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by Peter van Ooijen.
Original Post: (Mis-) understanding dataviews and databinding
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I was given a nice little Pocket PC application which worked well but took far too long to start. My first code walk through was a WTF. It was the common scenario, a well known major consulting firm producing code which demonstrated they did not quite understand what they were doing. But instead of just making fun of it in the WTF style, I'll approach it from the positive side as well with a walkthrough what I did with the problem.
The compact framework has no typed datasets. Working with a database in a CF application can be a branch of sports on itself. As a pleasant alternative you can work with plain XML files to store data and use dataviews in you code to sort and query this data. The DataView class provides a view on one table in an XML dataset. I never thought it was such a hard thing to use, but this project did show some people do completely misunderstand it.
The application was highly configurable. XML files contain things like the caption of labels and menu items. Like this
At startup the application loaded the xml-file in a dataset. Some snippets found to actually set a caption.
publicstatic DataView dvControls31;
.....
dvControls31 = new DataView(vfpdata.Tables["controls"]); dvControls31.RowFilter = "screen = 'screen3' AND object = 'btnNext.label'";
.....
if (Welcome1.dvControls31.Count != 0) btnNext.DataBindings.Add(new Binding("Text", Welcome1.dvControls31, "value"));
A dataview is declared, instantiated on a table after which the desired row is filtered out. After that the button gets its caption by data-binding it to the dataview.
This does work but gets more complicated when you want to set a property which is not data-bindable, like a menu item. It took a dummy label, leading to this
The dataview row is bound to the label caption after which the label caption is directly assigned to the menu text. Which is part of the solution:
Databindings are meant to use with data which changes. For setting a value just once they are a total waste of time. Use a plain assignment there.
Changing the databindings to straight assignments made the code leaner and faster. But the greatest WTF was where the data was. You might have noticed that the dataviews objects presented so far have different names. It turned out the application declared and instantiated a separate dataview for every configurable control property in the application. Well over 40. No wonder startup took so long.
You can do more with a dataview than just filtering out one row. The rows in a dataview can be sorted and after that you perform a find to get to a row. As a small demonstration, a part of a class to read the configuration files:
The dataset wraps up a controls and a fields config file. Their contents is queried using one dataview each. The complicated code to set up the controls now reads like
Dataviews are a dynamic view on underlying data. Do not create a new dataview for a new view on the same underlying data
Having made these changes the amount of code had dramatically shrunken. Startup time had halved.
But there was more to gain. The application worked with a larger set of data. It displayed just a subset. A dataview would be an way to get this subset. The application did use two but each of them was on another dataset. A large part of the code was dedicated to keeping these two datasets in sink. Throwing out one of the datasets and opening both dataviews on the one left was another boost in performance.
You can have multiple Dataviews on the same data, each providing a different set of row. Do not duplicate data
And now I'll stop kicking in open doors. It's all in Visual Studio's help files. WTFM ?
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