xamarin - ImagePicker MainActivity 实例未定义

Question

按照以下链接添加 ImagePicker

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

在这里，对于 Android 实现，问题是实例未在 MainActivity.cs 中定义

[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;
        }
    }
}

而且 MainActivity 没有实例字段，是否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:

internal static MainActivity Instance { get; private set; }

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

也感谢@SushiHangover's answer这里描述如何初始化Instance对象。

文档很可能缺少这个。