Xamarin Toast 消息错误 (C#) [英] Xamarin Toast Message error (C#)

问题描述

我想显示一个吐司消息.如果我在 onCreate() 中执行此操作，它会正常工作.但我想这样做，但出现错误:

I want to display a toast Message. If I'd do this in onCreate() it'd work fine. But I want to do it like this and I get an error:

Java.Lang.NullPointerException:尝试调用虚方法'android.content.res.Resources android.content.Context.getResources()'在空对象引用上

Java.Lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference

我该怎么办?

public  void textToast(string textToDisplay) {               
    Toast.MakeText(this,
    textToDisplay, ToastLength.Long).Show();
}
class SampleTabFragment : Fragment
{
    Button add;
    MainActivity main = new MainActivity();
    public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
    {
        base.OnCreateView(inflater, container, savedInstanceState);
        var view = inflater.Inflate(Resource.Layout.Tab, container, false);     
        add = view.FindViewById<Button>(Resource.Id.add);      
        add.Click += Click;
        return view;
    }
    void Click(object sender, EventArgs eventArgs)
    {      
        main.textToast( "I like Toast!"); 
    }
}

推荐答案

Java.Lang.NullPointerException 被触发，因为您正在手动创建和使用 MainActivity 的实例.

The Java.Lang.NullPointerException is triggered because you are manually creating and using an instance of MainActivity.

不要使用 MainActivity 的自定义实例在 Click 中显示您的 Toast 消息，而是简化您的代码以使用现有活动引用的片段:

Instead of using a custom instance of MainActivity to display your toast message in Click, simplify your code to use the fragments existing activity reference:

public  void textToast(string textToDisplay) {               
    Toast.MakeText(this,
    textToDisplay, ToastLength.Long).Show();
}

class SampleTabFragment : Fragment
{
    Button add;

    // Remove manual creation code
    // MainActivity main = new MainActivity();

    // ...

    void Click(object sender, EventArgs eventArgs)
    {      
        (Activity as MainActivity).textToast( "I like Toast!"); 
    }
}

此代码假定拥有的 Activity 始终是 MainActivity 的实例.

This code assumes that the owning activity is always an instance of MainActivity.

见:

• Fragment getActivity()
• 如何使用DialogFragment中Activity的方法?

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