There are quite a few articles about how to use the Xamarin SearchBar with MVVM pattern. The problem is that all these articles bind the PerformSearch event to an MVVM command.
I think these days, most users expect to see some results as they type and not once they tap "Search". So, I tried using the Xamarin Community Toolkit's EventToCommandBehavior to bind the TextChanged event to my MVVM command. It looks like this:
<SearchBar
x:Name="CompanySearch"
Placeholder="Search companies..."
PlaceholderColor="#777777"
BackgroundColor="{StaticResource PrimaryBackground}"
TextColor="{StaticResource SecondaryDark}">
<SearchBar.Behaviors>
<xct:EventToCommandBehavior
EventName="TextChanged"
Command="{Binding SearchTextChanged}"
CommandParameter="{Binding Text, Source={x:Reference CompanySearch}}"/>
</SearchBar.Behaviors>
</SearchBar>
This works nicely UNTIL the user taps "Cancel" on iOS. Then it crashes the app with the following error:
Invalid type for parameter. Expected Type System.String, but received
Type Xamarin.Forms.TextChangedEventArgs
That's because the method that handles the search in my model view is expecting string which looks like this:
async Task On_Search_Text_Changed(string keyword)
{
if(!string.IsNullOrEmpty(keyword) && keyword.Length > 3)
{
var data = await _myApi.CompanySearch(keyword);
if (data != null && data.Count > 0)
Suggestions = new ObservableRangeCollection<CompanyModel>(data);
}
}
Any idea how to handle the cancel event with EventToCommandBehavior? I'm also open to another approach for as long as I can perform the search on TextChanged and NOT on PerformSearch.
just wrap it in a try/catch to trap the exception, or try this
async Task On_Search_Text_Changed(object keyword)
{
if (keyword is string)
{
...
}
}
Related
My application viewModel responds to a user clicking a button to see test results:
private void AddDetailRows(List<QuizHistory> quizHistoryList)
{
quizDetails.Children.Clear();
quizDetails.Children.Add(AddData(quizHistoryList));
quizDetails.Children.Add(new LineTemplate());
}
Where quizDetails is the name of an element in the view.
But this doesn't work for me as the view model doesn't know what the view looks like and does not have access to the names of elements.
In a MVVM application, how is this problem solved?
You are completely right, that is not something that ViewModel is responsible of.
So, whatever you want to do with UI is not responsibility of the ViewModel.
If this is really the only option, then you can think of creating boolean properties in your VM and binding them to your views and then changing that boolean from false to true or vice versa on button click command which is binded to your VM.
To simplify it:
MyView.xaml
<StackLayout>
<Button Command="{Binding ShowHideQuizHistoryCommand}" ... />
<StackLayout x:Name="QuizHistory"
IsVisible={Binding ShowQuizHistory }>
//
</StackLayout>
</StackLayout>
MyViewModel.cs
private bool _showQuizHistory ;
public bool ShowQuizHistory
{
get { return _showQuizHistory ; }
set
{
_showQuizHistory = value;
OnPropertyChanged();
}
}
public ICommand ShowHideQuizHistoryCommand => new Command(() =>
{
ShowQuizHistory = !ShowQuizHistory;
});
So, this is just an example based on what you provided in question.
You can also use visual states, converters, triggers and behaviors in order to achieve this, but in my opinion this is the easiest way.
I have a form where a user can enter some data and save it to a database. I have been trying to add basic validation, making a user have to enter a value in every field. I want my save button to be unavailable until the user has entered some information in every form.
Here is my button:
<ToolbarItem Name="MenuItem1" Order="Primary" Text="Save" Priority="1" Command="{Binding SaveDataCommand}" IsEnabled="{Binding CanSaveData}"/>
I can access the isEnabled property in the XAML but where this gets infuriating is that I can't then reupdate this property. The button gets stuck in whatever state I tell it on load. I have checked my view model and it is returning a boolean which is correct (printing it to the console), it's just there is no way of updating the button state.
I even tried adding a new button that would force update the enabled state however this wouldn't work
<ToolbarItem x:Name="whyxamarinwhy" Name="MenuItem1" Order="Primary" Text="Save" Priority="1" Command="{Binding SaveEvent}" IsEnabled="{Binding CanCreateEvent}"/>
whyxamarinwhy.isenabled = true; //I cannot access this property because it doesn't exist.
Is the only way to implement this functionality going to be using a custom renderer?
MenuItem.IsEnabledProperty / For internal use by the Xamarin.Forms platform.
You should use the Command's CanExecute of the ToolbarItem to determine if the Execute method can be triggered.
Since you are already binding the command to SaveDataCommand, you can toggle the return of the CanExecute of that command to determine if the menu item button should be allowed to trigger the attached command.
You did not post your ViewModel, but in a inner-class ICommand implementation, something as simple as this works:
static bool SaveDataCommandCanExecute = true;
class SaveDataCommand : ICommand
{
public event EventHandler CanExecuteChanged;
public bool CanExecute(object parameter)
{
// your code needs to toggle SaveDataCommandCanExecute to determine if the Execute method can be triggered
return SaveDataCommandCanExecute;
}
public void Execute(object parameter)
{
// Do something
}
}
I have a really odd problem with variable scopes. A Listview named "TodoListView" is defined via xaml, and it's ItemSource populated from a SQListe database. Works. Inside the ListView I have a ViewCell to display the data row-wise.
<ContentPage ... x:Class="JanitorPro.MainPage" ...>
<StackLayout>
<ListView x:Name="TodoListView" Margin="20" ItemSelected="OnListItemSelected">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<StackLayout Orientation="Horizontal" HorizontalOptions="FillAndExpand">
<Label Text="{Binding name}" VerticalTextAlignment="Center" HorizontalOptions="StartAndExpand" />
<Switch HorizontalOptions="End" IsToggled="{Binding done}" Toggled="DoneSwitchToggled"/>
</StackLayout>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</StackLayout>
The codebehind looks like this (some irrelevant portions removed):
public partial class MainPage : ContentPage
{
public MainPage()
{
InitializeComponent();
}
protected override async void OnAppearing()
{
base.OnAppearing();
// load Database
TodoListView.ItemsSource = await App.TodoDatabase.GetItemsAsync("SELECT * FROM [TodoItem]");
}
async void OnReloadButtonClicked(object sender, EventArgs e)
{
Debug.WriteLine("Reload Button Click");
TodoListView.ItemsSource = await App.TodoDatabase.GetItemsAsync("SELECT * FROM [TodoItem]");
Debug.WriteLine("Reload done.");
}
async void OnListItemSelected(object sender, SelectedItemChangedEventArgs e)
{
if (e.SelectedItem != null)
{
await Navigation.PushAsync(new TodoItemPage
{
BindingContext = e.SelectedItem as TodoItem
});
}
}
private void DoneSwitchToggled(object sender, ToggledEventArgs e)
{
// TodoItem i = null;
TodoItem i = TodoListView.SelectedItem;
if (i != null)
{
Debug.WriteLine("Toggle: {0}", i.id);
}
}
}
}
The oddity has two stages. Before I inserted the DoneSwitchToggled event handler, every occurrance of TodoListView.ItemsSource got a red underline under TodoListView and a hint that "The name TodoListView does not exist in the current context". OK, I thought that VS was not smart enough to find a definition in the xaml file, because, despite of the warning, the program compiled and ran fine. TodoListView gets initialized and does correctly display the rows of the underlying database, so it does clearly exist at runtime.
Things went wild when I added the DoneSwitchToggled event handler to both XAML and the codebehind. All the sudden the program won't compile any longer but bail out with a CS0103 error "The name "TodoListView" does not exist in the current context". The error appears three times, with the line numbers pointing to the other occurrances of TodoListView in onAppearing() and OnReloadButtonClicked(). Huh? How the heck can the addition of a variable reference in an event handler render that variable invalid in completely different methods? OK, there was something fishy with the variable before (still don't know what ...), but it worked. Now it doesn't any more, whch doesn't make any sense for me. Furthermore, if I comment out the offending line in the DoneSwitchToggled event handler, and insert a dummy definition for i, like so:
TodoItem i = null;
// TodoItem i = TodoListView.SelectedItem;
everything is like before, VS still underlines the other appearances of TodoListView, but now the program builds and runs ok again.
Anyone who can explain this effect, and show me how correct my code? I think the objective is clear: DoneSwitchToggled is supposed to write back the switch value into the database (and do some other processing not shown in my stripped down sample), and though the "sender" object is correctly set to reference my button, I found no way to access the underlying data binding, since ToggledEventArgs unfortunately does seem to only pass the switch position "true" or "false", but - unlike the OnListItemSelected event handler - not pass any reference to the bound row through the second argument. So my idea was to use ListView.SelectedItem for this purpose.
Finally I figured it out myself. This seems to be a glitch in VS 2017. There is nothing wrong with TodoListView, so error CS0103 is misleading nonsense.
What VS really means is an error CS0266. TodoListView is defined by a generic list
List<TodoItem>
to access SelectedItem i need to typecast it:
TodoItem i = (TodoItem)TodoListView.SelectedItem;
This added, all errors are gone, app code builds OK.
Btw, unfortunately this approach to get at the item where the Switch has been flipped has proven not working, TodoListView does always return null for SelectedItem, seems that the ListView control doesn't see the button press. Need to find a different way to find the list item beneath the switch to get at the bound row id.
I am trying to figure out how the best way to use AutoCompleteBox with MVVM Light.
public ICommand AutoComplete
{
get
{
return new RelayCommand<KeyEventArgs>(e =>
{
var txtBox = e.OriginalSource as TextBox;
if (e.Key == Key.Unknown)
{
return;
}
string autoComplete = txtBox.Text + e.Key;
if (autoComplete.Length >= 3)
{
RestClient c = new RestClient("http://localhost:3333/api/store");
RestRequest r = new RestRequest("/GetStoreNames",Method.GET);
r.AddParameter("Name", autoComplete);
r.AddParameter("Latitude", "49");
r.AddParameter("Longitude", "49");
var d = c.BuildUri(r);
c.ExecuteAsync(r, response2 =>
{
var content = response2.Content;
});
}
});
}
}
<i:Interaction.Triggers>
<i:EventTrigger EventName="KeyUp">
<GalaSoft_MvvmLight_Command:EventToCommand Command="{Binding AutoComplete, Mode=OneWay}" PassEventArgsToCommand="True"/>
</i:EventTrigger>
</i:Interaction.Triggers>
<toolkit:AutoCompleteBox x:Name="acbStore" Margin="154,196,29,0" VerticalAlignment="Top" RenderTransformOrigin="0.6,0.083" Height="162" MinimumPopulateDelay="500"/>
I did the above but there is a couple problems.
Once I get the results back how do I show them in the auto complete area?
How can I delay it from doing to many requests at once? As you can see I don't want to hit the server before 3 characters are entered but after that it is fair game. I am kinda worried that like 20 requests will be done to the server before the 1st request makes it back leading to wasted bandwidth.
I am assuming you are using KeyDown event or similar? That is not the way you want to do it. Instead, bind the AutoCompleteBox Populating event and set the MinimumPrefixLength on your AutoCompleteBox to 3 so thatPopulating is fired only when you have 3+ characters. To show the list retrieved in your control, the list need to be bound to ItemsSource property then a method needs to be called, PopulateComeplte().
You can see my answer here on a similar Question.
However, it is not MVVM friendly since you need to call a method on your AutoCompleteBox to trigger the control to show the list from your webservice. Take a look at this article for a MVVM-friendly approach, scroll down to "Bonus: MVVM-friendly asynchronous filtering" section.
Scenario: I start out in my app's main page. I navigate to sub-page A, change a value, hit the back button and the bound TextBlock in the main page doesn't change. If I navigate to sub-page B, a TextBlock using that same binding changes. Likewise, if I go to page A again I see the changed value. If I exit the app, the new value shows up on the main page. It's just when using the back button that a refresh doesn't get triggered.
I've got all my INotifyPropertyChanged stuff working. Like I said, the binding works in every scenario besides navigating back to the main page. How do I send a message or otherwise trigger a refresh of the bindings on that page? Thanks!
Edit:
Based on the accepted answer from willmel, here's what I did:
My MainPage.xaml file has this markup:
<TextBlock Text="{Binding Title, Mode=OneWay}" />
My MainViewModel.cs file has this:
public string Title
{
get { return ProfileModel.Instance.DescriptionProfile.Title; }
}
And I added this to the MainViewModel constructor:
Messenger.Default.Register<PropertyChangedMessage<string>>(this,
(action) => DispatcherHelper.CheckBeginInvokeOnUI(
() => RaisePropertyChanged("Title")));
In another view I have the following markup:
<TextBox Grid.Row="1" Width="250" Height="100" Text="{Binding TitleEdit, Mode=TwoWay}" />
In its view model I use this when getting/setting a string:
public string TitleEdit
{
get { return ProfileModel.Instance.DescriptionProfile.Title; }
set
{
if (ProfileModel.Instance.DescriptionProfile.Title == value) return;
string oldValue = ProfileModel.Instance.DescriptionProfile.Title;
ProfileModel.Instance.DescriptionProfile.Title = value;
RaisePropertyChanged("Title", oldValue, value, true);
}
}
In your view model you want to be modified if a child page changes a property. (note here, the property is of type bool, but could be anything)
Messenger.Default.Register<PropertyChangedMessage<bool>>(this,
(action) => DispatcherHelper.CheckBeginInvokeOnUI(
() =>
{
MessageBox.Show(action.newValue.ToString());
//do what you want here (i.e. RaisePropertyChanged on a value they share)
}));
When you use RaisePropertyChanged in the child class, use the broadcasting overload.
RaisePropertyChanged("Preference", oldValue, value, true);
Finally, note that to use DispatcherHelper, you need to Add the following to your App constructor (App.xaml.cs)
DispatcherHelper.Initialize();