Today I came across an issue with the LongListSelector, part of the Silverlight for Windows Phone Toolkit. Solving the issue seemed pretty simple in the end, but first some background.
The LongListSelector is a control that can be used to create a list of groups. A good example of this is the contacts list in the people hub. In that list the contacts are organized based on the first letter of their name. Every group contains a header, in this case the first letter, and using this header you can very easily switch between groups. By using the LongListSelector from the toolkit, developers can create the same experiences in their apps.
The issue I encountered was the fact that I was getting ArgumentOutOfRange exceptions when an item was added to a group dynamically. This issue was supposed to be fixed in the November 2011 release, as stated in the release notes:
LongListSelector bug fixes around OutOfRange exceptions, wrong ordering of items, grouping issues, and scrolling events. ItemTuple is now refactored to be the public type LongListSelectorItem to provide users better access to the values in selection changed handlers.
Well this doesn't seem to be the case exactly. Most of those fixes were actually in changeset 71191 and 71199, which according to the codeplex site are included in the November release. This appears to be not so. When comparing some of the code in the installed Microsoft.Phone.Controls.Toolkit.dll to the source on codeplex it seems to be the case that the installed dll is compiled from a revision at least before 70993.
The solution is simple:
- Download the latest source from codeplex (this currently also includes some fixes to the PhoneTextBox, changeset 74775).
- Compile it.
- Use the resulting dll instead of the one installed by the toolkit.
Recently I came accross the issue where I wanted to limit the number of lines shown in my TextBlock on Windows Phone to for example 3. If the text was longer than 3 lines, the rest of the text should just not be shown. To do this I wanted to set the height of the TextBlock to a fixed height, to be exact 3 times the height of one line. This seems trivial, but to find the height of a line, you need to know something about the way Silverlight measures and calculates heights.
In Silverlight on the desktop and Silverlight for Windows Phone the measuring system is based on pixel units instead of Device Independent Pixel (DIP) units, as it is in WPF. Also the measuring system does not support unit measure string suffixes such as pt, cm or px. Silverlight measurements are always pixel units.
So if we specify the height of a Grid or a TextBlock to be 30 it actually is 30 pixel units. FontSize is no exception to this rule so when you specify 30 as the FontSize, you get a font that from the top of its ascenders to the bottom of its descenders measures approximately 36 pixels. Nevertheless the height of the corresonding TextBlock will be higher, because additional space is used to preserve space between successive lines. This concept is called leading.
In classical typography font sizes are expressed in units of points where a point is almost 1/72th of an inch. Digital typography assumes almost always a value of exactly 1/72th of an inch. This means that text with a font size of 72 points measures approximately 1 inch. Converting between pixels and points is difficult because it depends on the device you are using. Using a printer with 600 dpi (dots per inch) a font with size 72 will measure 600 pixels.
Windows assumes video displayes to have a dpi of 96. Using this assumption one can easily transform pixels into points and the other way around using the following two formulas:
points = 72/96 * pixels = 3/4 * pixels
pixels = 96/72 * points = 4/3 * points
Although Windows Phone screens have a much higher dpi than 96 these formulas also work for the phone. Suppose you want to set a 45 point font. Then you need to set the FontSize property to 60. On top of this Silverlight will add leading, which is approximately 33% of the size set as FontSize, which will result in the height of the TextBlock being 80 pixels.
It is possible to change this behavior by using the LineHeight and LineStackingStrategy properties of a TextBlock. You can read more about it here.
The last couple of months I've been working on my bachelor project to finish my bachelor computer science. Together with Tom Verhoeff, Jos Kraaijeveld, Jochem Toolenaar and Oana Nitu, I participated in the Imagine Cup 2011. We formed team O!ife and our project was called the City Cloud. The City Cloud is a cloud computing platform that allows different kinds of data generated by the city, its inhabitants, companies and its government to be easily accessed by developers to create new innovative technologies and solutions. You can find more about the City Cloud on the O!ife blog.
This post is one in a series on how we built the City Cloud and everything around it. The focus of these posts will be on the technical aspects of the City Cloud. It will also cover some problems we encountered during development and the solutions we used to tackle them. At this moment I have no idea how many parts will follow. We've came across lots of different problems and used lots of technologies, so I will write another part whenever I have the time and when I think the topic is interesting enough for other developers. The first part will cover the overall architecture of the system to establish a background for other posts.
When we started the City Cloud project we thought of it as a developer platform for a city. The main goal should be to offer the correct tools for developers to build next generation ubiquitous applications: it should be easy to expose new datasets through the system and the development of applications using these datasets should not require large investments in both terms of infrastructure and programming skills.
In the design phase we divided the system up into three parts: the cloud framework, the mobile platform and the website/webapplication. The framework was the heart of the system and was designed to run in the Azure cloud. Its responsibility was to present the data that is part of the system in an easy and consistent way and offering an all-in-one solution for data and app storage. The mobile platform and the website/webapplication functioned as two different portals to the data and the apps. The website should give general information on cities and should show the datasets and applications available. It should also provide developers with documentation and best practices on how to make use of the platform's capabilities in the best possible way. The mobile framework should be the main portal to the end user in a city. It should give an user specific information on data and apps based on his current location. With more and more smartphones sold every day, smartphones offered a great opportunity to be the number one entry point to the City Cloud.
The framework should expose data in an easy way. To achieve this we decided to go with an OData webservice running in an Azure webrole. Besides being a Microsoft technology which counts for the Imagine Cup, OData offers an excellent way to expose datasets and besides that offers great possibilities to perform transformations and filtering on the returned data. Another advantage is the great tools available to develop OData webservices and to consume them on almost all major platforms. To get data into the City Cloud framework we invented "connectors". To expose a new dataset through the framework all a developer would need to do is build a connector that allows the City Cloud to retrieve data from their data source. This connector should implement some interface, provide the system with some description of the data structures and it should all magically come to life.
Website and webapplication
To give developers and users an overview of the data and applications already in the system we built the website including a Silverlight webapplication. The webapplication was meant to be used as an explorer for the webservice. In this way everybody could take a quick look into the system to browse data. It also offered the possibility to plot these datasets on a map.
Developers could use the website to submit new applications and new connectors to the system. Because of the extensibility model we felt the need to building a validation step to verify the working of the newly submitted parts in the framework. To validate new submissions we made use of a core feature of the azure platform: a worker role. In Windows Azure a worker role is meant to be used for background processing or other long running tasks. In the cloud framework we used a worker role to validate a submission and to make the required changes to the framework to add a new connector or building block.
At the end of the design phase the overall system architecture looked a lot like a high level Model-View-Controller architecture. The website and the mobile platform functioned as the views, while the framework holds the current state of the system and functioned as the model. The worker role was used to change the state of the framework and can thus be seen as the controller. The following image shows the high level architecture in a graphical way:
I hope this gives a good overview of the system we built the last couple of months. Unfortunately no code yet, but I will save that for the next part.