Monotouch:在单独的线程中渲染PDF页面预览，有时会崩溃

我有一个UISlider。它用于快速浏览PDF。每当达到下一页的阈值时，我就在滑块的旋钮旁边显示一个UIView，其中包含目标页面的一个小预览。滑块代码看起来在下面(一些部分被剥离)。如果到达下一页，则生成一个新的预览，否则，现有的预览将沿着滑块移动。

我得到了各种各样的效果:

• 如果预览很多页面，应用程序在

  崩溃

  MonoTouch.CoreGraphics.CGContext。处置(bool) <0x00047>UIKitApplication:com.brainloop。brainloopbrowser[0x1a2d][2951]: at MonoTouch.CoreGraphics.CGContext.Finalize () <</p>

• 或者如果我在最后一个方法中删除对Dispose()的调用:[NSAutoreleasePool release]: This pool has already been released, do not drain it (double release).

从查看代码，有人知道出了什么问题吗?或者使用线程的整个方法是错误的?

this.oScrollSlider = new UISlider ();
this.oScrollSlider.TouchDragInside += delegate( object oSender, EventArgs oArgs )
{
this.iCurrentPage = (int)Math.Round (oScrollSlider.Value);
    if (this.iCurrentPage != this.iLastScrollSliderPage)
    {
        this.iLastScrollSliderPage = this.iCurrentPage;
        this.RenderScrollPreviewImage(this.iCurrentPage);
    }
};
this.oScrollSlider.ValueChanged += delegate
{
    if (this.oScrollSliderPreview != null)
    {
        this.oScrollSliderPreview.RemoveFromSuperview ();
        this.oScrollSliderPreview.Dispose();
        this.oScrollSliderPreview = null;
    }
    // Go to the selected page.
};

创建预览的方法正在启动一个新线程。如果用户在线程还在运行的时候更改了页面，那么线程将被终止，并预览下一页:

private void RenderScrollPreviewImage (int iPage)
{
// Create a new preview view if not there.  
if(this.oScrollSliderPreview == null)
    {
        SizeF oSize = new SizeF(150, 200);
        RectangleF oFrame = new RectangleF(new PointF (this.View.Bounds.Width - oSize.Width - 50, this.GetScrollSliderOffset (oSize)), oSize);
        this.oScrollSliderPreview = new UIView(oFrame);
        this.oScrollSliderPreview.BackgroundColor = UIColor.White;
        this.View.AddSubview(this.oScrollSliderPreview);    
        UIActivityIndicatorView oIndicator = new UIActivityIndicatorView(UIActivityIndicatorViewStyle.Gray);
        oIndicator.Center = new PointF(this.oScrollSliderPreview.Bounds.Width/2, this.oScrollSliderPreview.Bounds.Height/2);
        this.oScrollSliderPreview.AddSubview(oIndicator);
        oIndicator.StartAnimating();
}
            // Remove all subviews, except the activity indicator.
            if(this.oScrollSliderPreview.Subviews.Length > 0)
            {
                foreach(UIView oSubview in this.oScrollSliderPreview.Subviews)
                {
                    if(!(oSubview is UIActivityIndicatorView))
                    {
                        oSubview.RemoveFromSuperview();
                    }
                }
            }
// Kill the currently running thread that renders a preview.
            if(this.oRenderScrollPreviewImagesThread != null)
            {
                try
                {
                    this.oRenderScrollPreviewImagesThread.Abort();
                }
                catch(ThreadAbortException)
                {
                    // Expected.
                }
            }
// Start a new rendering thread.
            this.oRenderScrollPreviewImagesThread = new Thread (delegate()
            {
                using (var oPool = new NSAutoreleasePool())
                {
                    try
                    {
// Create a quick preview.
                        UIImageView oImgView = PdfViewerHelpers.GetLowResPagePreview (this.oPdfDoc.GetPage (iPage), new RectangleF (0, 0, 150, 200));
                        this.InvokeOnMainThread(delegate
                        {
                            if(this.oScrollSliderPreview != null)
                            {
                                oImgView.Center = new PointF(this.oScrollSliderPreview.Bounds.Width/2, this.oScrollSliderPreview.Bounds.Height/2);
// Add the PDF image to the preview view.                               
this.oScrollSliderPreview.AddSubview(oImgView);
                            }
                        });
                    }
                    catch (Exception)
                    {
                    }
                }
            });
// Start the thread.
            this.oRenderScrollPreviewImagesThread.Start ();
        }

要渲染PDF图像，我使用以下命令:

internal static UIImageView GetLowResPagePreview (CGPDFPage oPdfPage, RectangleF oTargetRect)
        {
            RectangleF oPdfPageRect = oPdfPage.GetBoxRect (CGPDFBox.Media);
            // If preview is requested for the PDF index view, render a smaller version.
            float fAspectScale = 1.0f;
            if (!oTargetRect.IsEmpty)
            {
                fAspectScale = GetAspectZoomFactor (oTargetRect.Size, oPdfPageRect.Size, false);
                // Resize the PDF page so that it fits the target rectangle.
                oPdfPageRect = new RectangleF (new PointF (0, 0), GetFittingBox (oTargetRect.Size, oPdfPageRect.Size));
            }
            // Create a low res image representation of the PDF page to display before the TiledPDFView
            // renders its content.
            int iWidth = Convert.ToInt32 ( oPdfPageRect.Size.Width );
            int iHeight = Convert.ToInt32 ( oPdfPageRect.Size.Height );
            CGColorSpace oColorSpace = CGColorSpace.CreateDeviceRGB();
            CGBitmapContext oContext = new CGBitmapContext(null, iWidth, iHeight, 8, iWidth * 4, oColorSpace, CGImageAlphaInfo.PremultipliedLast);
            // First fill the background with white.
            oContext.SetFillColor (1.0f, 1.0f, 1.0f, 1.0f);
            oContext.FillRect (oPdfPageRect);
            // Scale the context so that the PDF page is rendered 
            // at the correct size for the zoom level.
            oContext.ScaleCTM (fAspectScale, fAspectScale);
            oContext.DrawPDFPage (oPdfPage);
            CGImage oImage = oContext.ToImage();
            UIImage oBackgroundImage = UIImage.FromImage( oImage);
            oContext.Dispose();
            oImage.Dispose ();
            oColorSpace.Dispose ();
            UIImageView oBackgroundImageView = new UIImageView (oBackgroundImage);
            oBackgroundImageView.Frame = new RectangleF (new PointF (0, 0), oPdfPageRect.Size);
            oBackgroundImageView.ContentMode = UIViewContentMode.ScaleToFill;
            oBackgroundImageView.UserInteractionEnabled = false;
            oBackgroundImageView.AutoresizingMask = UIViewAutoresizing.None;
            return oBackgroundImageView;
        }