In this example, a unique Person is defined by their FirstName and LastName. PageA is a form that selects a unique Person. PageB is a list of unique FirstNames, and PageC is a list of all the LastNames that exist for a given FirstName.
I'm having a hard time solving a particular UX pattern using MvxNavigationService. Here's what I'm attempting to do, (psuedocode):
PageA.SelectedItem = NavigateTo(PageB) [list of Person, grouped
by Person.FirstName];
PageB.SelectedItem = NavigateTo(PageC) [for
Person.FirstName, list of Person.LastName, ];
PageC.Close(SelectedItem);
PageB.Close(SelectedItem);
When I actually try and implement this and run it on Android, the viewmodel logic executes, but the UI doesn't show PageA.
Update: Calling PageB.Close() navigates back to PageC, since PageC was the previous page. Perhaps the problem could be solved by ensuring that PageC is removed from the stack upon closing it. How might this be accomplished?
There's many ways of doing this, using a Custom ViewPresenter on iOS or using an Activity on Android.
One way I achieved this for a small pair (2 view models) was by adding an Instance static variable to the first ViewModel that opens the second ViewModel, like this:
public class FirstViewModel
{
public static FirstViewModel Instance;
public void FirstViewModel()
{
Instance = this;
...
}
}
And then in the second ViewModel's save/close command, I just closed both ViewModels like this and it worked:
public new MvvmCross.Commands.IMvxCommand SaveClickCommand
{
get
{
return new MvvmCross.Commands.MvxAsyncCommand(
async () =>
{
await Navigator.Close(this);
await Navigator.Close(FirstViewModel.Instance);
}
);
}
}
Related
Functionality written inside the MessagingCenter.Subscribe() is called multiple times when i navigate to and fro multiple times in the application. But each time before subscribing, i do unsubscribe to the same in constructor as follows, still it didn't worked.
MessagingCenter.Unsubscribe<SubmitPage>(this,"Save");
MessagingCenter.Subscribe<SubmitPage>(this, "Save", (sender) =>
{
DisplayToastOnSuccessfulSubmission();
});
In my application i have 6 pages(git) and i save the data in 6th page with MessagingCenter.Send and same will be subscribed in 2nd page and saved message will be displayed in 2nd page(after navigating to that page).
Now i navigate like 2->1->2->3->4->5->6 in this particular case DisplayToastOnSuccessfulSubmission() would be called two times(because Page2 constructor is called twice).
I even tried placing the same code in OnAppearing.
I can't unsubscribe in OnDisappear as I need the event wiring up to when I reach Page6 for save.
Reproduced the same behaviour in sample project and added here https://github.com/suchithm/MessageCenterSampleApp Drop box link
What is the proper way to do this?
But each time before subscribing, I do unsubscribe to the same in constructor as follows, still it didn't worked.
MessagingCenter.Subscribe() is called multiple times, because there are two instances of Page2 in your code, both of them use MessagingCenter.Subscribe() method, that's why the Unsubscribe didn't work.
You can modify page2() to a singleton to make sure there is only one instance of Page2 in your project, after that when you send a message,
the MessagingCenter.Subscribe() is called only once.
Page2.cs:
public static Page2 instance = new Page2();
public static Page2 GetPage2Instance()
{
if(instance == null)
{
return new Page2();
}
return instance;
}
private Page2()
{
InitializeComponent();
MessagingCenter.Unsubscribe<Page2>(this, "SaveToastPage2");
MessagingCenter.Subscribe<Page2>(this, "SaveToastPage2", (sender) =>
{
DisplayToastOnSuccessfulSubmission();
}
}
When you send a message :
MessagingCenter.Send(Page2.GetPage2Instance(), "SaveToastPage2");
EDIT :
Remember that declaring constructors of Page2 class to be private to make sure there is only one instance of Page2 in your project sure.
private Page2()
{
...
}
Modify your Page1.cs code :
async void Handle_Next(object sender, System.EventArgs e)
{
await App.NavigationRef.PushAsync(Page2.GetPage2Instance(), true);
}
I faced same issue. I solved issue by passing the same parameters inn subscribe and unsubscribing as well.
MessagingCenter.Subscribe<Page1, T>(this, "Listen", async (Page1 arg1, T
listenedString) =>
{
});
Unsubscribe like below
MessagingCenter.Unsubscribe<Page1, T>(this, "Listen");
I'm using this temporary solution.
I declared a static dictionary to storage my object (to this example I used an object type).
private static Dictionary<string, object> subscribedReferencePages = new Dictionary<string, object>();
And I always storage the last subscribed page reference.
Then I compare the page reference before triggering the message method to fire only the last one.
subscribedReferencePages[pageName] = this;
MessagingCenter.Subscribe<ViewModelBase>(this, pageName, async (sender) =>
{
if (!ReferenceEquals(sender, this))
{
return;
}
this.OnInitialized();
});
To call the message method I need to pass the dictionary as parameter (instead of the "this" reference).
MessagingCenter.Send(subscribedPages[pageName], keyPageName);
Instead of unsubscribing when you navigate TO a page,
unsubscribe when you navigate AWAY from the page. At that point your instance of 'this' is still the same 'this' you think it is.
I have a MVC.Net4 Application in which i have Longrunning backround operations which is why i use the System.Threading.Tasks.Task Class.
I start the Tasks after the User clicked a certain Button on the GUI, from that Task im going to use async methods from a intern API which i need to await. This is all working.
public ActionResult DoAsyncAction()
{
//ReturnValue that needs to be further populated by the async action in productive environment
var arv = new AsyncReturnValue
{
ProgressBar = new ProgressBar {Action = "SomeAction", User = "SomeUser"}
};
var t = new Task<AsyncReturnValue>(DoAction, arv);
//Add a Progressbar before Task starts so i can visualize the process on the view
HttpContext.GetSession().ProgressBars.Add(arv.ProgressBar);
//from my understanding this is similar to an event that gets triggered when my DoAction Method finished so i need to remove
//the progressbar there again since the process will be finished in that case
t.ContinueWith(DoActionkComplete);
t.Start();
//Returns the User to the Index Page while the Task is processing
return View("Index");
}
Now what i really want to do is visualizing the operation. I use jQuery Progressbars on the GUI and my Own ProgressBar Object in the Session for this. I have a List of ProgressBars on my Session and a PartialView strongly Typed to a List of those ProgressBars.
ProgressBar Class:
public class ProgressBar
{
public string Action { get; set; }
public string User { get; set; }
}
PartialView:
#using AsyncProj.Models
#model List<AsyncProj.Models.ProgressBar>
#{
ViewBag.Title = "ProgressPartial";
}
#{
var foo = (MySessionObject) HttpContext.Current.Session["__MySessionObject"];
foreach (var pb in foo.ProgressBars)
{
<div style="border: 1px solid black">
<p>#pb.Action</p>
<div id="progressbar">This will be turned into a ProgressBar via jQuery.</div >
</div>
}
}
And then the Object i have in my Session:
public class MySessionObject
{
public List<ProgressBar> ProgressBars { get; set; }
public string User { get; set; }}
Whenever i start a new Task i will add another ProgressBar to that List, which works just fine.
No where i get into Troubles is when i want to Remove the ProgressBars from Session again.
In the DoActionkComplete Method which i set in Task.ContinueWith() i want to Remove the ProgressBar corresponding to the finished action. I have the ProgressBar Ready there, its stored in my AsyncReturnValue Class which i have in the Task.Result at this point:
public class AsyncReturnValue
{
public ProgressBar ProgressBar { get; set; }
}
In this Method i would like to remove the Progressbar from the Session with
HttpContext.GetSession().ProgressBars.Remove(pbToRemove). But the problem with that im still operating on a different Thread so i have no valid HttpContext there and my SessionObject is null on that Thread.
This is what my DoActionComplete Method looks right now:
public void DoActionkComplete(Task<AsyncReturnValue> t)
{
//i set the user hardcode because its only a demo
DeleteProgress.Add(new ProgressBarDeleteObject {ProgressBar = t.Result.ProgressBar, User = "Asd123"});
}
I created a Workaround where i have a static List of Progressbars on my Controller. In the DoActionComplete Method i add the ProgressBars i want to delete to that List. I need to use polling (with jQuery $.get() and setinterval) in order to delete them.
I have a custom Class for the DeleteList on which i can set a Username so i know who is the Owner of that ProgressBar and only show it to him, else everyone would see it because its Static.
public class ProgressBarDeleteObject
{
public ProgressBar ProgressBar { get; set; }
public string User { get; set; }
}
Dont get me wrong, my workaround works just fine but i want to know the clean way. From what i know static Lists on Controllers could technically grow very big and slow the site down. Such Lists also lose its entries when the ApplicationPool restarts the Application.
So my Actual Question would be how can i access a HttpContext SessionObject from a different Thread like i'm using? And if its not possible, what would be the proper Way to achieve what i want?
So my Actual Question would be how can i access a HttpContext SessionObject from a different Thread like i'm using?
That's not possible.
And if its not possible, what would be the proper Way to achieve what i want?
First, let's back up to the original scenario. The problem is here:
I have a MVC.Net4 Application in which i have Longrunning backround operations which is why i use the System.Threading.Tasks.Task Class.
That's the wrong solution for that scenario. The proper solution is to have a reliable queue (Azure queue / MSMQ) with an independent background process (Azure webjob / Win32 service) doing the processing. This is a more reliable solution because any work you toss onto the ASP.NET thread pool may be lost (especially if you don't inform ASP.NET about that work).
Once you have this architecture set up, then you can use SignalR to communicate from your web server to your clients. SignalR will use polling if it has to, but it can also use more efficient methods (such as websockets).
You can specify the SynchroniztionContext that the ContinueWith task continues on and then you should be able to access the progress bars. Try changing your t.ContinueWith(DoActionkComplete); call to
t.ContinueWith(DoActionkComplete, TaskScheduler.FromCurrentSynchronizationContext());
If you are using .NET 4.5 you can rewrite your method with async\await
public async Task<ActionResult> DoAsyncAction()
{
//ReturnValue that needs to be further populated by the async action in productive environment
var arv = new AsyncReturnValue
{
ProgressBar = new ProgressBar {Action = "SomeAction", User = "SomeUser"}
};
//Add a Progressbar before Task starts so i can visualize the process on the view
HttpContext.GetSession().ProgressBars.Add(arv.ProgressBar);
var result = await Task.Run(DoAction());
//i set the user hardcode because its only a demo
DeleteProgress.Add(new ProgressBarDeleteObject {ProgressBar = result.ProgressBar, User = "Asd123"});
//Returns the User to the Index Page while the Task is processing
return View("Index");
}
And if you make the DoAction method async as well, you can remove the Task.Run part as that uses up a thread from the thread pool.
I have a list of items that goes to another page, That page is hooked up to a view model. In the constructor of this view model I have code that grabs data from the server for that particular item.
What I found is that when I hit the back button and choose another item fromt hat list and it goes to the other page the constructor does not get hit.
I think it is because the VM is now created and thinks it does not need a new one. I am wondering how do I force a cleanup so that a fresh one is always grabbed when I select from my list?
I faced the same issue, that's how i solved it.
Have a BaseView class, override OnNavigatedTo
protected override void OnNavigatedTo(NavigationEventArgs e)
{
base.OnNavigatedTo(e);
if (NavigatedToCommand != null && NavigatedToCommand.CanExecute(null))
NavigatedToCommand.Execute(null);
}
add DependencyProperty.
public static readonly DependencyProperty NavigatedToCommandProperty =
DependencyProperty.Register("NavigatedToCommand", typeof(ICommand), typeof(BaseView), null);
public ICommand NavigatedToCommand
{
get { return (ICommand)GetValue(NavigatedToCommandProperty); }
set { SetValue(NavigatedToCommandProperty, value); }
}
On the necessary pages, add to xaml (and, of course, inherit BaseView )
NavigatedToCommand="{Binding OnNavigatedToCommand}"
In the ViewModel, make command itself
public RelayCommand OnNavigatedToCommand
{ get { return new RelayCommand(OnNavigatedTo); } }
and implement method you want to call to update list
public async void OnNavigatedTo()
{
var result = await myDataService.UpdateMyList();
if (result.Status == OK)
MyList = result.List;
}
So, now, every time you navigate to page with list, inside of overriden OnNavigatedTo(), a NavigatedToCommand would be executed, which would execute OnNavigatedToCommand (which you set in xaml), which would call OnNavigatedTo, which would update your list.
A bit messy, but MVVM :)
EDIT: What about cleanings, they can be done in OnNavigatedFrom(), which works the same. Or OnNavigatingFrom(), which also can be useful in some cases.
I'm developing my first app and I'm trying to make it multilanguage.
Using AppHub example and some other link I created my resource files, fixed binding strings on my components and set a settings page.
First problem I had was that menu items and appbar buttons couldn't use localization strings (project complained when launched) so I have:
TextBlocks and other components binded with localized strings
Appbar buttons and items localized manually with a procedure loading localized strings
Now that I have my settings page, one item user can change is language.
Well, correct CultureInfo is selected according to user selection and then I use
Thread.CurrentThread.CurrentUICulture = Settings.Language;
When I press back button and return to main page, appbar items are localized correctly, while everything else is not.
The only workaround (that I really don't like, it's just to understand) is this:
public MainPage()
{
Thread.CurrentThread.CurrentUICulture = Settings.Language;
InitializeComponent();
// Everything else I need here
}
so I have to set language before components are created to make it work.
What's wrong? Which is the correct way to make a page refresh after changing language using binded strings?
I did not put a lot of code because I used basically the one provided in the link, but if you need more info I will edit my question.
I finally found a solution to automatically update my application components reacting to language change.
A good tutorial can be found here; briefly you must find a way to notify your app that localized resource is changed.
public class LocalizedStrings : ViewModelBase
{
private static AppResources localizedresources = new AppResources();
public AppResources LocalizedResources
{
get { return localizedresources; }
}
public void UpdateLanguage()
{
localizedresources = new AppResources();
RaisePropertyChanged(() => LocalizedResources);
}
public static LocalizedStrings LocalizedStringsResource
{
get
{
return Application.Current.Resources["LocalizedStrings"]
as LocalizedStrings;
}
}
}
With this when user change language, you should simply run
LocalizedStrings.LocalizedStringsResource.UpdateLanguage();
and the job is done.
Im working on a MVVM Windows phone app that displays weather info.
When the app loads up it opens MainPage.xaml. It makes a call the the service to get weather info and binds that data to the UI. Both Fahrenheit and Celcius info are returned but only one is displayed.
On the setting page, the user can select to view the temp in either Fahrenheit or Celcius.
The user can change this setting at any time and its stored in IsolatedStorageSettings.
The issue Im having is this:
when the user navigates to the Settings page and changes their preference for either Fahrenheit or Celcius, this change is not reflected on the main page.
This issue started me thinking about this in a broader context. I can see this being an issue in ANY MVVM app where the display depends on some setting in IsolatedStorage. Any time any setting in the IsoStore is updated, how does the ViewModels know this? When I navigate back in the NavigationStack from the settings page back to MainPage how can I force a rebind of the page?
The data in my model hasnt changed, only the data that I want to display has changed.
Am I missing something simple here?
Thanks in advance.
Alex
Probably you have code like this:
public double DisplayTemperature
{
get { return (IsCelsium) ? Celsium : Fahrenheit; }
}
And IsCelsium is:
public double IsCelsium
{
get { return (bool)settings["IsCelsium"]; }
set { settings["IsCelsium"] = value; }
}
So you need to add NotifyPropertyChanged event to notify UI to get new values from DisplayTemperature property:
public double IsCelsium
{
get { return (bool)settings["IsCelsium"]; }
set
{
settings["IsCelsium"] = value;
NotifyPropertyChanged("DisplayTemperature");
}
}
Take a look at Caliburn Micro. You could implement something similar or use CM itself. When using CM I don't even think about this stuff, CM makes it so simple.
When your ViewModel inherits from Screen there are life-cycle events that fire that you can override. For example, OnInitialize fires the very first time the ViewModel is Activated and OnActivate fires every time the VM is activated. There's also OnViewAttached and OnViewLoaded.
These methods are the perfect place to put logic to populate or re-populate data.
CM also has some special built in features for allowing one to easily tombstone a single property or an entire object graph into Iso or phone state.
ok, so Ive come up with a solution. Before I get to it, let me provide some background. The app that Im working on uses both MVVM Light and WP7Contrib. That being the case, I am using Funq for DI and the MVVMLight Toolkit. After I posted my initial question, I gave the question a bit more thought. I remembered a video that I watched a while back from MIX2011 called Deep Dive MVVM with Laurent Bugnion
http://channel9.msdn.com/Events/MIX/MIX11/OPN03
In it, he talks about just this problem (view models not living at the same time) on Windows Phone. The part in question starts around the 19 minute mark.
Anyway, after I remembered that and realized that the ViewModel locator is exposed in App.xaml, this became a trivial problem to solve. When the user changes the Fahrenheit/Celcius option on the setting page, I simply get a reference to the MainViewModel via the ViewModelLocator and reset the collection that is bound to the UI thus causing the bindings to update.
public bool AddOrUpdateValue(string Key, Object value)
{
bool valueChanged = false;
// If the key exists
if (settings.Contains(Key))
{
// If the value has changed
if (settings[Key] != value)
{
// Store the new value
settings[Key] = value;
valueChanged = true;
}
}
// Otherwise create the key.
else
{
settings.Add(Key, value);
valueChanged = true;
}
return valueChanged;
}
public bool ImperialSetting
{
get
{
return GetValueOrDefault<bool>(ImperialSettingKeyName, ImperialSettingDefault);
}
set
{
if (AddOrUpdateValue(ImperialSettingKeyName, value))
{
Save();
RaisePropertyChanged("ImperialSettingText");
var vml = new ViewModelLocator();
vml.MainViewModel.Cities = (App.Current as App).Cities;
}
}
}
It was a mistake on my part not to realize that I could get access to the viewModel via the ViewModelLocator. Hopefully this post saves someone else the time I burned on this issue.