R – Silverlight background thread to measure height

silverlight

Thread MeasureThread = new Thread(delegate()
{
  TextBlock tb = new TextBlock();
});
MeasureThread.Start();

This throws an invalid cross thread access exception, even though this particular TextBlock would never be added to the visual tree. I realize that I could probably wrap it with Dispatcher.BeginInvoke, but that seems to defeat the point of using a background thread. I wanted to use this textbox to calculate the height of some text, for 1000+ different texts. I was hoping to be able to do this calculation in a background thread.

Best Answer

Unfortunately you cannot do this. All changes to UIElements must occur on the UI-thread, regardless of whether or not any specific element is actually in the visual tree.

I presume the reason that you don't want to place this logic on the UI thread is that it would cause the UI to lock up while the calculation completes. One way around this is to do what you suggested; use Dispatcher.BeginInvoke. And rather than just invoking the calculation for 1000+ TextBlocks, you could invoke the calculation for a single TextBlock, and then when that completes, invoke the next, and so forth. You can also use DispatcherTimer to schedule when things occur. This way, you break up your single large calculation, so that the UI never freezes up completely; of course it will take a longer time for the calculation to complete, but you do it without locking up the UI thread for an extended period of time.

Related Solutions

Silverlight with Dynamic Height

Set the overflow to "auto"
Set the scroll to "no"
Set the margin to "0"
Set the containing div's width and height to "100%"
Set the Silverlight control's width and height to "100%"

This should cover all your bases. See below for some sample HTML.

Try this:

<style type="text/css"> 
html, body { overflow:auto } 
</style>
<head>
    <title>My App</title>
</head>
<body id="bodyId" style="margin:0;" scroll="no">
    <form id="form1" runat="server">
        <asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
         <div style="position: fixed; height: 100%; width: 100%">
            <avn:Silverlight 
                ID="xamlHost" 
                runat="server" 
                Source="~/ClientBin/THE.xap" 
                MinimumVersion="x.x.xxxxx" 
                Width="100%" 
                Height="100%" 
                />
         </div>
    </form>
</body>
</html>

C# – How to render text on a WriteableBitmap on a background thread, in Windows Phone 7

This method copies the letters from an pre-made image instead of using TextBlock, it's based on my answer to this question. The main limitation is requiring a different image for each font and size needed. A size 20 Font needed about 150kb.

Using SpriteFont2 export the font and the xml metrics file in the sizes you require. The code assumes they're named "FontName FontSize".png and "FontName FontSize".xml add them to your project and set the build action to content. The code also requires WriteableBitmapEx.

public static class BitmapFont
{
    private class FontInfo
    {
        public FontInfo(WriteableBitmap image, Dictionary<char, Rect> metrics, int size)
        {
            this.Image = image;
            this.Metrics = metrics;
            this.Size = size;
        }
        public WriteableBitmap Image { get; private set; }
        public Dictionary<char, Rect> Metrics { get; private set; }
        public int Size { get; private set; }
    }

    private static Dictionary<string, List<FontInfo>> fonts = new Dictionary<string, List<FontInfo>>();
    public static void RegisterFont(string name,params int[] sizes)
    {
        foreach (var size in sizes)
        {
            string fontFile = name + " " + size + ".png";
            string fontMetricsFile = name + " " + size + ".xml";
            BitmapImage image = new BitmapImage();

            image.SetSource(App.GetResourceStream(new Uri(fontFile, UriKind.Relative)).Stream);
            var metrics = XDocument.Load(fontMetricsFile);
            var dict = (from c in metrics.Root.Elements()
                        let key = (char) ((int) c.Attribute("key"))
                        let rect = new Rect((int) c.Element("x"), (int) c.Element("y"), (int) c.Element("width"), (int) c.Element("height"))
                        select new {Char = key, Metrics = rect}).ToDictionary(x => x.Char, x => x.Metrics);

            var fontInfo = new FontInfo(new WriteableBitmap(image), dict, size);

            if(fonts.ContainsKey(name))
                fonts[name].Add(fontInfo);
            else
                fonts.Add(name, new List<FontInfo> {fontInfo});
        }
    }

    private static FontInfo GetNearestFont(string fontName,int size)
    {
        return fonts[fontName].OrderBy(x => Math.Abs(x.Size - size)).First();
    }

    public static Size MeasureString(string text,string fontName,int size)
    {
        var font = GetNearestFont(fontName, size);

        double scale = (double) size / font.Size;

        var letters = text.Select(x => font.Metrics[x]).ToArray();

        return new Size(letters.Sum(x => x.Width * scale),letters.Max(x => x.Height * scale));
    }

    public static void DrawString(this WriteableBitmap bmp,string text,int x,int y, string fontName,int size,Color color)
    {
        var font = GetNearestFont(fontName, size);

        var letters = text.Select(f => font.Metrics[f]).ToArray();

        double scale = (double)size / font.Size;

        double destX = x;
        foreach (var letter in letters)
        {
            var destRect = new Rect(destX,y,letter.Width * scale,letter.Height * scale);
            bmp.Blit(destRect, font.Image, letter, color, WriteableBitmapExtensions.BlendMode.Alpha);
            destX += destRect.Width;
        }
    }
}

You need to call RegisterFont once to load the files then you call DrawString. It uses WriteableBitmapEx.Blit so if your font file has white text and a transparent background alpha is handled correctly and you can recolour it. The code does scale the text if you draw at a size you didn't load but the results aren't good, a better interpolation method could be used.

I tried drawing from a different thread and this worked in the emulator, you still need to create the WriteableBitmap on the main thread. My understanding of your scenario is that you want to scroll through tiles similar to how mapping apps work, if this is the case reuse the old WriteableBitmaps instead of recreating them. If not the code could be changed to work with arrays instead.

Best Answer

Related Solutions

Silverlight with Dynamic Height

C# – How to render text on a WriteableBitmap on a background thread, in Windows Phone 7

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