這篇文章主要介紹如何在Xamarin.Form實現(xiàn)跨平臺的調(diào)用系統(tǒng)圖片庫。
由于系統(tǒng)圖片庫是一個平臺相關(guān)性比較大的系統(tǒng)工具，在Xamarin.Form上沒有現(xiàn)成的統(tǒng)一API可用，所以我們只能利用DependencyService
來自己實現(xiàn)一個對系統(tǒng)圖片庫的調(diào)用。

下面，我們就一步一步來實現(xiàn)這個DependencyService調(diào)用。

聲明接口

namespace DependencyServiceSample
{
    public interface IPicturePicker
    {
        Task<Stream> GetImageStreamAsync();
    }
}

這里我們將方法聲明為異步的，因為只有等到用戶選擇完圖片之后這個方法才會返回，聲明成異步方法才不至于阻塞主線程。

iOS平臺下的實現(xiàn)

在iOS平臺下，我們要利用UIImagePickerController
來打開圖片庫。

[assembly: Dependency (typeof (PicturePickerImplementation))]

namespace DependencyServiceSample.iOS
{
    public class PicturePickerImplementation : IPicturePicker
    {
        TaskCompletionSource<Stream> taskCompletionSource;
        UIImagePickerController imagePicker;

        public Task<Stream> GetImageStreamAsync()
        {
            // 創(chuàng)建UIImagePickerController
            imagePicker = new UIImagePickerController
            {
                SourceType = UIImagePickerControllerSourceType.PhotoLibrary,
                MediaTypes = UIImagePickerController.AvailableMediaTypes(UIImagePickerControllerSourceType.PhotoLibrary)
            };

            // 添加事件處理
            imagePicker.FinishedPickingMedia += OnImagePickerFinishedPickingMedia;
            imagePicker.Canceled += OnImagePickerCancelled;

            // 顯示UIImagePickerController;
            UIWindow window = UIApplication.SharedApplication.KeyWindow;
            var viewController = window.RootViewController;
            viewController.PresentModalViewController(imagePicker, true);

            // 返回Task對象
            taskCompletionSource = new TaskCompletionSource<Stream>();
            return taskCompletionSource.Task;
        }
        void OnImagePickerFinishedPickingMedia(object sender, UIImagePickerMediaPickedEventArgs args)
        {
            UIImage image = args.EditedImage ?? args.OriginalImage;

            if (image != null)
            {
                //將 UIImage轉(zhuǎn)換成 .NET Stream對象
                NSData data = image.AsJPEG(1);
                Stream stream = data.AsStream();

                // 將Stream設(shè)置為Task的結(jié)果
                taskCompletionSource.SetResult(stream);
            }
            else
            {
                taskCompletionSource.SetResult(null);
            }
            imagePicker.DismissModalViewController(true);
        }

        void OnImagePickerCancelled(object sender, EventArgs args)
        {
            taskCompletionSource.SetResult(null);
            imagePicker.DismissModalViewController(true);
        }
    }
}

為了使程序可以正常運(yùn)行，我們還需要在plist文件中的dict段里添加如下聲明：

<key>NSPhotoLibraryUsageDescription</key>
<string>Picture Picker uses photo library</string>

Android平臺下的實現(xiàn)

在Android平臺下，我們要聲明一個Activity。

public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsApplicationActivity
{
    ...
    // 以下是為圖片選擇器而聲明的字段、屬性和方法
    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);

                // 將Stream設(shè)置為Task的結(jié)果
                PickImageTaskCompletionSource.SetResult(stream);
            }
            else
            {
                PickImageTaskCompletionSource.SetResult(null);
            }
        }
    }
}

還有圖片選擇器的實現(xiàn)：

[assembly: Dependency(typeof(PicturePickerImplementation))]

namespace DependencyServiceSample.Droid
{
    public class PicturePickerImplementation : IPicturePicker
    {
        public Task<Stream> GetImageStreamAsync()
        {
            // 定義一個用于獲取圖片的Intent
            Intent intent = new Intent();
            intent.SetType("image/*");
            intent.SetAction(Intent.ActionGetContent);

            // 獲取MainActivity的實例
            MainActivity activity = Forms.Context as MainActivity;

            //啟動圖片選擇的 activity (選擇結(jié)果在MainActivity.cs中處理)
            activity.StartActivityForResult(
                Intent.CreateChooser(intent, "Select Picture"),
                MainActivity.PickImageId);

            // 保存一個TaskCompletionSource 對象作為 MainActivity的屬性
            activity.PickImageTaskCompletionSource = new TaskCompletionSource<Stream>();

            // 返回 Task 對象
            return activity.PickImageTaskCompletionSource.Task;
        }
    }
}

在Xamarin.Form中調(diào)用圖片選擇器

 Stream stream = await DependencyService.Get<IPicturePicker>().GetImageStreamAsync();

 if (stream != null)
    {
        Image image = new Image
        {
            Source = ImageSource.FromStream(() => stream)
        };
    }

參考文獻(xiàn)

Picking a Photo from the Picture Library