Xamarin.Forms 在Prism 下实现导航栏回退按钮点击事件截留的正确姿势

1.定义一个用于实现菜单管理的容器基类 MenuContainerPage 继承自 ContentPage, IMenuContainerPage

public class MenuContainerPage : ContentPage, IMenuContainerPage
 2 {

        SlideMenuView slideMenu;
        public SlideMenuView SlideMenu
        {
            get
            {
                return slideMenu;
            }
            set
            {
                if (slideMenu != null)
                    slideMenu.Parent = null;
                slideMenu = value;
                if (slideMenu != null)
                    slideMenu.Parent = this;
            }
        }

        public Action ShowMenuAction { get; set; }

        public Action HideMenuAction { get; set; }

        public void ShowMenu ()
        {
            if (SlideMenu.IsShown)
            {
                HideMenuAction?.Invoke();
            }
            else
            {
                ShowMenuAction?.Invoke();
            }
        }

        public void HideMenu ()
        {
            HideMenuAction?.Invoke();
        }


        public virtual void OnSoftBackButtonPressed()
        {
        }
        public virtual bool NeedOverrideSoftBackButton { get; set; } = false;
    }

2. 定义一个泛型视图基类BaseContentPage<T> 继承自  MenuContainerPage  其中T派生自 ViewModelBase，该类用于模型绑定上下文， 申明 

OnSoftBackButtonPressed 和 NeedOverrideSoftBackButton 用来接受点击事件和开关

    public abstract class BaseContentPage<T> : MenuContainerPage where T : ViewModelBase
    {
        public T ViewModel => (T)this.BindingContext;
        public BaseContentPage()
        {
            Padding = 0;
            this.SlideMenu = new RightSideMasterPage();
            BackgroundColor = Color.White;
            DeviceDisplay.KeepScreenOn = true;
        }
        protected override void OnAppearing()
        {
            base.OnAppearing();
        }
        protected override void OnDisappearing()
        {
            base.OnDisappearing();
        }


        public override void OnSoftBackButtonPressed()
        {
            var bindingContext = BindingContext as ViewModelBaseCutom;
            bindingContext?.OnSoftBackButtonPressed();
        }
        public override bool NeedOverrideSoftBackButton { get; set; } = false;

    }

3. 要实现导航栏回退按钮侦听事件，需要自定义 NavigationPageRenderer ，这里我们定义一个 CustomNavigationPageRenderer 实现 Android.Views.View.IOnClickListener 接口

 public class CustomNavigationPageRenderer : NavigationPageRenderer, Android.Views.View.IOnClickListener
    {

        private static readonly FieldInfo ToolbarFieldInfo;
        private bool _disposed;
        private Toolbar _toolbar;
        static CustomNavigationPageRenderer()
        {
            ToolbarFieldInfo = typeof(NavigationPageRenderer).GetField("_toolbar", BindingFlags.NonPublic | BindingFlags.Instance);
        }
        public CustomNavigationPageRenderer(Context context) : base(context) { }

        public override void OnViewAdded(Android.Views.View child)
        {
            base.OnViewAdded(child);

            if (child.GetType() == typeof(Toolbar))
            {
                _toolbar = (Toolbar)child;
                _toolbar.ChildViewAdded += Toolbar_ChildViewAdded;
            }
        }

        private void Toolbar_ChildViewAdded(object sender, ChildViewAddedEventArgs e)
        {
            var view = e.Child.GetType();
            if (view == typeof(AppCompatTextView))
            {
                var textView = (AppCompatTextView)e.Child;
                textView.TextSize = 14;
                textView.SetTypeface(null, TypefaceStyle.Bold);
                textView.RequestLayout();
                if (_toolbar != null)
                {
                    _toolbar.ChildViewAdded -= Toolbar_ChildViewAdded;
                }
            }
        }

        public new void OnClick(Android.Views.View v)
        {
            if (!(Element.CurrentPage is MenuContainerPage curPage))
            {
                Element.PopAsync();
            }
            else
            {
                if (curPage.NeedOverrideSoftBackButton)
                    curPage.OnSoftBackButtonPressed();

                else Element.PopAsync();
            }
        }

        protected override void OnLayout(bool changed, int l, int t, int r, int b)
        {
            base.OnLayout(changed, l, t, r, b);
            UpdateToolbarInstance();
        }

        protected override void OnConfigurationChanged(Configuration newConfig)
        {
            base.OnConfigurationChanged(newConfig);
            UpdateToolbarInstance();
        }

        protected override void Dispose(bool disposing)
        {
            if (disposing && !_disposed)
            {
                _disposed = true;
                RemoveToolbarInstance();
            }

            base.Dispose(disposing);
        }

        private void UpdateToolbarInstance()
        {
            RemoveToolbarInstance();
            GetToolbarInstance();
        }

        private void GetToolbarInstance()
        {
            try
            {
                _toolbar = (Toolbar)ToolbarFieldInfo.GetValue(this);
                _toolbar.SetNavigationOnClickListener(this);
            }
            catch (Exception exception)
            {
                System.Diagnostics.Debug.WriteLine($"Can't get toolbar with error: {exception.Message}");
            }
        }

        private void RemoveToolbarInstance()
        {
            if (_toolbar == null) return;

            _toolbar.ChildViewAdded -= Toolbar_ChildViewAdded;
            _toolbar.SetNavigationOnClickListener(null);
            _toolbar = null;
        }

    }

 其中重写 OnClick(Android.Views.View v)

 public new void OnClick(Android.Views.View v)
{
            if (!(Element.CurrentPage is MenuContainerPage curPage))
            {
                Element.PopAsync();
            }
            else
            {
                if (curPage.NeedOverrideSoftBackButton)
                    curPage.OnSoftBackButtonPressed();

                else Element.PopAsync();
            }
        }

4. 定义测试页面 SaleOrderBillPage 继承  BaseContentPage<SaleOrderBillPageViewModel>  其中 SaleOrderBillPageViewModel 表示视图模型，在OnAppearing 中 开启当前页面是否启用回退截留

 NeedOverrideSoftBackButton = true;

protected override void OnAppearing()
        {
            base.OnAppearing();
            if (Content == null)
            {
                try
                {
                    InitializeComponent();

                    ToolbarItems.Clear();
                    NeedOverrideSoftBackButton = true;

                    string btnIco = "uf0c7";
                    foreach (var toolBarItem in this.GetToolBarItems(ViewModel, true, btnIco).ToList())
                    {
                        ToolbarItems.Add(toolBarItem);
                    }
                }
                catch (Exception ex) { Log.Write(ex); }
                return;
            }
        }

5.  在ViewModelBase 中重写 OnSoftBackButtonPressed

   public async virtual void OnSoftBackButtonPressed()
        {
            var ok = await _dialogService.ShowConfirmAsync("你是否要保存单据？", "提示", "确定", "取消");
            if (!ok)
            {
                await _navigationService.GoBackAsync();
            }
        }

当点击返回按钮时，提示表单是否要存储，效果如下：

protected override void OnAppearing()        {            base.OnAppearing();            if (Content == null)            {                try                {                    InitializeComponent();
                    ToolbarItems.Clear();                    NeedOverrideSoftBackButton = true;
                    string btnIco = "uf0c7";                    foreach (var toolBarItem in this.GetToolBarItems(ViewModel, true, btnIco).ToList())                    {                        ToolbarItems.Add(toolBarItem);                    }                }                catch (Exception ex) { Log.Write(ex); }                return;            }        }