Xamarin, Bindable Property

Dear all

It looks like I do misunderstand something in bindable properties. I would appreciate if somebody could explain me, how to do it correctly and why it works like that.

Why does the Binding on the "MeetingControl for "Meeting.Subject" not work?

GitHub Link: https://github.com/PatrickRainer/BindablePropertyExample

This is my Main Page:

"1.0" encoding="utf-8"?>
"MyPage"
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:SimpleSfTimePicker"
x:Class="SimpleSfTimePicker.MainPage"
BindingContext="{x:Reference Name=MyPage}">
"LightCyan">
"{Binding Meeting.Subject}"/>
"MeetingControl"
Subject="{Binding Meeting.Subject}"/>

This is the Code Behind (For ease I used no Vm):

public partial class MainPage {
Meeting _meeting;
public MainPage() {
InitializeComponent();
Meeting = new Meeting {
Subject = "A Meeting",
};
}
public Meeting Meeting {
get =>_meeting;
set {
_meeting = value;
OnPropertyChanged();
}
}
}

The Meeting:

public sealed class Meeting: INotifyPropertyChanged {
string _subject;
public string Subject {
get =>_subject;
set {
if (value == _subject) return;
_subject = value;
OnPropertyChanged();
}
}
public event PropertyChangedEventHandler PropertyChanged;
[NotifyPropertyChangedInvocator]
void OnPropertyChanged([CallerMemberName] string propertyName = null) {
PropertyChanged ? .Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}

The Meeting Control Xaml:

"1.0" encoding="utf-8"?>
"MyContentView"
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:til="clr-namespace:Syncfusion.XForms.TextInputLayout;assembly=Syncfusion.Core.XForms"
x:Class="SimpleSfTimePicker.MeetingControl"
BindingContext="{x:Reference MyContentView}">
Hint="Subject">
"SubjectEntry"/>

The MeetingControl Code Behind:

[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class MeetingControl {
public static readonly BindableProperty SubjectProperty = BindableProperty.Create(nameof(Subject), typeof(string), typeof(MeetingControl), "default", BindingMode.TwoWay, null, PropertyChanged);
static void PropertyChanged(BindableObject bindable, object oldvalue, object newvalue) {
var str = (string) newvalue;
var control = (MeetingControl) bindable;
control.SubjectEntry.Text = str;
}
public string Subject {
get {
return (string) GetValue(SubjectProperty);
}
set {
SetValue(SubjectProperty, value);
}
}
public MeetingControl() {
InitializeComponent();
}
}

2 Replies

Know the answer? Post it — somebody with the same question will find it here.

Patrick Rainer6y ago
@Maria Scheeren, I think I do understand Mvvm, but all the different binding possibilities not. Especially in XAML.

Anyway, here is the solution:

Modify your code as below
"SubjectEntry" Text="{Binding Source={x:Reference MyContentView},Path=Subject}" />
and remove the binding context

BindingContext="{x:Reference MyContentView}"

Set BindingContext to custom control itself will break the binding setting in MainPage.
0
Mario Scheeren6y ago
You should try to understand the MVVM pattern first. The main problem (besides naming conventions) relies in not unterstanding how binding works, where to instanciate the object and how the datacontext / bindingcontext works.

https://docs.microsoft.com/en-us/xamarin/xamarin-forms/xaml/xaml-basics/data-binding-basics
https://docs.microsoft.com/en-us/xamarin/xamarin-forms/xaml/xaml-basics/data-bindings-to-mvvm
0