如何拦截Xamarin表单中单击的导航栏后退按钮? [英] How to intercept Navigation Bar Back Button Clicked in Xamarin Forms?

问题描述

我有一个xamarin表单页面，用户可以在其中更新表单中的某些数据.我需要截取单击的导航栏后退按钮，以警告用户是否尚未保存某些数据.该怎么办?

I have a xamarin form page where a user can update some data in a form. I need to intercept the Navigation Bar Back Button Clicked to warn the user if some data have not been saved.How to do it?

我能够使用Android.MainActivity.OnBackPressed()截获Android中单击的硬件后退"按钮，但是该事件仅在硬件后退"按钮单击时引发，而不会在导航"后退按钮单击时引发.

I'm able to intercept the hardware Bar Back Button Clicked in Android using the Android.MainActivity.OnBackPressed(), but that event is raised only on hardware Bar Back Button Clicked, not on Navigation Bar Back Button Clicked.

我也尝试覆盖Xamarin.Forms.NavigationPageOnBackButtonPressed()，但是它不起作用.为什么? 有人已经解决了这个问题吗?

I tried also to override Xamarin.Forms.NavigationPageOnBackButtonPressed() but it doesn't work. Why? Any one have already solved that issue?

我也尝试通过覆盖OnDisappear()来解决两个问题:

I also tried by overriding OnDisappear(), there are two problems:

1. 该页面已经在视觉上消失了，因此您确定吗?"对话框出现在上一页上.
2. 无法取消后退动作.

那么，有可能拦截导航栏的后退按钮按下吗?

So, is it possible to intercept the navigation bar back button press?

推荐答案

我能够显示一个确认对话框，该对话框可以通过重写FormsApplicationActivity中的以下方法来取消导航.

I was able to show a confirmation dialog that could cancel navigation by overriding the following methods in the FormsApplicationActivity.

  // navigation back button
  public override bool OnOptionsItemSelected(IMenuItem item)
  {
     if (item.ItemId == 16908332)
     {
        var currentViewModel = (IViewModel)navigator.CurrentPage.BindingContext;
        currentViewModel.CanNavigateFromAsync().ContinueWith(t =>
        {
           if (t.Result)
           {
              navigator.PopAsync();
           }
        }, TaskScheduler.FromCurrentSynchronizationContext());
        return false;
     }
     else
     {
        return base.OnOptionsItemSelected(item);
     }
  }

  // hardware back button
  public async override void OnBackPressed()
  {
     var currentViewModel = (IViewModel)navigator.CurrentPage.BindingContext;

     // could display a confirmation dialog (ex: "Cancel changes?")
     var canNavigate = await currentViewModel.CanNavigateFromAsync();
     if (canNavigate)
     {
        base.OnBackPressed();
     }
  }

navigator.CurrentPage是INavigation服务的包装.我不必从模式页面取消导航，因此我只处理NavigationStack.

The navigator.CurrentPage is a wrapper around the INavigation service. I do not have to cancel navigation from modal pages so I am only handling the NavigationStack.

this.navigation.NavigationStack[this.navigation.NavigationStack.Count - 1];

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