I have an (non-virtualized) ItemsControl that binds its ItemsSource to a ObeservableCollection of ViewModel instances. Now once the large amount Model instances is loaded all the ViewModel complemnents needs to be added to that ObservableCollection. How can I add a large amount of ViewModels without making the UI Thread hang?
I suppose the UI Thread hangs because each time a new item is added the ItemsControl needs to update itself and does layout etc. over and over again.
Should I suspend the binding add all
items and then resume? If so, how?
Should I override the
ObservableCollection to implement an
AddRange so only 1 CollectionChanged
Event is fired for adding multiple
items? Or alternatively just replace
the whole collection?
Or is it better
to add each items separately and call
Dispatcher.Invoke for each item
separately? So I would unblock
frequently.
How do you handle large dynamic lists that can not be virtualized?
You can create a a class derived from ObservableCollection which allows you to temporarily suspend CollectionChanged events like this:
public class SuspendableObservableCollection : ObservableCollection
{
private bool suspended;
public bool Suspended
{
get
{
return this.suspended;
}
set
{
this.suspended = value;
OnCollectionChanged(new NotifyCollectionChangedEventArgs(
NotifyCollectionChangedAction.Reset));
}
}
protected override void OnCollectionChanged(
NotifyCollectionChangedEventArgs args)
{
if (!Suspended)
{
base.OnCollectionChanged(args);
}
}
}
<ItemsControl IsAsync="True" ... />
Related
I have seen this coding style:
public CustomTextCell()
{
}
public static readonly BindableProperty IsCheckedProperty =
BindableProperty.Create(
"IsChecked", typeof(bool), typeof(CustomTextCell),
defaultValue: false);
public bool IsChecked
{
get { return (bool)GetValue(IsCheckedProperty); }
set { SetValue(IsCheckedProperty, value); }
}
}
and this:
public class ExtViewCell : ViewCell
{
public bool NoTap { get; set; }
}
Can someone help explain the difference. Is one serving a different function from the other? In my case all I need is to pass to a custom renderer the value of NoTap. Should I code it like in the first or second example?
The second one is a POCO - a plain old C# object - that is relatively self-explanatory, but serves not much more purpose that holding data - and not that much in this case.
The first one is a bit more interesting, especially in the context of MVVM. SetValue does more than just setting the value, but will (in most cases) raise PropertyChanged event (see INotifyPropertyChanged), to notify subscribers that, well, a property has changed.
Now how does this relate to your custom renderer? You could implement the property in your view as a plain property - i.e. without notifications - and it might work (cannot tell, though, since I do not know your custom renderer) when setting IsChecked initially (and without binding). Anyway, imagine you'll update the value of IsChecked. You do so from your code and wonder, why this change is not reflected in your custom renderer. But how is your renderer supposed to know? Polling each and every property might be possible for smaller forms, but is a terrible waste of resources. (And Xamarin.Forms just does not work this way.) You'll page/view has to tell your custom renderer, that something has changed. INotifyPropertyChanged to the rescue. In your custom renderer you can subscribe to PropertyChanged event and react to IsChecked being changed, updating your native view.
I need to recreate new page instance on every page load (also when user pressed Back button).
So I overrided OnBackKeyPress method:
protected override void OnBackKeyPress(CancelEventArgs e)
{
base.OnBackKeyPress(e);
if (NavigationService.CanGoBack) {
e.Cancel = true;
var j = NavigationService.RemoveBackEntry();
NavigationService.Navigate(j.Source);
NavigationService.RemoveBackEntry();
}
}
The problem is that I can't handle case when user press back button to close CustomMessageBox dialog. How can I check it? Or is there any way to force recreation of page instance when going back through history state?
Why do you need to recreate the page instance? If you are simply trying to re-read the data to be displayed, why not put the data loading logic into OnNavigatedTo()?
Assuming that is what you are actually trying to achieve, try something like this...
public partial class MainPage : PhoneApplicationPage
{
// Constructor
public MainPage()
{
InitializeComponent();
// don't do your data loading here. This will only be called on page creation.
}
protected override void OnNavigatedTo(NavigationEventArgs e)
{
LoadData();
base.OnNavigatedTo(e);
}
MyViewModel model;
async void LoadData()
{
model = new MyViewModel();
await model.LoadDataAsync();
}
}
If you also have specific logic that you need to run on first construction of the page vs. on a back key navigation, check the NavigationMode property of the NavigationEventArgs object that gets passed to OnNavigatedTo.
if(e.NavigationMode == NavigationMode.New)
{
//do what you need to do specifically for a new page instance
}
if (e.NavigationMode == NavigationMode.Back)
{
// do anything specific for back navigation here.
}
Ha, in the near thread, i have opposite question :)
What about MessageBox - it depends, which one are you using. It can be custom message box, for example. Anyway, try to check MessageBox.IsOpened (or alternative for your MessageBox) in your OnBackKeyPress().
Another solution is to use OnNavigatedTo() of the page you want to be new each time.
Third solution: in case you works with Mvvm Light, add some unique id in ViewModel getter, like
public MyViewModel MyViewModel
{
get
{
return ServiceLocator.Current.GetInstance<MyViewModel>((++Uid).ToString());
}
}
This would force to recreate new ViewModel each time, so you'd have different instance of VM, so you would have another data on the View.
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 have a property and depending upon it's state (say A and B) I either show a usercontrol of animation or a image.
Now, if the property changes, I want to trigger the datatemplate selector again. On searching, I found that in WPF I could have used DataTemplate.Trigger but it's not available in WP.
So, my question is
Is their a way to trigger datatemplate selector so when property changes from state A to B, then appropriate usercontrol gets selected. If yes, then please give some example how to implement it.
Also, as there are only two states, if think I can use Converter to collapse the visibility. For basic if else situation, I will need to write two converters.( Can I somehow do it using one converter only?)
Here is the exact situation.
If state == A :
select userControl_A
else : select userControl_B
Also,
Will the animation usercontrol will effect the performance when it's in Collapsed state?
EDIT- Just realized, I can use parameter object to write just one converter.
You could implement a DataTemplateSelector like described here.
I use it and it works pretty well.
EDIT:
If you need to update the DataTemplate when the property changes, you should subscribe to the PropertyChanged event of the data object in the TemplateSelector and execute the SelectTemplate method again.
Here is the code sample:
public override DataTemplate SelectTemplate(object item, DependencyObject container)
{
City itemAux = item as City;
// Subscribe to the PropertyChanged event
itemAux.PropertyChanged += new System.ComponentModel.PropertyChangedEventHandler(itemAux_PropertyChanged);
return GetTemplate(itemAux, container);
}
private DataTemplate GetTemplate(City itemAux, DependencyObject container)
{
if (itemAux != null)
{
if (itemAux.Country == "Brazil")
return BrazilTemplate;
if (itemAux.Country == "USA")
return UsaTemplate;
if (itemAux.Country == "England")
return EnglandTemplate;
}
return base.SelectTemplate(itemAux, container);
}
void itemAux_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
{
// A property has changed, we need to reevaluate the template
this.ContentTemplate = GetTemplate(sender as City, this);
}
I'm loading pivot items based on a call a webservice call. Given that all I have is asynchronous calls available, how do I go about catching when it's finished?
My main reason is that I'd like to keep a loading dialog up while it's waiting for the callback. However, I'm loading in a viewmodel class, and obviously the loading bar is in the page class.
Honestly, if I could just know when one pivot item was loaded, that would be fine, however setting an event handler on loadedpivotitem never seems to trigger.
I assume you are databinding your View to your ViewModel. In that case all you need to do is create a bool property and set it to true while loading/awaiting the async call. You could do something like this:
private bool isSyncing;
public bool IsSynchronizing
{
get { return this.isSyncing; }
set
{
this.isSyncing = value;
this.RaisePropertyChanged(() => this.IsSynchronizing); //Use appropriate RaisePropertyChanged method for your MVVM implementation
}
}
Before starting the async call you would set IsSynchronizing = true. At the end of the eventhandler set IsSynchronizing = false;
From your view you can bind to this bool. For the loadingbar it could be like this:
<ProgressBar Visibility="{Binding IsSynchronizing, Converter={StaticResource booleanToVisibilityConverter}}" IsIndeterminate="{Binding IsSynchronizing}" Style="{StaticResource PerformanceProgressBar}" />
In your scenario you can use an inverted BooleanToVisibilityConverter to hide the pivot while it is still loading.
Hope this helps, let me know if you need more info on using the BooleanToVisibilityConverters
You would need to hook up an event handler similar to as shown in this block of code:
public void LoadData()
{
SampleDataServiceClient client = new SampleDataServiceClient();
client.GetDataCompleted += new EventHandler<GetDataCompletedEventArgs>(client_GetDataCompleted);
client.GetDataAsync();
}
void client_GetDataCompleted(object sender, GetDataCompletedEventArgs e)
{
this.DataContext = e.Result;
}