未定义ImagePicker MainActivity实例

Question

按照下面的链接添加ImagePicker

https://docs.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/dependency-service/photo-picker

在这里，对于Android实现，问题是MainActivity.cs中没有定义实例

[assembly: Dependency(typeof(PicturePickerImplementation))]

namespace DependencyServiceSample.Droid
{
    public class PicturePickerImplementation : IPicturePicker
    {
        public Task<Stream> GetImageStreamAsync()
        {
            // Define the Intent for getting images
            Intent intent = new Intent();
            intent.SetType("image/*");
            intent.SetAction(Intent.ActionGetContent);

            // Start the picture-picker activity (resumes in MainActivity.cs)
            MainActivity.Instance.StartActivityForResult(
                Intent.CreateChooser(intent, "Select Picture"),
                MainActivity.PickImageId);

            // Save the TaskCompletionSource object as a MainActivity property
            MainActivity.Instance.PickImageTaskCompletionSource = new TaskCompletionSource<Stream>();

            // Return Task object
            return MainActivity.Instance.PickImageTaskCompletionSource.Task;
        }
    }
}

而MainActivity没有实例字段，有没有其他方法来获取实例？

public class MainActivity : FormsAppCompatActivity
{
    ...
    // Field, property, and method for Picture Picker
    public static readonly int PickImageId = 1000;

    public TaskCompletionSource<Stream> PickImageTaskCompletionSource { set; get; }

    protected override void OnActivityResult(int requestCode, Result resultCode, Intent intent)
    {
        base.OnActivityResult(requestCode, resultCode, intent);

        if (requestCode == PickImageId)
        {
            if ((resultCode == Result.Ok) && (intent != null))
            {
                Android.Net.Uri uri = intent.Data;
                Stream stream = ContentResolver.OpenInputStream(uri);

                // Set the Stream as the completion of the Task
                PickImageTaskCompletionSource.SetResult(stream);
            }
            else
            {
                PickImageTaskCompletionSource.SetResult(null);
            }
        }
    }
}

Answer 1

在MainActivity类中添加以下内容：

 internal static MainActivity Instance { get; private set; }

文档很可能缺少这一点。

Answer 2

缺少静态变量定义及其赋值。

向MainActivity类添加一个名为Instance的静态变量：

public static Activity Instance;

然后在OnResume覆盖中指定它：

protected override void OnResume()
{
    Instance = this;
    base.OnResume();
}