How can I pass a command to a template and have it execute in my back end code and pass the parameter? - xamarin

Question

I have this template:
<?xml version="1.0" encoding="utf-8"?>
<Grid Padding="20,0" xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:Japanese;assembly=Japanese"
x:Class="Japanese.Templates.DataGridTemplate"
x:Name="this" HeightRequest="49" Margin="0">
<Grid.GestureRecognizers>
<TapGestureRecognizer
Command="{Binding TapCommand, Source={x:Reference this}}"
CommandParameter="1"
NumberOfTapsRequired="1" />
</Grid.GestureRecognizers>
<Label Grid.Column="0" Text="{Binding Test" />
</Grid>
Behind this I have:
public partial class DataGridTemplate : Grid
{
public DataGridTemplate()
{
InitializeComponent();
}
public static readonly BindableProperty TapCommandProperty =
BindableProperty.Create(
"Command",
typeof(ICommand),
typeof(DataGridTemplate),
null);
public ICommand TapCommand
{
get { return (ICommand)GetValue(TapCommandProperty); }
set { SetValue(TapCommandProperty, value); }
}
}
and I am trying to call the template like this in file: Settings.xaml.cs
<template:DataGridTemplate TapCommand="openCFSPage" />
hoping that it will call my method here in file: Settings.cs
void openCFSPage(object sender, EventArgs e)
{
Navigation.PushAsync(new CFSPage());
}
The code compiles but when I click on the grid it doesn't call the openCFSPage method.
1) Does anyone have an idea what might be wrong?
2) Also is there a way that I can add a parameter to the template and then have that parameter passed to my method in the CS back end code?
Note that I would like to avoid adding a view model if possible. The application is small and I'd like to just have the code I need in the CS code of the page that calls the template.

Answer 1

Please note that the simplest way to implement this would be through MVVM (i.e. a view-model), but if you want to side-step this option (as you mentioned in the question) then you can use one of the following options
Option1 : Wrap delegate into command object
If you look at it from the perspective of a XAML parser, you are technically trying to assign a delegate to a property of type ICommand. One way to avoid the type mismatch would be to wrap the delegate inside a command-property in the page's code-behind.
Code-behind [Settings.xaml.cs]
ICommand _openCFSPageCmd;
public ICommand OpenCFSPageCommand {
get {
return _openCFSPageCmd ?? (_openCFSPageCmd = new Command(OpenCFSPage));
}
}
void OpenCFSPage(object param)
{
Console.WriteLine($"Control was tapped with parameter: {param}");
}
XAML [Settings.xaml]
<!-- assuming that you have added x:Name="_parent" in root tag -->
<local:DataGridView TapCommand="{Binding OpenCFSPageCommand, Source={x:Reference _parent}}" />
Option2 : Custom markup-extension
Another option (a bit less mainstream) is to create a markup-extension that wraps the delegate into a command object.
[ContentProperty("Handler")]
public class ToCommandExtension : IMarkupExtension
{
public string Handler { get; set; }
public object Source { get; set; }
public object ProvideValue(IServiceProvider serviceProvider)
{
if (serviceProvider == null)
throw new ArgumentNullException(nameof(serviceProvider));
var lineInfo = (serviceProvider?.GetService(typeof(IXmlLineInfoProvider)) as IXmlLineInfoProvider)?.XmlLineInfo ?? new XmlLineInfo();
object rootObj = Source;
if (rootObj == null)
{
var rootProvider = serviceProvider.GetService<IRootObjectProvider>();
if (rootProvider != null)
rootObj = rootProvider.RootObject;
}
if(rootObj == null)
{
var valueProvider = serviceProvider.GetService<IProvideValueTarget>();
if (valueProvider == null)
throw new ArgumentException("serviceProvider does not provide an IProvideValueTarget");
//we assume valueProvider also implements IProvideParentValues
var propInfo = valueProvider.GetType()
.GetProperty("Xamarin.Forms.Xaml.IProvideParentValues.ParentObjects",
BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
if(propInfo == null)
throw new ArgumentException("valueProvider does not provide an ParentObjects");
var parentObjects = propInfo.GetValue(valueProvider) as IEnumerable<object>;
rootObj = parentObjects?.LastOrDefault();
}
if(rootObj != null)
{
var delegateInfo = rootObj.GetType().GetMethod(Handler,
BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public);
if(delegateInfo != null)
{
var handler = Delegate.CreateDelegate(typeof(Action<object>), rootObj, delegateInfo) as Action<object>;
return new Command((param) => handler(param));
}
}
throw new XamlParseException($"Can not find the delegate referenced by `{Handler}` on `{Source?.GetType()}`", lineInfo);
}
}
Sample usage
<local:DataGridView TapCommand="{local:ToCommand OpenCFSPage}" />

Answer 2

You have 2 options depending on the the use case :
FYI, there's no way to call another method directly from the view (its a bad design pattern to do so)
Using Event Aggregator :
Create interface
public interface IEventAggregator
{
TEventType GetEvent<TEventType>() where TEventType : EventBase, new();
}
All you have to do is call it from you TapCommand
_eventAggregator.GetEvent<ItemSelectedEvent>().Publish(_selectedItem);
Then in your Settings.cs you can Create a method that can receive the data
this.DataContext = new ListViewModel(ApplicationService.Instance.EventAggregator);
Inheritance and Polymorphism / Making openCFSPage a service :
Creating a interface / service that links both models
public interface IOpenCFSPage
{
Task OpenPage();
}
and a method :
public class OpenCFSPage : IOpenCFSPage
{
private INavigationService _navigationService;
public OpenCFSPage(INavigationService navigationService){
_navigationService = navigationService;
}
public async Task OpenPage()
{
await _navigationService.NavigateAsync(new CFSPage());
}
}

Answer 3

Settings.xaml:
<template:DataGridTemplate TapCommand="{Binding OpenCFSPage}" />
<!-- Uncomment below and corresponding parameter property code in DataGridTemplate.xaml.cs to pass parameter from Settings.xaml -->
<!--<template:DataGridTemplate TapCommand="{Binding OpenCFSPage}" CommandParameter="A" />-->
Settings.xaml.cs:
public Settings()
{
InitializeComponent();
OpenCFSPage = new Command(p => OpenCFSPageExecute(p));
BindingContext = this;
}
public ICommand OpenCFSPage { get; private set; }
void OpenCFSPageExecute(object p)
{
var s = p as string;
Debug.WriteLine($"OpenCFSPage:{s}:");
}
DataGridTemplate.xaml:
<?xml version="1.0" encoding="UTF-8"?>
<Grid xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:Japanese;assembly=Japanese"
Padding="0,20"
HeightRequest="49" Margin="0"
x:Class="Japanese.DataGridTemplate">
<Grid.GestureRecognizers>
<TapGestureRecognizer
Command="{Binding TapCommand}"
CommandParameter="1"
NumberOfTapsRequired="1" />
</Grid.GestureRecognizers>
<Label Grid.Column="0" Text="Test" />
</Grid>
DataGridTemplate.xaml.cs:
public partial class DataGridTemplate : Grid
{
public DataGridTemplate()
{
InitializeComponent();
}
public static readonly BindableProperty TapCommandProperty =
BindableProperty.Create(
nameof(TapCommand), typeof(ICommand), typeof(DataGridTemplate), null,
propertyChanged: OnCommandPropertyChanged);
public ICommand TapCommand
{
get { return (ICommand)GetValue(TapCommandProperty); }
set { SetValue(TapCommandProperty, value); }
}
//public static readonly BindableProperty CommandParameterProperty = BindableProperty.Create(
// nameof(CommandParameter), typeof(string), typeof(DataGridTemplate), null);
//public string CommandParameter
//{
// get { return (string)GetValue(CommandParameterProperty); }
// set { SetValue(CommandParameterProperty, value); }
//}
static TapGestureRecognizer GetTapGestureRecognizer(DataGridTemplate view)
{
var enumerator = view.GestureRecognizers.GetEnumerator();
while (enumerator.MoveNext())
{
var item = enumerator.Current;
if (item is TapGestureRecognizer) return item as TapGestureRecognizer;
}
return null;
}
static void OnCommandPropertyChanged(BindableObject bindable, object oldValue, object newValue)
{
if (bindable is DataGridTemplate view)
{
var tapGestureRecognizer = GetTapGestureRecognizer(view);
if (tapGestureRecognizer != null)
{
tapGestureRecognizer.Command = (ICommand)view.GetValue(TapCommandProperty);
//tapGestureRecognizer.CommandParameter = (string)view.GetValue(CommandParameterProperty);
}
}
}
}

Answer 4

Check this code you help you. Here you have to pass a reference of list view and also you need to bind a command with BindingContext.
<ListView ItemsSource="{Binding Sites}" x:Name="lstSale">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<StackLayout Orientation="Vertical">
<Label Text="{Binding FriendlyName}" />
<Button Text="{Binding Name}"
HorizontalOptions="Center"
VerticalOptions="Center"
Command="{Binding
Path=BindingContext.RoomClickCommand,
Source={x:Reference lstSale}}"
CommandParameter="{Binding .}" />
</StackLayout>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>