问题
I'm writing an application in WPF (C#) which does long operations on a collection of Bitmaps. To keep my application responsive, I decided to use another thread to perform the operations on bitmaps and report progress on a progressbar in main UI thread. I thought BackgroundWorker would do anything for me, but looks like it won't be that easy.
I have the following code:
public class ImageProcessor
{
public Collection<WriteableBitmap> Pictures { get; private set; }
private BackgroundWorker _worker = new BackgroundWorker();
public ImageProcessor()
{
_worker.DoWork += DoWork;
}
public void DoLotsOfOperations()
{
_worker.RunWorkerAsync();
}
private void DoWork(object sender, DoWorkEventArgs e)
{
// operations on Pictures collection
}
}
At runtime I load images using a standard open file dialog into the Pictures collection and then I invoke DoLotsOfOperations() method. But as soon as I try to access any of the properties of a single bitmap I get InvalidOperationException: "The calling thread cannot access the object because different thread owns it".
It is obviosly true - I loaded bitmaps and populated the collection in the UI thread and I try to read collection elements in another thread. So i tried different approaches:
- I passed the whole collection as a parameter of RunWorkerAsync method and got it back in DoWork method from e.Argument but then when I tried to read properties of a single bitmap I still got the same exception.
- I tried the same thing, this time passing a single bitmap as backgroundworker's argument and still I couldn't get any of the bitmap's properties, let alone bitmap's pixels.
So how can I access bitmap's data inside another thread (and preferably using BackgroundWorker)?
I don't know, maybe my whole approach is wrong. The general idea I want to achieve is:
- User loads bitmaps which are then displayed in a window.
- User clicks a button and a long operation on the bitmaps is performed but the UI is responsive (which allows user for example to cancel the opration) and the progress is reported on a progess bar.
Thanks in advance for any help.
回答1:
1) Proxy class (without thread restriction)
public class WriteableBitmapProxy
{
public IntPtr BackBuffer { get; set; }
public int BackBufferStride { get; set; }
public int PixelHeight { get; set; }
public int PixelWidth { get; set; }
}
2) Extension methods (unsafe)
public class RGBColor
{
public byte R { get; set; }
public byte G { get; set; }
public byte B { get; set; }
public uint Value
{
get
{
return (uint)(((uint)R << 16) + ((uint)G << 8) + (B) + ((uint)255 << 24));
}
}
}
public static RGBColor GetPixel(this WriteableBitmap bmp, uint x, uint y)
{
unsafe
{
if (y >= bmp.PixelHeight) return default(RGBColor);
if (x >= bmp.PixelWidth) return default(RGBColor);
// Get a pointer to the back buffer.
uint pBackBuffer = (uint)bmp.BackBuffer;
// Find the address of the pixel to draw.
pBackBuffer += y * (uint)bmp.BackBufferStride;
pBackBuffer += x * 4;
byte* pCol = (byte*)pBackBuffer;
return new RGBColor() { B = pCol[0], G = pCol[1], R = pCol[2] };
}
}
public static void SetPixel(this WriteableBitmapProxy bmp, uint x, uint y, RGBColor col)
{
SetPixel(bmp, x, y, col.Value);
}
public static void SetPixel(this WriteableBitmapProxy bmp, uint x, uint y, uint value)
{
unsafe
{
if (y >= bmp.PixelHeight) return;
if (x >= bmp.PixelWidth) return;
// Get a pointer to the back buffer.
uint pBackBuffer = (uint)bmp.BackBuffer;
// Find the address of the pixel to draw.
pBackBuffer += y * (uint)bmp.BackBufferStride;
pBackBuffer += x * 4;
// Assign the color data to the pixel.
*((uint*)pBackBuffer) = value;
}
}
3) Procedure to fire long running operation in different thread
var image = sender as Image;
var bitmap = image.Source as WriteableBitmap;
var prx = new WpfImage.MyToolkit.WriteableBitmapProxy()
{
BackBuffer = bitmap.BackBuffer,
BackBufferStride = bitmap.BackBufferStride,
PixelHeight = bitmap.PixelHeight,
PixelWidth = bitmap.PixelWidth
};
bitmap.Lock();
Thread loader = new Thread(new ThreadStart(() =>
{
Global_Histogramm(prx);
Dispatcher.BeginInvoke(DispatcherPriority.Background,
(SendOrPostCallback)delegate { bitmap.AddDirtyRect(new Int32Rect(0, 0, prx.PixelWidth - 1, prx.PixelHeight - 1)); bitmap.Unlock(); }, null);
}
));
loader.Priority = ThreadPriority.Lowest;
loader.Start();
4) Long Running operation implementation
void Global_Histogramm(WpfImage.MyToolkit.WriteableBitmapProxy src)
{
int SrcX = src.PixelWidth;
int SrcY = src.PixelHeight;
double[] HR = new double[256];
double[] HG = new double[256];
double[] HB = new double[256];
double[] DR = new double[256];
double[] DG = new double[256];
double[] DB = new double[256];
uint i, x, y;
// wyzeruj tablice
for (i = 0; i < 256; i++) HB[i] = HG[i] = HR[i] = 0;
// wypelnij histogramy R G B
for (y = 0; y < SrcY; y++)
for (x = 0; x < SrcX; x++)
{
var color = src.GetPixel(x, y);
HB[color.B]++;
HG[color.G]++;
HR[color.R]++;
};
// oblicz histogramy znormalizowane i przygotuj dystrybuanty
int ilosc_punktow = SrcX * SrcY;
double sumaR = 0, sumaG = 0, sumaB = 0;
for (i = 0; i < 256; i++)
{
DB[i] = sumaB + HB[i] / ilosc_punktow;
DG[i] = sumaG + HG[i] / ilosc_punktow;
DR[i] = sumaR + HR[i] / ilosc_punktow;
sumaB = DB[i];
sumaG = DG[i];
sumaR = DR[i];
};
Dispatcher.BeginInvoke(DispatcherPriority.Background,
(SendOrPostCallback)delegate { progressBar1.Maximum = SrcY - 1; }, null);
// aktualizuj bitmape
for (y = 0; y < SrcY; y++)
{
for (x = 0; x < SrcX; x++)
{
var stmp = src.GetPixel(x, y);
var val = new WpfImage.MyToolkit.RGBColor()
{
B = (byte)(DB[stmp.B] * 255),
G = (byte)(DG[stmp.G] * 255),
R = (byte)(DR[stmp.R] * 255)
};
src.SetPixel(x, y, val);
};
Dispatcher.BeginInvoke(DispatcherPriority.Background,
(SendOrPostCallback)delegate { progressBar1.Value = y; }, null);
}
}
5) Hope it proves the thing.
回答2:
WriteableBitmap has explicit support for threading. But you have to follow the protocol, use the Lock() method in the thread to gain access to the BackBuffer.
来源：https://stackoverflow.com/questions/1855308/asynchronous-operations-on-writeablebitmap