With Xamarin Evolve coming up (I am currently on the way to Orlando!), it was time to release a new version and take care of a few bugs, improvements and new features.
- Main package including scaffolding
- Libraries only without scaffolding
- New package: Android RecyclerView support
In version 5.3, the majority of the activity was on the data binding system for Xamarin.iOS and Xamarin.Android, with many improvements in stability and performance, as well as a few new features. Existing code will continue to work as is, and at most you might have a few “deprecated” warnings, which will give you time to update your code. I also added 3 classes and a few helpers around them, which should help you greatly when you work with lists: ObservableRecyclerAdapter (for Android RecyclerView), ObservableTableViewSource (for iOS TableView) and ObservableCollectionViewSource (for iOS CollectionView). These objects will be detailed in separate blog posts.
In other areas not directly Xamarin-related, you will also find quite a few bug fixes. Here is list of changes . You will also find a few explanations below. I will publish additional blog posts with more details about the Observable list components mentioned above.
Removed old frameworks
In a desire of simplification, I removed the Silverlight 4, Windows Phone 7.1 and Windows 8 versions of MVVM Light from the repo. This does NOT affect the Silverlight 5, Windows Phone 8.0 and 8.1 as well as Windows 10 of course which are still very well supported. The main reason for not supporting these older versions is that they require Visual Studio 2012 and cannot be maintained or tested in VS2015. I hope that this doesn’t affect your projects too much. Should that be the case, please send me a note . We can always find a solution!
Fixing Nuget install.ps1 and documentation
There was a silly bug in the install.ps1 script that is run after the Nuget installation is performed, it is fixed now. Also, I added a more explicit description of the packages mvvmlight and mvvmlightlibs , in order to underline the differences between both.
Documentation: Closures not supported
In the same vein, I added explicit documentation warning that closures are not supported in the Messenger Register method, as well as in the RelayCommand Execute and CanExecute delegates. This is sometimes an issue. The reason is that these components are using WeakReference to avoid tight coupling with the objects using them. When we store the delegate that will be executed by the Messenger or respectively the RelayCommand, we store them as WeakAction instances. This causes the delegate to be “dehydrated” and stored as a MethodInfo. When we “rehydrate” the delegate, we notice that the closure has been lost, and the call fails. I am experimenting with ways to work around that limitation (i.e storing the delegates are actual Action or Func and not dehydrating them, which will of course cause a strong coupling to occur… but would be acceptable in some cases if the developer explicitly opts into this). For the moment, closures are not supported and the developer needs to find another way, for example storing the parameter in an attribute or a list of some kind.
I optimized the code a bit there and made it less redundant. I also made the RaisePropertyChanged methods public (was: protected) following a user’s request. This can make sense in certain scenarios when an external object wants to call this method on another VM.
Here too I fixed some bugs. I also added new features for iOS:
- Navigating with an unconfigured key in iOS : In iOS, the key passed to the NavigateTo method is often the same value as the corresponding Storyboard ID set in the Storyboard designer for a given controller. Until V5.3, you had to configure the key for the NavigationService, which was often leading to code such as
this didn’t quite make sense. As of V5.3, if you call NavigateTo with a key that is not configured, we will attempt to find a corresponding controller with this same Storyboard ID. Of course if you want to use a different ID, you can still configure the navigation service like before.
- Retrieving parameter without deriving from ControllerBase : Until now, the implementation used to retrieve a navigation parameter from the NavigationService forced you to derive your controller from ControllerBase. That was annoying, especially in cases where you had to derive from another base class anyway. I removed this restriction. Instead, you can now retrieve the NavigationService from the ServiceLocator in the ViewDidLoad method, and then call the GetAndRemoveParameter method to get the navigation parameter (if there is one). This is the same method than in the Android NavigationService.
I fixed a few bugs here which were causing the data binding to fail in certain scenarios. I also added a lot of unit tests to test current and new features (currently about 300 unit tests which run in both iOS and Android), as well as a few features. Expect samples very soon!
- FallbackValue : This value can be set in the SetBinding method (or in the Binding constructor) and will be used in case an error occurs when the Binding is being resolved. This can happen, for example, if you have a complex path expression and one of the elements is null, which would cause a NullReferenceException to happen inside the Binding. This exception is caught and nothing happens in the application, but the FallbackValue will be used as the Binding value if the user has set it. This can be used for information purposes. For example, if your source property is MyViewModel.SelectedItem.Name , and nothing is selected, the SelectedItem is null and the FallbackValue (for instance “Nothing selected yet”) is used.
- TargetNullValue : This value can also be set in the SetBinding method (or in the Binding constructor) and will be used in case the Binding value is null. This can also be used for information purposes.
- SetCommand with static parameter : I added an overload to the SetCommand extension method which can be used to pass a static parameter to the ICommand’s Execute and CanExecute methods. Prior to V5.3, you had to define a binding for this (and use the SetCommand(…, Binding) method) which was very cumbersome in case the binding value never changed. Now you can use the SetCommand method with a simple value (or if you want to observe the value you can still use the SetCommand method with a binding).
- SetCommand for ICommand : In previous versions, you could only use SetCommand with a RelayCommand (or RelayCommand<T>), which was an oversight. Now I modified this method to work with any ICommand.
- New name for UpdateSourceTrigger: This method is now named ObserveSourceEvent. The old method is still available but marked as deprecated. The old name was confusing for users.
- New name for UpdateTargetTrigger: Similarly, this method is now named ObserveTargetEvent. The old method is still available but marked as deprecated.
- Binding with implicit event names : When you set a binding on a UI element in Android and iOS, you must also specify which event should be observed for changes. This is necessary because properties in these elements, unlike in Windows, aren’t depedency properties. For instance, you can choose between FocusChange or TextChanged for an EditText, etc. In the huge majority of cases however, the same event is used over and over for a given element.
On Android, the TextChanged event is used for an EditText and the CheckedChange event for CheckBox ; on iOS, the ValueChanged event is used for UISwitch , the Changed event for UITextView , and the EditingChanged for UITextField . Note that existing code does not need to be changed unless of course you want to simplify it. And like before, you can continue to use ObserveSourceEvent and ObserveTargetEvent (formerly UpdateSourceTrigger and UpdateTargetTrigger) to specify a different event if needed.
- SetCommand with implicit event names: Like with the Binding class, when we set a Command on a control, we specify which event must be observed to trigger the command. For commonly used controls, this is mostly the same event. To simplify the code, you don’t have to explicitly specify these events anymore. The events that are observed implicitly are: On Android the Click event for Button , and the CheckedChange event for CheckBox . On iOS, the TouchUpInside event for UIButton , the Clicked event for UIBarButtonItem and the ValueChanged event for UISwitch . Of course you can also continue to specify these events explicitly, or any other event you wish to observe instead.
- Binding to private fields and local variables: In previous versions, you could only set a binding on public properties (for example public MainViewModel Vm , or public CheckBox MyCheck ; in V5.3, you can also set bindings on objects which are saved as private attributes. You can even create a new Binding (using the Binding constructor instead of the SetBinding method) on elements which are defined as local variables.
- Binding on RecyclerView, TableViewSource and CollectionViewSource cells : The feature above (binding on local variables) can be useful, for example to create a new binding between a data item and the cell that represents it in a list control. I’ll have blog posts for RecyclerView, TableViewSource and CollectionViewSource.
As you can see, this is a massive change set and I am really happy that your excellent feedback has led to these improvements. Of course the task is not over and there is more coming. V5.3 should greatly improve working with data binding in MVVM Light, in Xamarin.iOS and Xamarin.Android. As always, keep your feedback coming!
For more information about data binding in Xamarin.iOS and Xamarin.Android, you can watch my Xamarin Evolve 2016 session (slides, sample code and video recording will be posted ASAP).