Code With Controls

In this section, you will implement the core logic to update the Fahrenheit temperature based on the Celsius input.

Control Names

Avalonia creates objects for each control defined in the XAML hierarchy. Your code can access these controls at runtime, but should be named for easy access.

To add control names, follow this procedure:

• Stop the app if it is running.
• Locate the TextBox for Celsius.
• Add the Name attribute like this:

<TextBox ... Name="celsius"/>

• Repeat the above for the Fahrenheit input:

<TextBox ... Name="fahrenheit"/>

Get Control Values in Code-Behind

To access the Text value of the celsius input, follow this procedure:

• Switch to the MainView.axaml.cs code-behind file.
• Locate the ButtonClicked event handler.
• Alter the Debug statement to display the text property of the celsius input, like this:

Debug.WriteLine($"Click! Celsius={celsius.Text}");

• Run the app to confirm that you can see the value in the Celsius appear in the debug window.

Set Control Values in Code-Behind

To use the simple formula that converts Celsius temperature to Fahrenheit, you will first need to ensure that the Celsius input text converts to a number. The formula is then:

Tf = Tc * (9/5) + 32

To add the conversion formula, follow this procedure:

• Locate the ButtonClicked event handler.
• Validate the Celsius input text as a number.
• Use the conversion formula.
• Update the Text in the Fahrenheit input.
• Run the app to check your work.

One implementation of the above is as follows:

public void ButtonClicked(object source, RoutedEventArgs args)
{
    if (Double.TryParse(celsius.Text, out double C))
    {
        var F = C * (9d / 5d) + 32;
        fahrenheit.Text = F.ToString("0.0");
    }
    else
    {
        celsius.Text = "0";
        fahrenheit.Text = "0";
    }
}

You can check your work using the following conversion table:

Celsius	Fahrenheit
-10	14.0
0	32.0
10	50.0
21	69.8
25	77.0
32	89.6

Exercises

You have now used an event handler to get and set control properties at runtime. Try some of these exercises:

• Stop showing the gridlines (easy).
• Stop the user from changing the text in the Fahrenheit input by setting the IsReadOnly attribute (easy).
• Calculate the conversion as the user types into the Celsius input using the TextChanged event (moderate).

info

For full information about the complete range of Avalonia built-in controls, events and attributes, see the controls reference section here.