However, there are some patterns that are typical to model and view model classes, that can benefit from specific unit testing techniques. This project has a community site, on which you can post questions, and provide feedback. The community site is located on GitHub.
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Many enterprise apps are sufficiently complex to require more than one developer. It can be a significant challenge to decide how to design an app so that multiple developers can work effectively on different pieces of the app independently, while ensuring that the pieces come together seamlessly when integrated into the app.
The traditional approach to designing and building an app results in what is referred to as a monolithic app, where components are tightly coupled with no clear separation between them. Typically, this monolithic approach leads to apps that are difficult and inefficient to maintain, because it can be difficult to resolve bugs without breaking other components in the app, and it can be difficult to add new features or to replace existing features.
An effective remedy for these challenges is to partition an app into discrete, loosely coupled components that can be easily integrated together into an app.
Such an approach offers several benefits:. However, there are many issues that must be resolved when partitioning an app into discrete, loosely coupled components. These include:. This guide provides guidance on these issues, and focuses on the core patterns and architecture for building a cross-platform enterprise app using Xamarin. The guidance aims to help to produce adaptable, maintainable, and testable code, by addressing common Xamarin.
Forms enterprise app development scenarios, and by separating the concerns of presentation, presentation logic, and entities through support for the Model-View-ViewModel MVVM pattern. This guide includes a sample application, eShopOnContainers, that's an online store that includes the following functionality:.
These backend services are implemented as microservices using ASP. Collectively, these backend services are referred to as the eShopOnContainers reference application. Apps should be architected for the correct use of property change notification, by meeting the following requirements:. The eShopOnContainers mobile app uses the ExtendedBindableObject class to provide change notifications, which is shown in the following code example:. The ExtendedBindableObject class provides the RaisePropertyChanged method to invoke property change notification, and in doing so uses the functionality provided by the BindableObject class.
Therefore, each view model class uses the RaisePropertyChanged method in the ExtendedBindableObject class to provide property change notification. The following code example shows how the eShopOnContainers mobile app invokes property change notification by using a lambda expression:.
Note that using a lambda expression in this way involves a small performance cost because the lambda expression has to be evaluated for each call. Although the performance cost is small and would not normally impact an app, the costs can accrue when there are many change notifications. However, the benefit of this approach is that it provides compile-time type safety and refactoring support when renaming properties.
In mobile apps, actions are typically invoked in response to a user action, such as a button click, that can be implemented by creating an event handler in the code-behind file.
However, in the MVVM pattern, the responsibility for implementing the action lies with the view model, and placing code in the code-behind should be avoided. Commands provide a convenient way to represent actions that can be bound to controls in the UI.
They encapsulate the code that implements the action, and help to keep it decoupled from its visual representation in the view. Forms includes controls that can be declaratively connected to a command, and these controls will invoke the command when the user interacts with the control. Behaviors also allow controls to be declaratively connected to a command. However, behaviors can be used to invoke an action that's associated with a range of events raised by a control.
Therefore, behaviors address many of the same scenarios as command-enabled controls, while providing a greater degree of flexibility and control.
In addition, behaviors can also be used to associate command objects or methods with controls that were not specifically designed to interact with commands. View models typically expose command properties, for binding from the view, that are object instances that implement the ICommand interface.
A number of Xamarin. Forms controls provide a Command property, which can be data bound to an ICommand object provided by the view model. The ICommand interface defines an Execute method, which encapsulates the operation itself, a CanExecute method, which indicates whether the command can be invoked, and a CanExecuteChanged event that occurs when changes occur that affect whether the command should execute.
Execute method is invoked. The CanExecute method is an optional constructor parameter, and is a Func that returns a bool. The following code shows how a Command instance, which represents a register command, is constructed by specifying a delegate to the Register view model method:. The command is exposed to the view through a property that returns a reference to an ICommand. When the Execute method is called on the Command object, it simply forwards the call to the method in the view model via the delegate that was specified in the Command constructor.
An asynchronous method can be invoked by a command by using the async and await keywords when specifying the command's Execute delegate. This indicates that the callback is a Task and should be awaited. For example, the following code shows how a Command instance, which represents a sign-in command, is constructed by specifying a delegate to the SignInAsync view model method:.
If a delegate isn't specified, the Command will return true for CanExecute. However, the view model can indicate a change in the command's CanExecute status by calling the ChangeCanExecute method on the Command object. This causes the CanExecuteChanged event to be raised.
Any controls in the UI that are bound to the command will then update their enabled status to reflect the availability of the data-bound command. A command parameter can also be optionally defined using the CommandParameter property. The type of the expected argument is specified in the Execute and CanExecute target methods.
The TapGestureRecognizer will automatically invoke the target command when the user interacts with the attached control. The command parameter, if provided, will be passed as the argument to the command's Execute delegate.
Behaviors allow functionality to be added to UI controls without having to subclass them. Instead, the functionality is implemented in a behavior class and attached to the control as if it was part of the control itself.
Behaviors enable you to implement code that you would normally have to write as code-behind, because it directly interacts with the API of the control, in such a way that it can be concisely attached to the control, and packaged for reuse across more than one view or app.
In the context of MVVM, behaviors are a useful approach for connecting controls to commands. A behavior that's attached to a control through attached properties is known as an attached behavior. The guide comes with source code for the eShopOnContainers mobile app , and source code for the eShopOnContainers reference app. The eShopOnContainers mobile app is a cross-platform enterprise app developed using Xamarin.
Forms, which connects to a series of containerized microservices known as the eShopOnContainers reference app. However, the eShopOnContainers mobile app can be configured to consume data from mock services for those who wish to avoid deploying the containerized microservices.
This guide is aimed at readers who are already familiar with Xamarin. For a detailed introduction to Xamarin. Forms, see the Xamarin. Forms documentation , and Creating Mobile Apps with Xamarin. The guide is complementary to. NET Applications , which focuses on developing and deploying containerized microservices.
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