![]() The view-model also serves as an integration point with other services such as database-access code. The view-model can also define members for keeping track of data that is relevant to the UI but not to the model, such as the display order of a list of items. In many cases, the view-model exposes the model directly, or provides members that wrap specific model members. The view-model layer provides data binding targets for the view. ![]() Code-behind files are sometimes used as part of the view layer to contain additional code needed to customize or manipulate the UI, or to extract data from event handler arguments before calling a view-model method that performs the work. The markup includes data binding expressions (such as x:Bind) that define the connection between specific UI components and various view-model and model members. The view layer defines the UI using XAML markup.Given a fully implemented model layer, you can create multiple different client apps if you so choose, such as UWP and web apps that work with the same underlying data. This layer is completely independent of the view and view-model layers, and often resides partially in the cloud. This includes everything required to model the core app domain, and often includes core app logic. The model layer defines the types that represent your business data.When using the MVVM pattern, an app is divided into the following layers: With this architecture, code-behind files often accumulate code that isn't directly related to the UI, such as database-access code, which ends up being duplicated and modified for use with other pages. This makes it difficult or impossible to replace a control without having to update the event handling code. The event handlers are implemented in code-behind files (such as ), and are often tightly coupled to the controls, typically containing code that manipulates the UI directly. In contrast with MVVM, an app with a more conventional "code-behind" structure typically uses data binding for display-only data, and responds to user input by directly handling events exposed by controls. Fixing bugs in decoupled code is less likely to cause regressions in other code. Decoupled code that adheres to well-designed interfaces can be developed by separate individuals or teams, and integrated later. Code units that are isolated from one another can be tested individually and outside of production environments. Change that is isolated is less risky and easier to experiment with. Enabling an iterative, exploratory coding style.Benefits of MVVMĭecoupling your code has many benefits, including: This decoupling is an example of the separation of concerns, which is an important concept in many design patterns. This makes it easier to change individual code units (methods, classes, controls, etc.) without causing unintended side effects in other units. The data binding infrastructure provides a loose coupling that keeps the UI and the linked data synchronized and routes user input to the appropriate commands.īecause it provides loose coupling, the use of data binding reduces hard dependencies between different kinds of code. With MVVM, you define your UI declaratively in XAML and use data binding markup to link it to other layers containing data and commands. Model-View-ViewModel (MVVM) is a UI architectural design pattern for decoupling UI and non-UI code.
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