Getting public variable of popped page - xamarin

Question

I'm instancing another page, and I assign a value to one of its public properties ("SomeValue") like this:
_btnGotoOtherPage.Clicked += async (sender, e) =>
{
OtherPage _otherpage = new OtherPage;
_otherpage.SomeValue = 1033;
await Navigation.PushAsync(_otherpage);
return;
};
Within this "_otherpage", the user can modified this value.
When "_otherpage" is popped, I would like to have a look at the "SomeValue" variable and do something with it.
MessagingSystem wouldn't be what I need because I don't want to be notified on the value change. I only want to know what this value is when "_otherpage" is popped.
I would also like to not use Binding (if possible!) because I feel it's hard to organize when I'm dealing with many of such variables.
Is it possible to do this with an event perhaps?
My dream solution would be (pseudo code):
private void OnPagePopped()
{
int iNewValue = PoppedPage.SomeValue;
}
Thank you.

Answer 1

If you are looking for an ideal solution, I would suggest following the MVVM pattern and moving a lot of your code from your page behind to the view model.
I use an MVVM framework called FreshMvvm. This allows me to perform view model to view model navigation and to pass parameters between them like this:
await CoreMethods.PushPageModel<BPageModel>(myParameter, true);
This passes myParameter to the BPage which I can access in the Init method of the BPage View Model.
When I pop the B page (via the view model) I can pass a parameter back to the A Page
await CoreMethods.PopPageModel(myReturnParam, true);
which I can access in the ReverseInit method of APageViewModel.
Most MVVM frameworks have similar functionality.
Here are more details about FreshMvvm

Answer 2

This is how I do it with my popups, but it can be used with the page style in the same way.
Heres my little example, expecting MainPage = new NavigationPage(new Page1());
It is basically about having a Task on the Page where the public property sits. This task can return the value of the public property. The task will be completed in the OnDisappearing override and return the public property.
To get the value back, you push the page and await the Task page.PagePoppedTask
using System;
using System.Threading.Tasks;
using Xamarin.Forms;
namespace MVVMTests
{
/// <summary>
/// A base implementation for a page, that holds a task,
/// which can be completed and returns a value of type of the generic T
/// </summary>
/// <typeparam name="T"></typeparam>
public class ResultContentPage<T> : ContentPage
{
public Task<T> PagePoppedTask { get { return tcs.Task; } }
private TaskCompletionSource<T> tcs;
public ResultContentPage()
{
tcs = new TaskCompletionSource<T>();
}
/// <summary>
/// Completes the task and sets it result
/// </summary>
/// <param name="result"></param>
protected /* virtual */ void SetPopupResult(T result)
{
if (PagePoppedTask.IsCompleted == false)
tcs.SetResult(result);
}
}
/// <summary>
/// Page1 exists of one button, that creates the Page2, assigns a value to a prop on the Page2
/// and then awaits the PagePoppedTask of Page2
/// </summary>
public class Page1 : ContentPage
{
public Page1()
{
var button = new Button()
{
Text = "Go to page 2"
};
button.Clicked += Button_Clicked;
Content = button;
}
private async void Button_Clicked(object sender, EventArgs e)
{
//Push the page
var newPage = new Page2() { CoolInt = 123 };
await App.Current.MainPage.Navigation.PushAsync(newPage);
//Await the result
int result = await newPage.PagePoppedTask;
System.Diagnostics.Debug.WriteLine("Page result: " + result.ToString());
}
}
/// <summary>
/// Inherits from the ResultContentPage and sets the PagePoppedTask as soon as its Disappearing
/// </summary>
public class Page2 : ResultContentPage<int>
{
public int CoolInt { get; set; } //Your property on the page
public Page2()
{
var button = new Button()
{
Text = "Go back to page 1"
};
button.Clicked += Button_Clicked;
Content = button;
}
private async void Button_Clicked(object sender, EventArgs e)
{
CoolInt = 321; //assign dummy value to CoolInt prop and pop the page
await App.Current.MainPage.Navigation.PopAsync(); //pop the page
}
protected override void OnDisappearing()
{
base.OnDisappearing();
SetPopupResult(CoolInt); //set the result of the task (in ResultContentPage<T>)
}
}
}

Answer 3

In Page A:
MessagingCenter.Subscribe<string>(this, "SomeMessage", (s) => HandleMessage(s));
In Page B:
MessagingCenter.Send<string>("SenderOrAnythingElse", "SomeMessage");

Answer 4

In PageB there is public variable:
public int SomeValue { get; set; }
Now we show PageB:
PageB nPageB = new PageB;
await Navigation.PushAsync(nPageB);
In nPageB, the user can now change PageB's public variable
//we track the "Disappearing" event of the page.
//When it occurs, we get the value
nPageB.Disappearing += async (sender1, e1) =>
{
Debug.WriteLine("New value is: " + nPage.SomeValue.ToString());
};