Data-binding overview

One of the key concepts in DotVVM is the data-binding. The magic of building "rich client-side apps" without writing JavaScript is here thanks to the ability of DotVVM to translate binding expressions to JavaScript.

All binding expressions in DotVVM are strongly-typed and compiled, which helps to avoid many runtime errors caused by typos in identifiers and similar.

Therefore, each DotVVM page has to specify the @viewModel directive so the binding compiler can verify the correctness of all expressions, and pre-compile them.

Types of binding expressions

There are several types of binding expressions in DotVVM:

• {value: Property} - accesses the value of a specified property in the viewmodel.
• {resource: ResourceFile.ResourceKey} - evaluates an expression when the page is rendered.
• {command: Function()} - posts the viewmodel to the server and invokes the specified method.
• {staticCommand: Function()} - invokes either a static method, or a method in a static command service.

The value binding and resource binding are used to access the viewmodel properties.

The command and static commands are used to respond to user actions in buttons and other components.

There are also the controlCommand and controlProperty bindings, which are just a special cases of command and value bindings used in creating markup controls.

The commercial version of DotVVM for Visual Studio offers IntelliSense for expressions in all types of bindings.

Usage

You can use a data binding expression almost everywhere in DotHTML. The binding expression is enclosed in curly braces and has two parts:

• Binding type - e.g. value, command etc.
• Binding expression - a C#-like expression (with some restrictions and some enhancements).

For example, to bind some value from the viewmodel to a HTML attribute, you can use this syntax:

<a href="{value: Url}">...</a>

If you want to render the value as a text, you need to use double curly braces. That is because of the <script> and <style> elements - single curly braces have special meaning inside these elements.

<p>Hello {{value: YourName}}!</p>

It is not possible to use the binding expression in the HTML attribute value in combination with another content:

<!-- This does not work - the whole attribute value has to be a data binding! -->
<a class="this-does-not-work {value: DoNotUseThis}">...</a>

However, you can use expressions inside the binding, so you can combine the values easily:

<!-- This works - the whole attribute is a binding expression -->
<a class="{value: "tab " + AdditionalLinkClass}">...</a>

If you want to build complex class attributes, DotVVM has a special feature that allows to combine multiple CSS classes.

See also

• Value binding
• Resource binding
• Binding context
• Respond to user actions