I have a property representing an actor
private Actor _actor;
public Actor Actor
{
get => _actor;
set
{
if (_actor != value) {
_actor = value;
OnPropertyChanged("Actor");
}
}
}
and a list, with a checkmark that depends on the state of Actor. When I click over the label the state of Actor shall change the checkmark
private async void OnSelectionAsync(object item)
{
Actor = item;
but I cannot see the changes in my ListView, why?
Edit 1:
in my list, i am binding the actor Text="{Binding Actor.id} to send my converter and to change the item check
<Label IsVisible="False" x:Name="dTd" Text="{Binding Actor.id}" />
<ListView ItemsSource="{Binding Actors}">
...
<Image Source="" IsVisible="{Binding id , Converter={StaticResource MyList}, ConverterParameter={x:Reference dTd}}"/>
As far as I can tell, you are using the text of your label to determine whether an actor is selected or not. Anyway, you are not binding to that text in any way, but you are using the Label itself as a binding parameter. I do not know for sure, but it seems to me as if "binding" the value this way does not work as intended by you, because change notifications are not subscribed to this way. Anyway, when you refresh that list, the bindings are refreshed and the converter is used with the new value. Since ConverterParameter is not a bindable property I doubt that there is any chance to use it this way.
What I'd do is to either extend your Actor class or create a new ActorViewModel class with the property
public bool IsSelected
{
get => isSelected;
set
{
if (value == isSelected)
{
return;
}
isSelected = value;
OnPropertyChanged(nameof(IsSelected));
}
}
From your ListView you can now bind to that property
<Image Source="..." IsVisible="{Binding IsSelected}"/>
All you have to do now is setting the property accordingly
private async void OnSelectionAsync(object item)
{
Actor = item as Actor;
foreach(var actor in Actors)
{
Actor.IsSelected = actor.id == Actor.id;
}
}
this way, the change should reflect in the ListView.
Related
I have the following situation:
I have an app, where I want to create notes using a special page for it. I have VM with ObservableCollection NoteItems with all the notes. When I want to create a new note, I add a new note to this ObservableCollection and pass this new note using BindingContext
var editingPage = new EditingPage();
editingPage.BindingContext = NoteItems[NoteItems.Count - 1].Text;
Application.Current.MainPage.Navigation.PushAsync(editingPage);
In the editing page I have an Entry field
<Entry
x:Name="EdtingPageEntryField"
Text="{Binding Text}"
Placeholder="Enter a note"
/>
, which is Binding his text with the Text parameter of a NoteItem.
The problem is that if I change the text in entry field, it does not automatically apply to a Text parameter of a NoteItem. That is why I want to pass this text when I close this EditingPage(go back to the MainPage). So the question is, how can I pass this Text parameter to a NoteItem element from NoteItems ObservableCollection, which is located in VM.
UPD. The value of NoteItem, which is located in NoteItems does not change
UPD2. I was wrong about the value of NoteItem, it has been changed, but the new value does not display on MainPage, that is why I used INotifyPropertyChanged, but it did not work.
Here is Note Item class
public class NoteItem
{
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
private string text;
public string Text
{
get { return text; }
set
{
if(text != value)
{
text = value;
NotifyPropertyChanged();
}
}
}
And MainPage.xaml:
<ContentPage.BindingContext>
<local:NoteItemViewModel/>
</ContentPage.BindingContext>
<FlexLayout>
<ListView ItemsSource="{Binding NoteItems}">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<Label Text="{Binding Text}"/>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</FlexLayout>
'''
in order to dynamically update the UI, your model must implement INotifyPropertyChanged
public class NoteItem : INotifyPropertyChanged
{
...
}
simply adding a PropertyChanged method is not the same as implementing INotifyPropertyChanged. You must add the interface to the class definition so that the binding mechanism knows that your class has implemented it
When binding to a picker, you can use ItemDisplayBinding to bind the displayed value, but I do not see a way to map each item to a selection value. Because of this, I'm having to write some very convoluted code to keep my pickers in sync with data source changes.
Original Model
// NOTE: this implements INPC, just abbreviated for clarity
public class DataModel
{
public ICollection<DataItem> Items;
pubilc DataItem SelectedItem;
}
Original Picker:
<Picker Title="Select Item..."
ItemsSource="{Binding Items}"
ItemDisplayBinding="{Binding Name}"
SelectedItem="{Binding Path=SelectedItem}"></Picker>
New Model
// NOTE: this implements INPC, just abbreviated for clarity
public class DataModel
{
public ICollection<DataItems> Items;
public ICollection<string> ItemNames;
public DataItem SelectedItem;
public string SelectedItemName;
public DataModel()
{
this.PropertyChanged += (s, e) =>
{
// I feel like I shouldn't have to do this...
if(StringComparer.Ordinal.Equals(e.PropertyName, nameof(Items)))
{
if(!String.IsNullOrWhitespace(this.SelectedItemName))
{
this.SelectedItem = this.Items.FirstOrDefault(x => StringComparer.Ordinal.Equals(x.Name, this.SelectedItemName));
if (this.SelectedItem == null) { this.SelectedItemName = null; }
}
}
}
}
New Picker:
<Picker Title="Select Item..."
ItemsSource="{Binding ItemNames}"
SelectedItem="{Binding Path=SelectedItemName}"></Picker>
I would like to be able to do something like this:
<Picker Title="Select Item..."
ItemsSource="{Binding Items}"
ItemDisplayBinding="{Binding Name}"
ItemValueBinding="{Binding Name}"
SelectedItem="{Binding Path=SelectedItemName}"></Picker>
I do not need a reference to the item, I need a property off of it. In this way, when the Items collection changes, it automatically reselects the correct item if it's still present. I find that I'm adding a second collection everywhere with just the properties I want to choose and doing all this mapping. Every other platform I've worked on, this is pretty straight forward, so I feel like I have to be missing something with Xamarin.Forms.
I think you don't need to do this.The SelectedItem property data binds to the SelectedItem(in your original model) property of the connected view model, which is of type DataItem. Therefore, when the user selects an item in the Picker, the SelectedItem property will be set to the selected DataItem object automatically.
You could test it in its SelectedIndexChanged event like:
<Picker Title="Select Item..."
ItemsSource="{Binding Items}"
ItemDisplayBinding="{Binding Name}"
SelectedItem="{Binding Path=SelectedItem}"
SelectedIndexChanged="Picker_SelectedIndexChanged">
</Picker>
private void Picker_SelectedIndexChanged(object sender, EventArgs e)
{
Picker picker = sender as Picker;
DataItem dt = picker.SelectedItem as DataItem ;
Console.WriteLine(dt.Name); // you will see when you select a item,the SelectedItem will be changed automatically
}
And i suggest you use ObservableCollection<Item> Items, so it will automatically update your Items when it changes.
I have a ListView which is bound to an ObservableCollection.
Is there a way to update a single cell whenever a property of a SomeModel item changed, without reloading the ListView by changing the ObservableCollection?
(Question is copied from https://forums.xamarin.com/discussion/40084/update-item-properties-in-a-listviews-observablecollection, as is my answer there.)
As I can see you are trying to use MVVM as a pattern for your Xamarin.Forms app. You are already using the ObservableCollection for displaying a list of the data. When a new item is added or removed from collection UI will be refreshed accordingly and that is because the ObserverbleCollection is implementing INotifyCollectionChanged.
What you want to achieve with this question is next behaviour, when you want to change the particular value for the item in the collection and update the UI the best and simplest way to achieve that is to implement INotifyPropertyChanged for a model of the item from your collection.
Bellow, I have a simple demo example on how to achieve that, your answer is working as I can see but I am sure this example would be nicer for you to use it.
I have simple Button with command and ListView which holds my collection data.
Here is my page, SimpleMvvmExamplePage.xaml:
<StackLayout>
<Button Text="Set status"
Command="{Binding SetStatusCommand}"
Margin="6"/>
<ListView ItemsSource="{Binding Cars}"
HasUnevenRows="True">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<StackLayout Orientation="Vertical"
Margin="8">
<Label Text="{Binding Name}"
FontAttributes="Bold" />
<StackLayout Orientation="Horizontal">
<Label Text="Seen?"
VerticalOptions="Center"/>
<CheckBox IsChecked="{Binding Seen}"
Margin="8,0,0,0"
VerticalOptions="Center"
IsEnabled="False" />
</StackLayout>
</StackLayout>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
</StackLayout>
The basic idea from this demo is to change the value of the property Seen and set value for the CheckBox when the user clicks on that Button above the ListView.
This is my Car.cs class.
public class Car : INotifyPropertyChanged
{
private string name;
public string Name
{
get { return name; }
set
{
name = value;
OnPropertyChanged();
}
}
private bool seen;
public bool Seen
{
get { return seen; }
set
{
seen = value;
OnPropertyChanged();
}
}
// Make base class for this logic, something like BindableBase
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
In the full demo example which is on my Github, I am using my BindableBase class where I handle raising the INotifyPropertyChanged when some property value is changed with this SetProperty method in the setter of the props.
You can find the implementation here: https://github.com/almirvuk/Theatrum/tree/master/Theatrum.Mobile/Theatrum.Mobile
The last thing to show is my ViewModel for this page, and inside of the ViewModel, I will change the value of Seen property to True for items in the collection when the user clicks on the Button above the ListView. Here is my SimpleMvvmExamplePageViewModel.cs
public class SimpleMvvmExamplePageViewModel
{
public ObservableCollection<Car> Cars { get; set; }
public ICommand SetStatusCommand { get; private set; }
public SimpleMvvmExamplePageViewModel()
{
// Set simple dummy data for our ObservableCollection of Cars
Cars = new ObservableCollection<Car>()
{
new Car()
{
Name = "Audi R8",
Seen = false
},
new Car()
{
Name = "BMW M5",
Seen = false
},
new Car()
{
Name = "Ferrari 430 Scuderia",
Seen = false
},
new Car()
{
Name = "Lamborghini Veneno",
Seen = false
},
new Car()
{
Name = "Mercedes-AMG GT R",
Seen = false
}
};
SetStatusCommand = new Command(SetStatus);
}
private void SetStatus()
{
Car selectedCar = Cars.Where(c => c.Seen == false)
.FirstOrDefault();
if (selectedCar != null)
{
// Change the value and update UI automatically
selectedCar.Seen = true;
}
}
}
This code will help us to achieve this kind of behaviour: When the user clicks on the Button we will change value of the property of the item from collection and UI will be refreshed, checkbox value will be checked.
The final result of this demo could be seen on this gif bellow.
P.S. I could combine this with ItemTapped event from ListView but I wanted to make this very simple so this example is like this.
Hope this was helpful for you, wishing you lots of luck with coding!
Any UI associated with a model item will be refreshed, if replace the item with itself, in the Observable Collection.
Details:
In ViewModel, given property:
public ObservableCollection<Item> Items { get; set; } = new ObservableCollection<Item>();
Where Item is your model class.
After adding some items (not shown), suppose you want to cause item "item" to refresh itself:
public void RefreshMe(Item item)
{
// Replace the item with itself.
Items[Items.IndexOf(item)] = item;
}
NOTE: The above code assumes "item" is known to be in "Items". If this is not known, test that IndexOf returns >= 0 before performing the replacement.
In my case, I had a DataTemplateSelector on the collection, and the item was changed in such a way that a different template was required. (Specifically, clicking on the item toggled it between collapsed view and expanded/detailed view, by the TemplateSelector reading an IsExpanded property from the model item.)
NOTE: tested with a CollectionView, but AFAIK will also work with the older ListView class.
Tested on iOS and Android.
Technical Note:
This replacement of an item presumably triggers a Replace NotifyCollectionChangedEvent, with newItems and oldItems both containing only item.
I have developed the xamarin forms project. I need to truncate the text as default then show the remaining text once the button clicked. Again the text need to be truncated when click the button. It is like "See more" & "See less" functionality. Please anyone suggest me how to achieve this. I have added the screenshot for further reference.
This is how it should look when expanded:
And this is how it should look like when collapsed:
First of all I think you have the carets wrong. The downward caret is usually the one for an expanded control and vice versa. At the end it's up to you, but you should stick to common UX idioms unless you have a very good reason not to.
If one line of preview text would suffice there is quite an easy solution for your issue: You can set the LineBreakMode of the label when your view is clicked:
XAML:
<?xml version="1.0" encoding="UTF-8"?>
<ContentView xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="Test_SO.ExpansibleControl">
<ContentView.GestureRecognizers>
<TapGestureRecognizer Command="{Binding ClickCommand}" />
</ContentView.GestureRecognizers>
<ContentView.Content>
<Grid RowSpacing="10" Padding="10">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"></RowDefinition>
</Grid.RowDefinitions>
<Label Text="{Binding Text}" LineBreakMode="{Binding LineBreakMode}" HorizontalOptions="StartAndExpand" />
<Label Grid.Column="1" Text="{Binding CaretCharacter}" HorizontalOptions="End" />
</Grid>
</ContentView.Content>
</ContentView>
Just a short outline of what I've done
Added a Grid to ExpansibleControl
Within the grid there is a Label for the text
The Text is bound to the Text property of a viewmodel (see below)
The LineBreakMode is bound to the LineBreakMode of a viewmodel
Furthermore a second Label is added for the caret
This would be an image in a real app, but for the purpose of demonstration a Label will suffice
The Text is bound to the CaretCharacter property of our viewmodel
Furthermore I've added a TapGestureRecognizer and bound its Command to the ClickCommand property of out viewmodel.
Viewmodel
As stated above, the views in our ExpansibleControl are bound to a viewmodel. Here is the respective code
public class ExpansibleControlViewModel : INotifyPropertyChanged
{
private char _caretCharacter;
private bool _isCollapsed;
private LineBreakMode _lineBreakMode;
private string _text;
public ExpansibleControlViewModel()
{
ClickCommand = new Command(OnClick);
}
public event PropertyChangedEventHandler PropertyChanged;
public char CaretCharacter
{
get => _caretCharacter;
private set
{
if (_caretCharacter == value)
{
return;
}
_caretCharacter = value;
OnPropertyChanged();
}
}
public Command ClickCommand { get; private set; }
public bool IsCollapsed
{
get => _isCollapsed;
set
{
_isCollapsed = value;
if (_isCollapsed)
{
LineBreakMode = LineBreakMode.TailTruncation;
CaretCharacter = '<';
}
else
{
LineBreakMode = LineBreakMode.WordWrap;
CaretCharacter = 'v';
}
}
}
public LineBreakMode LineBreakMode
{
get => _lineBreakMode;
private set
{
if (_lineBreakMode == value)
{
return;
}
_lineBreakMode = value;
OnPropertyChanged();
}
}
public string Text
{
get
{
return _text;
}
set
{
if (_text == value)
{
return;
}
_text = value;
OnPropertyChanged();
}
}
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
private void OnClick()
{
ToggleIsCollapsed();
}
private void ToggleIsCollapsed()
{
IsCollapsed = !IsCollapsed;
}
}
The viewmodel implements INotifyPropertyChanged to inform the view that properties have changed (otherwise the bound views would not be updated when a value changes). Due to CallerMemberNameAttribute the name of the property that calls OnPropertyChanged is automatically passed in the propertyName parameter when the optional parameter is omitted.
IsCollapsed:bool is rather a helper, to simplify the code and reveal intention. When the callback for ClickCommand is called, IsCollapsed is inverted. When it has changed, we set the LineBreakMode and the CaretCharacter to the according values. The setters of those in turn call OnPropertyChanged to raise the PropertyChanged event and the UI updates. By setting the LineBreakMode we "tell" the label either to truncate the text or to break text at words (there are other options).
What if you need a larger portion of text?
There are some options you can go with here.
You could for example fix the LineBreakMode to WordWrap and set the HeightRequest of the Label to a value >0 to show just a portion of the text and to -1 to show the whole text. In that case you could overlay a gradient from transparent to your background color, to fade out text from top to bottom, as a hint that there is more text to show.
Another option is to manipulate the text itself. You could have a writeable property OriginalText and a read-only property Text and just truncate the text "by hand".
Anyway, for both options you will have to take care that it scales across platforms, which may or may not be an easy endeavour.
With latest Xamarin Forms 3.3 there is a new attribute "MaxLines" for Labels, see https://blog.verslu.is/xamarin/xamarin-forms-xamarin/maxlines-label-xamarin-forms/ . This should help if you need to display on multiple lines.
Not sure if I formatted the question appropriately, please let me know if I did not. But I am trying to simply bind a background color to a value in my viewcell. I have this working, actually. The issue is when I update a value, I don't see the change in background color. The implementation is a bit complicated, but here's my code.
ViewCell (OnBindingContextChanged)
...
ShowReadOverlay.SetBinding(Xamarin.Forms.VisualElement.BackgroundColorProperty, new Xamarin.Forms.Binding(".", Xamarin.Forms.BindingMode.TwoWay, new XamarinMobile.Converters.GridCellBackgroundColorConverter(), null, null, null));
...
So essentially I just build my layout. I decided to only post the relevant code that sets the binding in my OnBindingContextChanged method. If anyone needs any other code I'd be glad to add it, just don't know if it's relevant. My ViewCell class is a simple class that just inherits ViewCell.
Here's my converter:
public class GridCellBackgroundColorConverter : Xamarin.Forms.IValueConverter
{
#region IValueConverter Members
public object Convert(object value, Type targetType, object parameter,
System.Globalization.CultureInfo culture)
{
try
{
var cell = (XamarinMobile.ViewModels.GridCellViewModel)value;
if(cell.HasRead)
{
//return with shadow
return Xamarin.Forms.Color.FromRgba(0,0,0,0.6);
} else
{
//return no shadow
return Xamarin.Forms.Color.FromRgba(0, 0, 0, 0.0);
}
} catch(System.Exception ex)
{
return null;
}
}
public object ConvertBack(object value, Type targetType, object parameter,
System.Globalization.CultureInfo culture)
{
throw new NotSupportedException();
}
#endregion
}
Simple. It works. Now here's the tricky part. So the grid I'm describing, is a listview that contains cells of stories. A user will click on an image which will take them to a story page. When the user is in the story page, they can either go back to the grid to go to another story, or swipe left or right and they can get to another story that way. When a user goes to a story page from our grid, then the cell gets updated fine. BUT if a user swipes to another story NOT from the grid, that's where my issue is. In my story page I have logic that iterates through the grid cells, and finds the story you're currently on (the story you swiped to) and sees if it's in the grid, if it's in the grid, I update the cell's HasRead property. As such:
//find the cell in the grid (if exists)
ViewModels.GridCellViewModel cell = App.GridCells.Where(x => x.StoryId == App.Story.StoryId).FirstOrDefault();
if (cell != null)
{
cell.HasRead = true;
}
This works but... it doesn't trigger the value converter to change the property. What am I doing wrong? How can I get it so that I can update a property, and have it trigger my value converter?
My guess is that you're converter isn't triggering because you've technically bound to the viewcell itself, not the HasRead property. When you set HasRead, it will (assuming it's implementing INotifyPropertyChanged) fire a PropertyChangedEvent which would trigger the binding and call the value converter. However, since your binding is pointing to the viewcell itself, it will only trigger when that changes and ignore property changes elsewhere on that object.
A possible solution is to change the binding to point to HasRead (instead of '.'), and update your converter to expect the boolean directly rather than taking in a viewcell. This would be a better practice for a converter regardless.
That said, this is not really following the mvvm pattern that is generally recommended for xamarin forms apps. My suggestion would be to have a viewmodel that has a property that holds your story models (wrapped in their own StoryViewModels if you need logic there) and make sure the VM and Model classes implement INotifyPropertyChanged. Make the VM the datacontext for the page, bind the list to your listview source and your listview itemtemplate contents will bind to each individual story. Each story can have a HasRead property that binds to the background color via your updated converter.
Like this:
<ContentPage
x:Class="Stack_Stories.MainPage"
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:Stack_Stories">
<ContentPage.BindingContext>
<local:StoriesViewModel x:Name="VM" />
</ContentPage.BindingContext>
<ContentPage.Resources>
<ResourceDictionary>
<local:StoryReadBackgroundColorConverter x:Key="HasReadColor" />
</ResourceDictionary>
</ContentPage.Resources>
<ListView ItemsSource="{Binding Stories}">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<Grid x:Name="StoryGrid" BackgroundColor="{Binding HasRead, Converter={StaticResource HasReadColor}}">
<Button Command="{Binding ToggleReadCommand}" Text="{Binding Name}" />
</Grid>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
public class StoryViewModel : INotifyPropertyChanged
{
private string _name = "";
public string Name
{
get { return _name; }
set { _name = value; OnPropertyChanged(); }
}
private bool _hasRead = false;
public bool HasRead
{
get { return _hasRead; }
set { _hasRead = value; OnPropertyChanged(); }
}
private Command _toggleRead;
public Command ToggleReadCommand
{
get
{
return _toggleRead
?? (_toggleRead = new Command(() => HasRead = !HasRead));
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
public class StoriesViewModel : INotifyPropertyChanged
{
public StoriesViewModel()
{
// add sample stories
Stories.Add(new StoryViewModel { Name = "First Story" });
Stories.Add(new StoryViewModel { Name = "Second Story", HasRead=true });
}
private ObservableCollection<StoryViewModel> _stories = new ObservableCollection<StoryViewModel>();
public ObservableCollection<StoryViewModel> Stories
{
get { return _stories; }
set { _stories = value; OnPropertyChanged(); }
}
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
public class StoryReadBackgroundColorConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (!(value is bool)) return null;
return (bool)value ? Color.FromRgba(0, 0, 0, 0.6) : Color.FromRgba(0, 0, 0, 0.0);
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}