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UWP C# MVVM How To Access ViewModel from Other Page

I am tying to further understand MVVM with some example scenario. I have a rootpage with a 'maindisplay' textblock. I would like to display 'status' or 'scenarios' from activation of any form of UI eg. togglebutton on the 'maindisplay' textblock.
I am able to bind the the page navigation info in the rootpageviewmodel to the textblock. However, I am not able to achieve the result when displaying info from different page.
I have checked another post multiple-viewmodels-in-same-view & Accessing a property in one ViewModel from another it's quite similar but it didn't work.
Please help. Thanks.
While accessing the RootPageViewModel should retain the instance?
View
<TextBlock Text="{x:Bind RootViewModel.MainStatusContent, Mode=OneWay}"/>
RootPage.xaml.cs
public sealed partial class RootPage : Page
{
private static RootPage instance;
public RootPageViewModel RootViewModel { get; set; }
public RootPage()
{
RootViewModel = new RootPageViewModel();
this.InitializeComponent();
// Always use the cached page
this.NavigationCacheMode = NavigationCacheMode.Required;
}
public static RootPage Instance
{
get
{
if (instance == null)
{
instance = new RootPage();
}
return instance;
}
}
private void nvTopLevelNav_ItemInvoked(NavigationView sender, NavigationViewItemInvokedEventArgs args)
{
if (args.IsSettingsInvoked)
{
contentFrame.Navigate(typeof(SettingsPage));
RootViewModel.MainStatusContent = "Settings_Page";
}
else
{
var navItemTag = args.InvokedItemContainer.Tag.ToString();
RootViewModel.MainStatusContent = navItemTag;
switch (navItemTag)
{
case "Home_Page":
contentFrame.Navigate(typeof(HomePage));
break;
case "Message_Page":
contentFrame.Navigate(typeof(MessagePage));
break;
}
}
}
}
RootPage ViewModel:
public class RootPageViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private static RootPageViewModel instance = new RootPageViewModel();
public static RootPageViewModel Instance
{
get
{
if (instance == null)
instance = new RootPageViewModel();
return instance;
}
}
public RootPageViewModel()
{
}
private string _mainStatusContent;
public string MainStatusContent
{
get
{
return _mainStatusContent;
}
set
{
_mainStatusContent = value;
OnPropertyChanged();
}
}
protected void OnPropertyChanged([CallerMemberName] string name = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
}
}
MessagePage.xaml.cs - to access RootPage ViewModel
public sealed partial class MessagePage : Page
{
public MessagePageViewModel MessageViewModel { get; set; }
public MessagePage()
{
MessageViewModel = new MessagePageViewModel();
this.InitializeComponent();
// Always use the cached page
this.NavigationCacheMode = NavigationCacheMode.Required;
}
private void Message1_Checked(object sender, RoutedEventArgs e)
{
RootPageViewModel.Instance.MainStatusContent = "Message 1 Selected";
}
private void Message1_Unchecked(object sender, RoutedEventArgs e)
{
RootPageViewModel.Instance.MainStatusContent = "Message 1 De-Selected";
}
}
When I debug the value did write to the instance but did't update the TextBlock. Did I do anything wrong in my XAML binding?

UWP C# MVVM How To Access ViewModel from Other Page
The better way is make static variable for RootPage, but not make singleton instance for RootPage and RootPageViewModel.
For example:
public RootPage ()
{
this.InitializeComponent();
this.NavigationCacheMode = NavigationCacheMode.Required;
Instance = this;
RootViewModel = new RootPageViewModel();
}
public static RootPage Instance;
Usage
private void Message1_Checked(object sender, RoutedEventArgs e)
{
RootPage.Instance.RootViewModel.MainStatusContent = "Message 1 Selected";
}
private void Message1_Unchecked(object sender, RoutedEventArgs e)
{
RootPage.Instance.RootViewModel.MainStatusContent = "Message 1 De-Selected";
}

Xamarin / SQLite: update doubles in a database

I am working on a Xamarin.Forms app that will store a list of receipes for some chemicals. The chemicals have names (not user changeable) and concentrations. I want the user to be able to change the concentrations in the form and store them.
I have it to where I can modify values in the list with the UPDATE button, I can add (or delete) members of the list, all persistent. However, I cannot figure out how to change the values in the list within the Entry box, itself.
I tried to do something like Arvind Chourasiya did here, but I could not quite figure out the equivalent "connection.Update." I think the SQLite can't be connected to because it's not in the right event, but I am not sure.
My most functional C# code is...
using System;
using System.ComponentModel;
using Xamarin.Forms;
using SQLite;
using System.Collections.ObjectModel;
using System.Runtime.CompilerServices;
namespace contactBook
{
public class Recipe : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
[PrimaryKey, AutoIncrement]
public int Id { get; set; }
private string _name;
[MaxLength(255)]
public string Name
{
get { return _name; }
set
{
if (_name == value)
return;
_name = value;
OnPropertyChanged();
}
}
private double _concentration;
public double Concentration
{
get
{ return _concentration; }
set
{
if (_concentration == value)
return;
_concentration = value;
OnPropertyChanged();
}
}
private void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
// Learn more about making custom code visible in the Xamarin.Forms previewer
// by visiting https://aka.ms/xamarinforms-previewer
[DesignTimeVisible(false)]
public partial class MainPage : ContentPage
{
private SQLiteAsyncConnection _connection;
private ObservableCollection<Recipe> _recipes;
public MainPage()
{
InitializeComponent();
_connection = DependencyService.Get<ISQLiteDb>().GetConnection();
//setBasicReceipies();
}
protected override async void OnAppearing()
{
await _connection.CreateTableAsync<Recipe>();
var recipes = await _connection.Table<Recipe>().ToListAsync();
_recipes = new ObservableCollection<Recipe>(recipes);
recipesListView.ItemsSource = _recipes;
base.OnAppearing();
}
async void setBasicReceipies() // worked during tests
{
var recipe1 = new Recipe { Name = "NH3", Concentration = 0.0 };
var recipe2 = new Recipe { Name = "H2SO4", Concentration = 0.1 };
var recipe3 = new Recipe { Name = "NaCl", Concentration = 0.2 };
await _connection.InsertAsync(recipe1);
await _connection.InsertAsync(recipe2);
await _connection.InsertAsync(recipe3);
}
async void OnAdd(object sender, System.EventArgs e)
{
var recipe = new Recipe { Name = "test ", Concentration = 0.0 };
await _connection.InsertAsync(recipe);
_recipes.Add(recipe);
}
async void OnUpdate(object sender, System.EventArgs e)
{
var recipe = _recipes[0];
recipe.Concentration += 0.05;
await _connection.UpdateAsync(recipe);
}
async void OnDelete(object sender, System.EventArgs e)
{
var recipe = _recipes[0];
await _connection.DeleteAsync(recipe);
_recipes.Remove(recipe);
}
//async void Entry_PropertyChanged(System.Object sender, System.ComponentModel.PropertyChangedEventArgs e)
//{
// await _connection.UpdateAllAsync();
//}
}
}

How to switch between pages with Xamarin.Forms.INavigation.InsertPageBefore

I'm using Xamarin Forms and I'm having an issue use InsertPageBefore() method with existing objects of Pages.
Here is my view code:
private FirstPage firstPage;
private SecondPage secondPage = new SecondPage();
private ThirdPage thirdPage = new ThirdPage();
private async void ItemSelectedMethod()
{
var root = App.NavigationPage.Navigation.NavigationStack[0];
if (SelectedItem == Items[0])
{
if (!IsFirstChoose)
{
App.NavigationPage.Navigation.InsertPageBefore(firstPage, root);
await App.NavigationPage.PopToRootAsync(false);
}
}
if (SelectedItem == Items[1])
{
App.NavigationPage.Navigation.InsertPageBefore(secondPage, root);
await App.NavigationPage.PopToRootAsync(false);
}
if (SelectedItem == Items[2])
{
App.NavigationPage.Navigation.InsertPageBefore(thirdPage, root);
await App.NavigationPage.PopToRootAsync(false);
}
IsFirstChoose = false;
rootPageViewModel.IsPresented = false;
}
It's throw exception "System.ArgumentException: 'Cannot insert page which is already in the navigation stack'". How to switch between existing objects of pages? I don't want create new object in InsertPageBefore(). I tried use it code, before call InsertPageBefore():
foreach (var item in App.NavigationPage.Navigation.NavigationStack.ToList())
App.NavigationPage.Navigation.RemovePage(item);
But it's not working... Can anyone help me?

It didn't work with UWP. Here is agly workaround for you but you really need to read how to work with Master-Detail pages.
public partial class App : Application
{
public static RootPage RootPage { get; private set; } //DON'T DO THIS,
//FIND A BETTER WAY
public App()
{
InitializeComponent();
RootPage = new RootPage();
MenuPage menuPage = new MenuPage(RootPage.vm);
RootPage.Master = menuPage;
RootPage.Detail = new NavigationPage(new MainPage());// NavigationPage;
MainPage = RootPage;
}
protected override void OnStart()
{
// Handle when your app starts
}
protected override void OnSleep()
{
// Handle when your app sleeps
}
protected override void OnResume()
{
// Handle when your app resumes
}
}
Then
private async void ItemSelectedMethod()
{
if (SelectedItem == Items[0])
{
App.RootPage.Detail = new NavigationPage(mainPage);
}
if (SelectedItem == Items[1])
{
App.RootPage.Detail = new NavigationPage(secondPage);
}
if (SelectedItem == Items[2])
{
App.RootPage.Detail = new NavigationPage(thirdPage);
}
rootPageViewModel.IsPresented = false;
}

How to pass data between WPF forms

I need help passing data from one WPF form to another. I have a main window with two other windows that will prompt the user for information. I want to end up with all the information in the first form so that I can store the data later on. The second form must return the Reservation and Room information when you click the OK button on the second form. The third form must return the Person information when you click OK.
public partial class MainWindow : Window
{
private string message;
public MainWindow()
{
InitializeComponent();
}
protected void Exit_Click(object sender, RoutedEventArgs e)
{
Application.Current.Shutdown();
}
private void Window_Loaded(object sender, RoutedEventArgs e)
{
}
protected void Create_Reservation_Click(object sender, RoutedEventArgs e)
{
Reservation PersonReservation = new Reservation();//Create a reservation instance
Room PersonRoom = new Room(); //Create an instance of a room
Person myPerson = new Person();//Create an instance of a person
CreateResRoom createReservationRoom = new CreateResRoom();//Create a instance of the CreateReservation WPF Form
createReservationRoom.Show();
Here it is supposed to set the room, reservation and person instance that I created equil to their corresponding instances in the CreateResRoom class.
I think the problem lies here, because it keeps continuing before it opens the CreateResRoom form.
PersonRoom = createReservationRoom.myRoom;
PersonReservation = createReservationRoom.myReservation;
}
}
That was my first class, the second and third will follow.
public partial class CreateResRoom : Window
{
Person myPerson;
public CreateResRoom()
{
InitializeComponent();
myReservation = new Reservation();
myRoom = new Room();
myPerson = new Person();
}
public Room myRoom
{
get;
set;
}
public Reservation myReservation
{
get;
set;
}
private void btnCancel_Click(object sender, RoutedEventArgs e)
{
this.Close();
}
private void btnOk_Click(object sender, RoutedEventArgs e)
{
myRoom.RoomBeds = txtHeadCount.Text;
myRoom.RoomNumber = 1;
myRoom.RoomPrice = 20;
myRoom.RoomType = cboRoomType.Text;
myReservation.ResEndDate = dpEnd.ToString();
myReservation.ResStartDate = dpStart.ToString();
CreateRes createReservation = new CreateRes();
createReservation.Show();
//I think the same problem lies here that is in the MainWindow.
myPerson = createReservation.myPerson;
this.Close();
}
}
And the last class follows:
public partial class CreateRes : Window
{
public Person myPerson
{
get;
set;
}
public CreateRes()
{
InitializeComponent();
myPerson = new Person();
}
private void Window_Loaded(object sender, RoutedEventArgs e)
{
}
private void btnOk_Click(object sender, RoutedEventArgs e)
{
myPerson.FirstName = txtFName.Text;
myPerson.LastName = txtLName.Text;
myPerson.IdNumber = Convert.ToInt32(txtIdNumber.Text);
myPerson.PhoneNumber = Convert.ToInt32(txtPhoneNumber.Text);
myPerson.AddressCity = txtAddressCity.Text;
myPerson.AddressStreet = txtAddressStreet.Text;
myPerson.AddressProvince = txtAddressProvince.Text;
myPerson.AddressPostalCode = txtAddressPostalCode.Text;
this.Close();
}
private void btnCancel_Click(object sender, RoutedEventArgs e)
{
this.Close();
}
}

Just make a overload constructor which takes parameters of the window in which you want to retrieve.
Example:
Suppose we want a user to login from our MainWindow( i.e Login Window ) and we want to pass an int ID / string Email to our second form to retrieve data of logging user.
Than We have to first overload our second wpf form constructor. You can either make default constructor to do this or make an overload constructor for this work.
SecondForm:
public secondForm()
{
//Your Default Constructor Logic
}
public secondForm(string email_ )
{
//Your Overload Constructor Logic
}
Now in MainWindow from where we are logging and passing our EMail
MainWindow:
public void btnLogin()
{
//On Success
SecondWindow sw = new SecondWindow(txtBoxEMail.Content);
sw.Show();
}

A pattern you can use for this sort of thing is to have each form be responsible for creating the instance on ok click and then provide the object via a property get.
public partial class SomeForm: Window
{
public SomeClass MyProperty { get; private set; }
private void btnOk_Click(object sender, RoutedEventArgs e)
{
this.MyProperty = new SomeClass();
//additional setter logic here
this.Close();
}
}
Then you would access it from a parent form like this (notice the use of ShowDialog() http://msdn.microsoft.com/en-us/library/system.windows.window.showdialog(v=vs.110).aspx for easy checking of whether ok was clicked or not).
protected void Create_Reservation_Click(object sender, RoutedEventArgs e)
{
SomeClass myObj;
SomeOtherClass myOtherObj;
SomeForm myForm = new SomeForm();
if(myForm.Show().Value)
{
myObj = myForm.MyProperty;
}
SomeOtherForm myOtherForm = new SomeOtherForm();
if(myOtherForm.ShowDialog().Value)
{
myOtherObj = myOtherForm.MyOtherProp;
}
//save myObj & myOtherObj or whatever you need to do with them

Use the "normal way", here is a short overview.
First create a Data Context:
public class DC_Reservation() : INotifyPropertyChanged {
protected Reservation _PersonReservation ;
public Reservation PersonReservation {
get { return _PersonReservation ; }
set {
_PersonReservation = value;
NotifyPropertyChanged("PersonReservation ");
}
}
protected Room _PersonRoom ;
public Room PersonRoom {
get { return _PersonRoom ; }
set {
_PersonRoom = value;
NotifyPropertyChanged("PersonRoom");
}
}
protected Person _myPerson ;
public Person myPerson {
get { return _myPerson ; }
set {
_myPerson = value;
NotifyPropertyChanged("myPerson ");
}
}
public event PropertyChangedEventHandler PropertyChanged;
public void NotifyPropertyChanged( string PropertyName ) {
if ( PropertyChanged != null ) {
PropertyChanged( this, new PropertyChangedEventArgs( PropertyName ) );
}
}
}
In the MainWindows you can assign and use the dataContext :
public partial class MainWindow : Window {
DC_Reservation dataContext {
get { return DataContext as DC_Reservation; }
}
private string message;
public MainWindow() {
InitializeComponent();
DataContext = new DC_Reservation();
}
protected void Create_Reservation_Click(object sender, RoutedEventArgs e) {
dataContext.PersonReservation = new Reservation();//Create a reservation instance
dataContext.PersonRoom = new Room(); //Create an instance of a room
dataContext.myPerson = new Person();//Create an instance of a person
CreateResRoom createReservationRoom = new CreateResRoom();//Create a instance of the CreateReservation WPF Form
// I'm not sure whether the next line is required.
createReservationRoom.DataContext = DataContext;
createReservationRoom.Show();
}
}
You can assign the DataContext in the constructor, but I think the better way is to define the DataContext in the MainWindow, in the other windows you can use the DesignContext:
<Window.DataContext>
<local:DC_Reservation />
</Window.DataContext>
So you can use the same DataContext over all forms ...
With DataBindings you can bind the input to the field:
<TextBox Text="{Binding FirstName, Path=myPerson, Mode=TwoWay}" />

I found another answer that Zarathos posted Jan 16 '13 at 21:43
for a different question
Use a public static class and access it from anywhere.
public static class Globals
{
public static String s_Name = "Mike"; //Modifiable in Code
public const int32 VALUE = 10; // unmodifiable
}
Then you can use it anywhere, provided you are working on the same namespace
string name = Globals.s_Name;

Panel does not contain a constructor that takes 0 arguments

I need to load a User Control in my panel1 inside Form1.cs, the problem is that the UserControl (AudioPlaybackPanel) contains an ImportingConstructor ([ImportMany]IEnumerable<>) and I can't figure out what two arguments I should have in the Form1 AudioPlaybackPanel(????).
The error I get is: 'NAudio.App.AudioPlaybackPanel' does not contain a constructor that takes 0 arguments
Here is the Form1.cs
namespace NAudio.App
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void panel1_Paint(object sender, PaintEventArgs e)
{
}
private void button1_Click(object sender, EventArgs e)
{
AudioPlaybackPanel myPanel = new AudioPlaybackPanel(????);
panel1.Controls.Add(myPanel);
}
}
}
And this is my User Control Panel (AudioPlaybackPanel.cs):
namespace NAudio.App
{
[Export]
public partial class AudioPlaybackPanel : UserControl
{
private IWavePlayer waveOut;
private string fileName = null;
private WaveStream fileWaveStream;
private Action<float> setVolumeDelegate;
[ImportingConstructor]
public AudioPlaybackPanel([ImportMany]IEnumerable<IOutputDevicePlugin> outputDevicePlugins)
{
InitializeComponent();
LoadOutputDevicePlugins(outputDevicePlugins);
}
[ImportMany(typeof(IInputFileFormatPlugin))]
public IEnumerable<IInputFileFormatPlugin> InputFileFormats { get; set; }
private void LoadOutputDevicePlugins(IEnumerable<IOutputDevicePlugin> outputDevicePlugins)
{
comboBoxOutputDevice.DisplayMember = "Name";
comboBoxOutputDevice.SelectedIndexChanged += new EventHandler(comboBoxOutputDevice_SelectedIndexChanged);
foreach (var outputDevicePlugin in outputDevicePlugins.OrderBy(p => p.Priority))
{
comboBoxOutputDevice.Items.Add(outputDevicePlugin);
}
comboBoxOutputDevice.SelectedIndex = 0;
}
void comboBoxOutputDevice_SelectedIndexChanged(object sender, EventArgs e)
{
panelOutputDeviceSettings.Controls.Clear();
Control settingsPanel;
if (SelectedOutputDevicePlugin.IsAvailable)
{
settingsPanel = SelectedOutputDevicePlugin.CreateSettingsPanel();
}
else
{
settingsPanel = new Label() { Text = "This output device is unavailable on your system", Dock=DockStyle.Fill };
}
panelOutputDeviceSettings.Controls.Add(settingsPanel);
}
private IOutputDevicePlugin SelectedOutputDevicePlugin
{
get { return (IOutputDevicePlugin)comboBoxOutputDevice.SelectedItem; }
}
// The rest of the code continues from here on...
}
}
Here is the Interface:
namespace NAudio.App
{
public interface IOutputDevicePlugin
{
IWavePlayer CreateDevice(int latency);
UserControl CreateSettingsPanel();
string Name { get; }
bool IsAvailable { get; }
int Priority { get; }
}
}
And just in case, here is one of the plugins:
DirectSoundOutPlugin.cs
namespace NAudio.App
{
[Export(typeof(IOutputDevicePlugin))]
class DirectSoundOutPlugin : IOutputDevicePlugin
{
private DirectSoundOutSettingsPanel settingsPanel;
private bool isAvailable;
public DirectSoundOutPlugin()
{
this.isAvailable = DirectSoundOut.Devices.Count() > 0;
}
public IWavePlayer CreateDevice(int latency)
{
return new DirectSoundOut(settingsPanel.SelectedDevice, latency);
}
public UserControl CreateSettingsPanel()
{
this.settingsPanel = new DirectSoundOutSettingsPanel();
return this.settingsPanel;
}
public string Name
{
get { return "DirectSound"; }
}
public bool IsAvailable
{
get { return isAvailable; }
}
public int Priority
{
get { return 3; }
}
}
}
Please help!

The error doesn't say it expects two arguments... it just says it doesn't take 0.
The constructor expects a single parameter - an IEnumerable<IOutputDevicePlugin>:
public AudioPlaybackPanel([ImportMany]IEnumerable<IOutputDevicePlugin> outputDevicePlugins)
{
...
}
You need to find something that implements the IOutputDevicePlugin interface and pass a collection of it, even if it's just an empty collection. (Passing null to the constructor will allow it to compile but will throw a runtime exception when you hit the loop in LoadOutputDevicePlugins.)
Considering the update to your question, something like this will get you up and running (although I doubt it means very much to pass an empty list):
var myPanel = new AudioPlaybackPanel(new List<DirectSoundOutPlugin>());
panel1.Controls.Add(myPanel);

It's worth asking whether you actually need to copy AudioPlaybackPanel.cs from the NAudio demo in its entirety. The reason it has this constructor is that it tries to demonstrate how you can use each and every one of NAudio's IWavePlayer implementations. But in a normal real-world application you would just select the one that was most appropriate for your use. e.g.
this.waveOut = new WaveOut();
waveOut.Init(new AudioFileReader("my file.mp3");
waveOut.Play();
So there's no need to incorporate the plug-in architecture from that particular demo, if all you want is just to play audio files.