Thursday, 26 July 2012

Data-Binding Pitfalls on WPF, WP7, Silverlight and WinRT Part 1

I've been doing a lot of work on all four platforms (WPF, WP7, Silverlight and WinRT), and am puzzled by how data-binding is supposed to be implemented.

For example, in David Anson's Data Visualization toolkit, you can be penalized with an extraordinarily long delay if property changes happen in the wrong order.

The problem in that specific instance is that the control sets the default range to 1 year, but the order of changing properties can cause the problem to occur at any point.  For example, if you were to change a date time axis from a range of 1 day to 1 year and back, with intervals of 1 hour and 1 month respectively, on the way to 1 year, you'd need to set the interval first, and the range second.  In the other direction, it's the opposite.

Now with Data Binding, it's not clear how you'd arrange this.  In my case in WPF everything appeared to be working just fine, until I newed up a control for printing.  Printing worked fine, but PrintPreview had a hugely long delay, before the preview window appeared.  The culprit?  The DateTimeAxis.

It turned out the problem was provoked by my code behind setting the Interval and Interval type.  When I changed it to data-binding, the problem disappeared.

Now, I suspect the reason this fixed the problem has to do with the options on Dispatcher.BeginInvoke and the Dispatcher itself.  If you look there (in WPF) the DispatcherPriorities includes Render and DataBind levels in addition to Normal.  Shawn Wildermuth wrote an article a long time ago about the dispatcher that provides some insights.

If you examine the code in the DateTimeAxis, you’ll see that on a property change, most of what happens is a simple call to Invalidate().  That pushes the refresh down to the Render level, allowing all of the other Invalidate calls to occur before doing any work for changes resulting from property changes.  I’ll come back to that scheduling trick in a part 2 tomorrow.

Now, my code didn’t use data binding for all of it’s properties.  It all ran on the UI thread in response to the user asking to print.  As a result, (if my analysis is correct) the initial data binding has not yet run, as we haven’t ceded control to the dispatcher.

Eventually, my UI control is asked to Measure and Arrange from my code on the UI thread, and finally UpdateLayout is called by my code.  Because I had set some of the properties in code (the Interval, and Interval Types) but not the Max and Min, when the control is being laid out it does all the work to create the stupid number of ticks for each hour in a year.

Then, inside UIElement.UpdateLayout() I have this call stack:

PresentationFramework.dll!MS.Internal.Data.DataBindEngine.Task.Run(bool lastChance) + 0x31 bytes   
PresentationFramework.dll!MS.Internal.Data.DataBindEngine.Run(object arg) + 0xb6 bytes   
PresentationFramework.dll!MS.Internal.Data.DataBindEngine.OnLayoutUpdated(object sender, System.EventArgs e) + 0x1e bytes   
PresentationCore.dll!System.Windows.ContextLayoutManager.fireLayoutUpdateEvent() + 0x154 bytes   
PresentationCore.dll!System.Windows.ContextLayoutManager.UpdateLayout() + 0x926 bytes   
PresentationCore.dll!System.Windows.UIElement.UpdateLayout() + 0x16 bytes   

So, it finally data-binds the rest of my values, and lays out the axis correctly with the intended values for Max and Min.

By switching everything to use data-binding instead mixing it with code-behind, the default values in the DateTimeControl work correctly (though the work is still wasted), and then the data-binding sets them to the new values.

It’s this wasted effort that concerns me, so future instalments will examine what can be done about it.

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