So I've been wanting to bind the items of two pickers in Xamarin.Forms to my ViewModel. I have mainly used binding for textfields, where I just write something like:
<Label Text="{Binding CurrentDate}" />
and simply by setting the binding context, defining a property in the viewmodel
public System.DateTime CurrentDate{
get { return currentDate; }
set { currentDate = value; PropertyChanged(this, new PropertyChangedEventArgs("CurrentDate")); }
}
I am done binding. Now I have two pickers. The pickers represent a map/dictionary. Dictionary>. "A" is mapped to {"1","2"} and "B" is mapped to {"4","5"}. The first picker should show "A" and "B" as options. The second one should display the values associated with the chosen value from the first picker.
So there are two questions. 1) How do I bind a picker? 2) How do I bind a picker that has data that depends on another pickers selection?
I tried
<Picker Items="{Binding ItemsA}"></Picker>
With a matching property
public List<string> ItemsA
{
get { return itemsA;}
set { itemsA = value;PropertyChanged(this, new PropertyChangedEventArgs("ItemsA")); }
}
Am I missing out on something here? Help would be appreciated.
This functionality is unfortunately missing from the standard component, but relatively easy to add as detailed at https://forums.xamarin.com/discussion/30801/xamarin-forms-bindable-picker. Using this derived component you will then be able to bind ItemsSource and SelectedItem properties. It's also relatively easy to add WPF-like DisplayMemberPath and ValueMemberPath properties if required.
Looks like this functionality is built into the regular Xamarin.Forms Picker now. It is currently in the pre-release NuGet package for version 2.3.4-pre1, but should be in the stable 2.3.4+ versions once it is released. Instead of binding to Items you bind to ItemsSource and SelectedItem.
Related
I'm adding a slider into my xamarin app, but im using view models to capture values. How can I move the slider on the GUI and show the value incrementing up and down and then capture its value?
or is there a way I can include the "MySlider_ValueChanged" event and apply it to the view model page rather than the content page code behind file?
is there a way I can include the "MySlider_ValueChanged" event and apply it to the view model page rather than the content page code behind file?
You have to find a BindableProperty which could be bound inside view model(ICommand) ,it should do the same work as the event ValueChanged .
For exmaple
We could handle Tapped event of TapGestureRecognizer in page code behind , we also could create new ICommand inside view model , and bind it with Command of TapGestureRecognizer .
However, Slider does not have command whose function is detecting the value changed , there is only DragStartedCommand and DragCompletedCommand ,
so the only solution is to trigger your method inside the setter method of Value .
Xaml
<Slider Value="{Binding CurrentProgress}" />
View Model
public double CurrentProgress
{
get { return currentProgress; }
set
{
currentProgress = value;
NotifyPropertyChanged("CurrentProgress");
YourMethod(value);
}
}
Refer https://stackoverflow.com/a/25141043/8187800.
Model:
public class Question : INotifyPropertyChanged
{
private float? _answer;
public float? Answer
{
get => _answer;
set
{
_answer = value;
NotifyPropertyChanged();
}
}
protected void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
public event PropertyChangedEventHandler PropertyChanged;
}
View model:
public class QuestionViewModel
{
private ObservableCollection<Question> _questions;
public ObservableCollection<Question> Questions
{
get => _questions;
set
{
if (_questions != value)
{
_questions = value;
}
}
}
}
XAML:
<ListView x:Name="ListViewQuestions" SelectionMode="Single" HasUnevenRows="True" HeightRequest="250" VerticalOptions="FillAndExpand">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<Entry x:Name="EntryAnswer" Text="{Binding Answer,Mode=TwoWay}" Keyboard="Numeric" FontSize="Medium" VerticalOptions="End"
HorizontalOptions="FillAndExpand" Grid.Row="0" Grid.Column="1" >
<Entry.Behaviors>
<behaviors:EntryMaxValueBehavior MaxValue="{Binding MaxVal}" BindingContext="{Binding BindingContext, Source={x:Reference EntryAnswer}}" />
<behaviors:EntryMinValueBehavior MinValue="{Binding MinVal}" BindingContext="{Binding BindingContext, Source={x:Reference EntryAnswer}}" />
</Entry.Behaviors>
</Entry>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
In my page OnAppearing method, I set the ListViewQuestions like this:
var questions = await DataStore.GetQuestions(_inspection.Id);
var questionsViewModel = new QuestionViewModel { Questions = new ObservableCollection<Question>(questions) };
ListViewQuestions.ItemsSource = null;
ListViewQuestions.ItemsSource = questionsViewModel.Questions;
However, when values are entered into EntryAnswer, the setter in the Question model is not called, as I would expect. I thought that maybe this was because the BindingContext for the ListView needed to be set, so I set it like this:
ListViewQuestions.BindingContext = questionsViewModel;
However, the setter in the Question model is still not called. I also tried implementing INotifyPropertyChanged in the QuestionViewModel, but still no joy. I checked that the ObservableCollection in the View Model is set correctly, with actual data, and it is. Can anyone spot what might be going wrong here?
Edit 1: I also tried not setting the ItemSource, but only setting the ListViewQuestions.BindingContext to the view model, but then the ListView was not being populated with any data.
Here is how this works together.
The BindingContext is the object that will be the scope for whatever bindings that are in the page or it's children, unless you specify a different context for a certain child object, but let's not overcomplicate things for now.
This means, that when you have set the BindingContext, all Bindings will now start looking into the object referenced in the BindingContext. In your case, you set the BindingContext to an instance of QuestionViewModel.
You want your ListView, to get its items from the QuestionViewModel.Questions property. So, you set a binding like this:
<ListView x:Name="ListViewQuestions" ItemsSource="{Binding Questions}" ...>.
Questions needs to be a public property in the BindingContext, in our case QuestionViewModel. You got this right already.
Now, whenever you assign something to Questions this should also propagate to your ListView because of the binding.
Inside your ListView you are using a ViewCell, now note, that the scope does change here. Each cell represents an instance of an object inside the ItemsSource. In our case, each cell will hold a Question. You are using this:
<Entry x:Name="EntryAnswer" Text="{Binding Answer,Mode=TwoWay}" ...>
This means Answer needs to be a public property inside Question. You got this right already.
When you implement it like this, basically the only thing you do is fill your view model and assign that to the BindingContext of your page. If you are using an MVVM framework, this might happen automatically.
At some point, you might run into some trouble that the UI doesn't update and you will have to implement the INotifyPropertyChanged interface. Have a close look at what object doesn't update on screen and implement the interface on that object along with the needed plumbing, but from what I can see in this code, this isn't needed right now. And besides, you have implemented it the right way in your Question right now.
I hope this makes sense! It's a bit hard to wrap your head around the first time, but once you get the swing of it, it is pretty easy!
In your Answer Setter try:
set
{
float? temp = null;
if(float.TryParse(value, out temp)
{
_answer = temp;
NotifyPropertyChanged("Answer");
}
}
It seems like for this to work though your setter would have to be called, and you indicate that it is not, so I think it must be the min, max binding where this is kicking out the error. For now perhaps get rid of that and see if the setter will get called.
In WPF using a converter is typical and I think will work with the Xamarin as well. See this for a good example of how to implement IValueConverter.
I am trying to set the title of my view dynamically with no success so far.
I am trying something like this:
<div data-role="view"
id="mt-details-view"
data-title="#= pageTitle #" <---- this one
data-layout="mt-main-layout"
data-init="X.details.onInit"
data-before-show="X.details.beforeShow"
data-show="X.details.onShow"
data-model="X.details.viewModel"
data-use-native-scrolling="true">
I tried using a function, tried setting a viewModel variable, tried passing the title from the view.params, tried also to set the title on the onShow function like that:
function onShow(e) {
X.debug.dbg2(e.view.id, "onShow");
viewModel.setViewParams(e.view.params);
e.view.title = e.view.params.pageTitle;
e.view.options.title = e.view.params.pageTitle;
fetchSomeDetails();
}
nothing works.
Enlighten me please!
Here is one approach you can try. After declaring your viewmodel, bind a function to its 'set' method. Within here, check whether the field being set is the page title property on the viewmodel. If it is, find the dom element holding the title text and set its html to the value being 'set':
X.details.viewModel.bind("set", function(e) {
if (e.field == "pageTitle") {
$("#mt-details-view [data-role='view-title']").html(e.value);
}
})
Whenever that property is changed on the viewmodel, you will now see it reflected on the page. However there is still the issue of setting the value in the UI initially. You can do this in your onShow function which of course happens after the view is rendered and all the dom elements have been created:
function onShow(e) {
var temp = viewModel.pageTitle;
viewModel.set("pageTitle", null);
viewModel.set("pageTitle", temp);
}
That will force the 'set' method on the viewmodel to run and the UI should then update.
As you can see in this plunkr (https://plnkr.co/edit/3EDk5xxSLRolv2t9br84?p=preview) I have two selects: one in the main component behaving as usual, and one in a custom component, inheriting the ngModel settings.
The following code links the innerNgModel to the component ngModel.
ngAfterViewInit() {
//First set the valueAccessor of the outerNgModel
this.ngModel.valueAccessor = this.innerNgModel.valueAccessor;
//Set the innerNgModel to the outerNgModel
//This will copy all properties like validators, change-events etc.
this.innerNgModel = this.ngModel;
}
It works, since the name property is updated by both selects.
However when it first loads the second select has no selection.
I guess I'm missing something, a way to initialize the innerNgModel with the initial value.
This is a weird situation to do something like this, but I believe to get this working they need to implement another life-cycle hook. AfterModelSet or something like that :)
Anyways, you can solve this with a simple setTimeout and a setValue:
ngAfterViewInit() {
this.ngModel.valueAccessor = this.innerNgModel.valueAccessor;
this.innerNgModel = this.ngModel;
setTimeout(() => {
this.innerNgModel.control.setValue(this.ngModel.model);
})
}
plunkr
I am using the excellent Mvvmcross and Ninja Coder for Mvvmcross for building a cross platform app. For my windows store app I have created a view and a view model using Ninja coder. I have also created a UserControl which will be referenced in the view. Hence I need to bind the same viewmodel to the User control also. I have been trying to set the Data context of the user control to the singleton instance of viewmodel. I have set the data context of the user control like below.
public sealed partial class SearchResultsGridViewControl : UserControl
{
private SearchresultsViewModel _viewModel;
public SearchResultsGridViewControl()
{
this.InitializeComponent();
_viewModel = Mvx.IocConstruct<SearchresultsViewModel>();
this.DataContext = _viewModel;
}
}
But when I refer this User Control in my main view, it throws an error in XAML saying "Object Reference not set to an instance of an object. Cannot create an instance of SearchResultsGridViewControl".
This is my viewmodel:
public class SearchresultsViewModel : BaseViewModel
{
private ISearchResultsService _searchResultsService;
public SearchresultsViewModel(ISearchResultsService searchResultsService)
{
_searchResultsService = searchResultsService;
var items = _searchResultsService.DisplaySearchResults();
SchoolDetails = new ObservableCollection<School>(items);
}
private ObservableCollection<School> _schoolDetails;
public ObservableCollection<School> SchoolDetails
{
get { return _schoolDetails; }
set
{
_schoolDetails = value;
RaisePropertyChanged(() => SchoolDetails);
}
}
public ICommand RefineCommand
{
get
{
refineCommand = refineCommand ?? new MvxCommand(FilterSearchResultsBasedOnRefine);
return refineCommand;
}
}
public void FilterSearchResultsBasedOnRefine()
{
SchoolDetails = new ObservableCollection<School>(_searchResultsService.FilterSchoolsBasedOnRefine(MidDayMeals, PlayGround, DigitalClassroom, DayBoarding, TransportationFacility));
}
}
The grid view in my usercontrol is getting populated when it loads for the first time. But when RefineCommand is called to update the collection from the main view, the grid view in usercontrol is not getting updated. And I am guessing its because of that error earlier in setting the data context of the user control to view model. Please let me know what could be going wrong. I have been banging my head about it for days.
I've been using MVVMCross with Windows Store fairly recently. Without looking back at my code, I'm pretty sure that the Datacontext will inherit from it's parent unless overridden.
So as long as the MvxPage that you have presented has a viewmodel, any user control that you add to it, either in XAML or in code-behind, should share the same data context. If you are looking at doing some MVVMCross data-binding from the User Control, you should probably make sure your User Control implements IMvxStoreView, and ensure that the ViewModel property is set to the value of DataContext.
Hope that help.
Cheers,
Tristan
I think your first problem "Object Reference not set to an instance of an object" is a design time only issue - because you are trying to set the viewmodel using Mvx. at design time. You can workaround this issue if you want to by using design time viewmodels and possibly also by using one of the design time helpers (see https://github.com/MvvmCross/MvvmCross/blob/v3.1/CrossCore/Cirrious.CrossCore.Wpf/Platform/MvxDesignTimeHelper.cs).
I've no idea what your second problem is "The grid view in my usercontrol is getting populated when it loads for the first time. But when RefineCommand is called to update the collection from the main view, the grid view in usercontrol is not getting updated" - this sounds like an issue either in your xaml or in the results returned from FilterSearchResultsBasedOnRefine. From the current level of detail, I can't see what it is. My "gut feeling" is that the problem won't be MvvmCross specific - it'll just be a general Mvvm/data-binding issue.