Value Binding vs Data Binding
This article explains the difference between
@bind-Value, and between Value Binding and Data Binding.
This article contains the following sections:
- Value Binding
- Data Binding
- Terminology in MSDN vs Telerik Documentation
In this documentation, we will refer to Value Binding as the process of passing values between a component and its parent. Value binding tells the component to take the value of the corresponding field from the view-model and use it.
Value binding has two forms:
One-way binding - syntax like
<ParameterName>="@someField", for example
Two-way binding - syntax like
@bind-<ParameterName>="@someField", for example
One-way binding does not update the view-model if the component changes the value.
When using one-way binding, you can use the
<ParameterName>Changed event. For example, you can use both
To update the view-model in a one-way binding scenario, handle the corresponding
<Parameter>Changed event. In the handler, update the view-model with the new data. This allows you to apply some synchronous logic to that new value and effectively cancel the update if you don't set the new value to the view-model.
If you use one-way binding inside forms, you may get an error similar to
Unhandled exception rendering component: Telerik.Blazor.Components.TelerikTextBox requires a value for the 'ValueExpression'. This will not happen when you use two-way binding. You can find more details and ways to solve this in the Requires a value for ValueExpression article.
Two-way binding updates the view-model field if the component changes the value.
When using two-way binding, you cannot use the
<ParameterName>Changed event. If you use
@bind-Value and then add
ValueChanged, the framework will throw an exception. With two-way binding in Blazor, the event has already been handled to populate the view-model, so you cannot handle it again.
In this documentation, we will refer to Data Binding as the process of providing a component with a collection of data (models) that it will use and display.
There is no two-way binding in this case, the flow of the data is from the parent to the child component. Sometimes the parent component needs to be aware of data changes in the child component. The usual approach is to raise a specific event that the parent can handle and apply the necessary changes to its own view-model.
Data binding includes the following steps:
Creating a collection with the desired set of models/primitive types in the view-model. You would usually get it from a data service specific to the app.
Dataparameter of the Telerik component to that collection.
Setting other necessary parameters that instruct the component how to work with that data (such as specifying ID fields, text and value fields, etc.).
Terminology in MSDN vs Telerik Documentation
In the MSDN documentation, the
@bind-Value syntax is referred to as "Data Binding" because it carries a piece of data to/from the view-model to an input component (like a simple
<input /> or its form equivalent - the
In the Telerik UI for Blazor documentation, the
@bind-Value syntax is referred to as "Value Binding" because it carries field values to and from the view-model.
In the Telerik UI for Blazor documentation, we call "Data Binding" the process of providing a collection of models to a component so that this component can work with them. For example, display them - rows in a grid, appointments in a scheduler, items in a dropdown.
In the MSDN documentation, providing data to a component like that is not covered - it uses simple HTML elements and
@foreach loops to generate
<option> elements for dropdowns or
<tr> elements for grids.
This lack of terminology in the framework for a scenario that is considered basic for a component suite prompts the difference in conventions that we use.