Jesse Liberty

Part 1 of Being a Visual Studio Champ – is now on video!

This is a 3 part conference streaming from Denmark and world-wide. vschamp.com

Do not miss the next one on Add-ons with (among others) Mads Kristensen. April 27, 10am Eastern.

One of my secret wish items was the ability to set the background color differently for different projects so I always know where I am. The problem was that it tells you your project in the upper right, so I didn’t want to whine about it. Well guess what, I installed VS2022 Preview today and saw this:

Awww snap! It works across projects (yay) but not across solutions (boo). Maybe next time.

In this post we will examine one of the aspects of advanced databinding that many people find confusing. Binding . (that is, Binding dot). However, it is surprisingly easy to explain.

Binding . allows you to access the entire binding context.

This is best understood with an example

<StackLayout BindingContext="{x:Static sys:DateTime.Now}"
                 HorizontalOptions="Center"
                 VerticalOptions="Center">

            <Label Text="{Binding Year, StringFormat='The year is {0}'}" />
            <Label Text="{Binding StringFormat='The month is {0:MMMM}'}" />
            <Label Text="{Binding Day, StringFormat='The day is {0}'}" />
            <Label Text="{Binding StringFormat='The time is {0:T}'}" />
            <Label Text="{Binding ., StringFormat='The full date is {0}'}" />
            <Label Text="{Binding}" />

        </StackLayout>

Here the stacklayout sets its BindingContext to DateTime.Now. That will allow us to bind to properties of the DateTime object that has the current local time. The first few lines are straight forward, we bind to properties of the DateTime object. For example,

You want to bind to a collection of values and display each in turn, but it is possible that some of the objects in the list have null properties, or some properties are missing altogether.

You can handle this with value converters, but there is a better (easier to write, easier to understand when reviewing) way.

In your Xaml, if you suspect a value may be null, you can makr it TargetNullValue and give it a string to display

TargetNullValue='Age unknown'

Similarly, if the value may be missing you can provide a default value,

FallbackValue='No School Found'

In this posting we’ll show how to use both.

En Español

There are times when you need to bind to a source but the source is not in the right format or otherwise needs to be manipulated.

For example, suppose, as we’ll show below, that you have a text entry and a button, but you only want the button enabled as long as there are one or more characters in the text entry, and of course if there is no text entered you want to disable button.

You could bind the button to a boolean property in your View Model and bind the text to another property and then on text changed you could test to see if there is text in the entry control and update the button.  Yuck.

This knocked me out. Let’s say you have a value in one view model, and when you navigate to another page you want that value in the new page’s view model. Enter IQueryAttributable. (tough to say, even tougher to spell)

I have created a simple example. We’re going to start with an out of the box Xamarin.Forms project. As part of that we’ll get an AboutPage. At the bottom of the about page I’m going to add a button:

<Button Text="ShowId" Command="{Binding ShowIdCommand}" />

Nothing surprising here. Let’s follow that ShowIdCommand. For that we turn to the associated view model (also out of the box)