LongListSelector ListFooterTemplate {Binding} Resolves to NULL

It’s quite common to have a ‘More’ button at the end of long lists within a LongListSelector. The problem is the template inexplicably doesn’t cascade the DataContext, so you have no access to the collection to trigger a more command. Fortunately the workaround is simple, albeit you can’t use pre-defined DataTemplates.

<toolkit:LongListSelector x:Name="SimilarList" ItemTemplate="{StaticResource GeneralCatalogListItemTemplate}" SelectionChanged="SimilarList_SelectionChanged" DataContext="{StaticResource dummyCatalogTitlesList}" ItemsSource="{Binding}" IsFlatList="True">
            <bindings:CommandButton Content="More..." Command="{StaticResource moreCommand}" DataContext="{Binding ElementName=SimilarList, Path=DataContext}" CommandParameter="{Binding}" />

You need to give your list a name, then when setting the DataContext for your controls make sure to provide the ElementName as seen above.

I’m not sure why the other templates cascade correctly and this one doesn’t, but I’m hopeful it’ll be fixed in a future release of the toolkit. As an aside there is a fairly clean workaround if/when MS add the Freezable class to WP7.

Posted by Dan in C#, Windows Phone, 2 comments

Isolated Storage Settings In WP7

Recently I had a problem with Isolated Storage Settings on WP7 – the entire collection appeared to be wiped every single time I restarted the app. The problem turned out to be the deserialization stage of IsolatedStorageSettings, in essence if any object can’t be re-instantiated, then the entire collection is inaccessible. The collection will work correctly until you attempt to retrieve something that can’t be instantiated.

At the bare minimum the constructor of your class must be public.

All non-public properties will also fail to be restored, resulting in a rather empty object instance when you come to load the values that should have been saved (and have been… they just don’t come back). Be very careful with this issue, you may not notice the problem until it’s too late.

Posted by Dan in C#, Windows Phone, 1 comment

NLipsum for Windows Phone 7 (Auto Generate Lorem Ipsum For WP7)

While developing a Silverlight WP7 app, it’s often handy to display demo content in design view so you have an idea of how real content will look. This is one of the biggest advantages of the MVVM architecture – separate data for design and runtime, as demonstrated below:

Fortunately Alex Pendleton has created a Lorem Ipsum generator in .NET 2.0 called NLipsum. Unfortunately it doesn’t work directly on WP7 without a little massaging. Fear not, for I have done the work for you! You can download the project files here, or the binary here.

The dll contains some raw XML files for generating the lipsums, and these are loaded at runtime. For this reason it’s best not to have the binary included with release builds of your app, lest you get a slower startup and increased memory usage. I’ve put in some caching that should mean you can call the generator as much as you like without worrying too much about performance. To use the generator, include it into your project and import the namespace:

using NLipsum.Core;

Then, it’s a simple case of calling the generator to do your bidding:

return LipsumGenerator.Generate(1, Features.Paragraphs, null, Lipsums.LoremIpsum);

return LipsumGenerator.Generate(1, Features.Sentences, null, Lipsums.TheRaven);

return LipsumGenerator.Generate(2, Features.Words, null, Lipsums.LeMasque);

If the content doesn’t show in design view, try re-building your app. This will refresh your design-time databinding as well as bringing in any changes you’ve made to the model (such as adding the lorem ipsum).

Posted by Dan in C#, Windows Phone, 1 comment

Resizing WP7 WebBrowser Height to Fit Content

At some point you’re probably going to need to have the height of a WebBrowser on your Windows Phone app resize automatically to fit the content, so that the other content on the page can wrap nicely around it. Fortunately MS have provided all of the functionality you need to pull this off, albeit in a somewhat roundabout way.

Adding the browser

First of all put a WebBrowser in to your XAML file, or alternatively add it pragmatically. Make sure IsScriptEnabled is set to true, by default it’s false.

If you’re adding the browser in code, make sure the height is at least 1 pixel.

Hooking up events

You’ll need to hook up the ‘ScriptNotify’ event, use the following code in the event handler:

protected void WebBrowser_ScriptNotify(object sender, NotifyEventArgs e)
	WebBrowser thisBrowser = (WebBrowser)sender;
	int height = Convert.ToInt32(e.Value);
	double newHeight = height * 1.5;

	thisBrowser.Height = newHeight;

This gets the browser that fired the event (so feel free to re-use the event for multiple browsers), gets the height the browser thinks it needs to display the content. The height value is actually set by the JavaScript, so due to the built-in scaling of the content you’ll need to multiply it by 1.5. Finally the browser’s height is set to the correct value to fit the content.

Add a teaspoon of JavaScript

Lastly, you need to add some JavaScript to the HTML you pass into the browser:

<script type="text/javascript">
    window.onload = function () {
        var elem = document.getElementById('content');
        window.external.Notify(elem.scrollHeight + '');

This code will find an element with the ID of ‘content’, and send the height to your program. This means you’ll need to wrap all of your content in a div like so:

<div id="content">
    <p>Hello World</p>

You may be wondering why we don’t just get the scrollHeight of body. While experimenting I found this was the most reliable – but if body works for you then it is the more elegant solution. You may also be wondering why we add ” to the scrollHeight in the JavaScript. This is due to window.external.Notify only supporting text strings, however the height is a number. By adding ” we convert the number to a string meaning window.external.Notify will work as intented. If you ever find Notify not working – check the datatype, it fails silently.

Finishing up

This should be enough for you to get started. You’ll probably want to customise the CSS to match the phone’s current theme, but that’s another guide for another day. One tweak you should probably think about doing now is to add some padding to the content div, otherwise the HTML you output tends to look out of place rammed right up against the border of the WebBrowser.

If you find the height returned isn’t correct, it’s most likely due to the scaling of the web content. Set the viewport width to a fixed value (you can pass in the width of the WebBrowser if generating the HTML on the fly, or you have the ability to process it before the browser reads in the HTML). You may also need to set the width of the div as well, using the aforementioned method.

Setting the viewport:

<meta name="viewport" content="width=320" />

Setting the div width + padding:

<style type="text/css">#content { padding: 10px; width: 300px; }</style>
Posted by Dan in C#, Guides, Windows Phone, 1 comment