get my height, width, row, etc. from the state
int[] bitmapData = new int[width];
// load the data for the row into the bitmap
Dispatcher.BeginInvoke(() =>
{
var bitmap = ImagePanel.Source as WriteableBitmap;
Array.Copy(bitmapData, 0, bitmap.Pixels, row * width, bitmapData.Length);
if (row < height - 1)
{
var newState = ... // create new state
ThreadPool.QueueUserWorkItem(PaintImage, newState);
}
});
}
If I add these lines after the Array.Copy above, the bitmap progressively is drawn to screen (although in reality it is just replacing the bitmap every time):
var newBitmap = new WriteableBitmap(width, height);
Array.Copy(bitmap.Pixels, newBitmap.Pixels, newBitmap.Pixels.Length);
ImagePanel.Source = newBitmap;
It seems like I need to manually have the WriteableBitmap fire some property changed notification so that the Image that has it. I'm guessing this problem will go away if I bind the image to a WriteableBitmap in a ViewModel?
I think you should call Invalidate() to request for a redraw. Ref: http://msdn.microsoft.com/en-us/library/system.windows.media.imaging.writeablebitmap.invalidate(v=vs.95).aspx
Just add a dirty rectangle
_myBitmap.Lock();
_myBitmap.AddDirtyRect(new Int32Rect(0, 0, _myBitmap.PixelWidth, _myBitmap.PixelHeight));
_myBitmap.Unlock();
or if you are on a background thread
Application.Current.Dispatcher.InvokeAsync(() =>
{
_myBitmap.Lock();
_myBitmap.AddDirtyRect(new Int32Rect(0, 0, _myBitmap.PixelWidth, _myBitmap.PixelHeight));
_myBitmap.Unlock();
});
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