// Licensed to the .NET Foundation under one or more agreements.

// The .NET Foundation licenses this file to you under the MS-PL license.

// See the LICENSE file in the project root for more information.

using System;

using System.Reactive.Linq;

using Splat;

using Xamarin.Forms;

namespace ReactiveUI.XamForms

{

/// <summary>

/// This content view will automatically load and host the view for the given view model. The view model whose view is

/// to be displayed should be assigned to the <see cref="ViewModel"/> property. Optionally, the chosen view can be

/// customized by specifying a contract via <see cref="ViewContractObservable"/> or <see cref="ViewContract"/>.

/// </summary>

public class ViewModelViewHost : ContentView, IViewFor

{

/// <summary>

/// The view model whose associated view is to be displayed.

/// </summary>

public object ViewModel

{

get { return GetValue(ViewModelProperty); }

set { SetValue(ViewModelProperty, value); }

}

/// <summary>

/// Identifies the <see cref="ViewModel"/> property.

/// </summary>

public static readonly BindableProperty ViewModelProperty = BindableProperty.Create(

nameof(ViewModel),

typeof(object),

typeof(ViewModelViewHost),

default(object),

BindingMode.OneWay);

/// <summary>

/// The content to display when <see cref="ViewModel"/> is <see langword="null"/>.

/// </summary>

public View DefaultContent

{

get { return (View)GetValue(DefaultContentProperty); }

set { SetValue(DefaultContentProperty, value); }

}

/// <summary>

/// Identifies the <see cref="DefaultContent"/> property.

/// </summary>

public static readonly BindableProperty DefaultContentProperty = BindableProperty.Create(

nameof(DefaultContent),

typeof(View),

typeof(ViewModelViewHost),

default(View),

BindingMode.OneWay);

/// <summary>

/// The contract to use when resolving the view for the given view model.

/// </summary>

public IObservable<string> ViewContractObservable

{

get { return (IObservable<string>)GetValue(ViewContractObservableProperty); }

set { SetValue(ViewContractObservableProperty, value); }

}

/// <summary>

/// Identifies the <see cref="ViewContractObservable"/> property.

/// </summary>

public static readonly BindableProperty ViewContractObservableProperty = BindableProperty.Create(

nameof(ViewContractObservable),

typeof(IObservable<string>),

typeof(ViewModelViewHost),

Observable<string>.Never,

BindingMode.OneWay);

private string viewContract;

/// <summary>

/// A fixed contract to use when resolving the view for the given view model.

/// </summary>

/// <remarks>

/// This property is a mere convenience so that a fixed contract can be assigned directly in XAML.

/// </remarks>

public string ViewContract

{

get { return this.viewContract; }

set { ViewContractObservable = Observable.Return(value); }

}

/// <summary>

/// Can be used to override the view locator to use when resolving the view. If unspecified, <see cref="ViewLocator.Current"/> will be used.

/// </summary>

public IViewLocator ViewLocator { get; set; }

public ViewModelViewHost()

{

// NB: InUnitTestRunner also returns true in Design Mode

if (ModeDetector.InUnitTestRunner()) {

ViewContractObservable = Observable<string>.Never;

return;

}

ViewContractObservable = Observable<string>.Default;

var vmAndContract = Observable.CombineLatest(

this.WhenAnyValue(x => x.ViewModel),

this.WhenAnyObservable(x => x.ViewContractObservable),

(vm, contract) => new { ViewModel = vm, Contract = contract, });

this.WhenActivated(() => {

return new[] {

vmAndContract.Subscribe(x => {

this.viewContract = x.Contract;

if (x.ViewModel == null) {

this.Content = this.DefaultContent;

return;

}

var viewLocator = ViewLocator ?? ReactiveUI.ViewLocator.Current;

var view = viewLocator.ResolveView(x.ViewModel, x.Contract) ?? viewLocator.ResolveView(x.ViewModel, null);

if (view == null) {

throw new Exception(String.Format("Couldn't find view for '{0}'.", x.ViewModel));

}

var castView = view as View;

if (castView == null) {

throw new Exception(String.Format("View '{0}' is not a subclass of '{1}'.", view.GetType().FullName, typeof(View).FullName));

}

view.ViewModel = x.ViewModel;

this.Content = castView;

})};

});

}

}

}