In the previous post I demonstrated the concept of automatic template selection in a very very simple way. The post was designed to demonstrate the concepts that will be applied in this post – you really shouldn’t use the methods described in the previous post in real projects!
This series is starting out covering off some very basic concepts that are important to understand before progressively up the complexity until we jump in the a full solution – so be patient if you find this stuff a little simple for your tastes!
In this post I will demonstrate a very simple way to apply convention to automatically select a View for the current ViewModel.
Convention is great – as long as you follow the convention (creating files with the correct names, and putting them in the correct spot) this it will “just work”. Yeah I know, never say it will “just work” – phrases like that curse software projects
Once again to reiterate, the ViewModels do not have any idea what View will be displayed… and you should always try to achieve this in your projects. As soon as the ViewModel knows about the View you are risking breaking all manner of software dev rules, but more importantly your code will lack elegance and be far harder to create and maintain. With this in mind, we “publish” the current ViewModel through a property in the “frame” – the frame being the VM (ViewModel) that looks after creating and displaying child VM’s, and eventually in later posts navigation and tombstoning etc. The VM is then picked up by a ContentControl which will then figure out which View to show. The frame is on the VM side of the fence, the ContentControl is on the View side of the fence. The link between the fence is DataBinding.
Example – automatic view selection using convention
First, you can download the example here.
You can view a video run through of the example here.
The code examines how to pass in a VM to the ContentControl and automatically resolve the view to show.
There are two concepts that are important to take away from this example: first is convention. Creating Views called xxxView and VMs called xxxViewModel. The ContentTemplateSelector is then called upon to look at the type names, figure out the View to show (by replacing ViewModel with View from the original VM type name), then dynamically building some XAML which contains a DataTemplate.
The second concept is of course the dynamic generation of a DataTemplate. It’s important to note that child controls in DataTemplates are not ingratiated when the DataTemplate is created – it’s not until the DataTemplate is applied against some content (in this case the VM) in the ContentControl. This is nice, it means we can cache the DataTemplate to negate the need to re-generate them when the same VM type is used next time.
The XamlReader is used to process the dynamic XAML and returns a DataTemplate, which the ContentControl sets to it’s ContentTemplate property at runtime.
So what’s the benefit?
Convention is great for writing code and building projects *fast*. To add a new View using this system takes minutes (if not seconds). Navigating to the new view is very simple also.
In the next post we will example some further benefits of letting the “View” side of the fence handle control resolution – mainly automatic, designer friendly, centrally managed content transitions!