Prevent DatePicker and TimePicker fields from being overriden

[CollinAlpert]

What does the pull request do?

The text displayed for a DatePicker or TimePicker which has no value set is hardcoded. This PR only sets the respective TextBlock's text to the hardcoded value if another value has not yet been supplied (i.e. through styling).

What is the current behavior?

Currently any text set through styling gets overwritten.

What is the updated/expected behavior with this PR?

Adding the following style:

<Style Selector="DatePicker:hasnodate /template/ TextBlock#MonthText">
    <Setter Property="Text" Value="Monat" />
</Style>

This should cause an unset DatePicker to display the text "Monat" (German for "month") in the month's column.

[amwx]

This will fail if you clear the selected date (or time) after having it previously set since the TextBlock text won't be null. WinUI probably has localized strings here to prevent English only.
TextBlock.Text is also a DirectProperty, not a StyledProperty, so there's a chance the Style lookup in the OP will fail, and may not even work here to allow custom strings for :hasnodate (or :hasnotime)

[maxkatz6]

Yes, property localization for internal avalonia strings is something that we must have. It needs a proper design though.

[maxkatz6]

<Style Selector="DatePicker:hasnodate /template/ TextBlock#MonthText">
    <Setter Property="Text" Value="Monat" />
</Style>

Note, that setters to a direct properties won't be allowed in the future. Still you can create edit this field directly without styles with some code. See #7982 . Although TextBlock.Text specifically might be styleable in the future.

[CollinAlpert]

@maxkatz6 could you elaborate on that? How is it currently possible to change the value of the "month" column without styling?

[maxkatz6]

@CollinAlpert sure:

myTimePicker.TemplateApplied += (s, a) =>
{
    var hourTextBlock = a.NameScope.Find<TextBlock>("HourTextBlock");
    hourTextBlock.Text = "час";
};

[maxkatz6]

But unfortunately, it won't work if value will be null again. So it needs to be re-refreshed with a hack...:

private static void EnsureLocalization(TimePicker picker, string tempaltePartName, string value)
{
    picker.TemplateApplied += (s, a) =>
    {
        var textBlock = a.NameScope.Find<TextBlock>(tempaltePartName);
        textBlock.Text = value;

        textBlock.PropertyChanged += (s, a) =>
        {
            if (a.Property == TextBlock.TextProperty
                && picker.SelectedTime is null)
            {
                textBlock.Text = value;
            }
        };
    };
}

and from ctor:

EnsureLocalization(Picker, "HourTextBlock", "час");
EnsureLocalization(Picker, "MinuteTextBlock", "минуты");

[CollinAlpert]

@maxkatz6 thanks. Although this implies a code-behind. Any way using MVVM?

[maxkatz6]

@CollinAlpert that's purely View logic, so should work with MVVM as it is. Unless you want to read localized values from the ViewModel, but in this case I don't understand how styles would solve same problem.