c# - Xamarin:Android - 大位图的 OutOfMemory 异常 - 如何解决？

Question

我正在拍摄图像，解码由 TakePicture 方法生成的 byte array，然后旋转 bitmap 270度。问题是我似乎内存不足，我不知道如何解决。这是代码:

Bitmap image = BitmapFactory.DecodeByteArray (data, 0, data.Length);
data = null;
Bitmap rotatedBitmap = Bitmap.CreateBitmap (image, 0, 0, image.Width,
    image.Height, matrix, true);

Answer 1

请看Load Large Bitmaps Efficiently来自官方 Xamarin 文档，该文档解释了如何通过在内存中加载较小的二次采样版本将大图像加载到内存中，而应用程序不会抛出 OutOfMemoryException。

读取位图尺寸和类型

async Task<BitmapFactory.Options> GetBitmapOptionsOfImageAsync()
{
    BitmapFactory.Options options = new BitmapFactory.Options
    {
        InJustDecodeBounds = true
    };

    // The result will be null because InJustDecodeBounds == true.
    Bitmap result=  await BitmapFactory.DecodeResourceAsync(Resources, Resource.Drawable.someImage, options);

    int imageHeight = options.OutHeight;
    int imageWidth = options.OutWidth;

    _originalDimensions.Text = string.Format("Original Size= {0}x{1}", imageWidth, imageHeight);

    return options;
}

将按比例缩小的版本加载到内存中

public static int CalculateInSampleSize(BitmapFactory.Options options, int reqWidth, int reqHeight)
{
    // Raw height and width of image
    float height = options.OutHeight;
    float width = options.OutWidth;
    double inSampleSize = 1D;

    if (height > reqHeight || width > reqWidth)
    {
        int halfHeight = (int)(height / 2);
        int halfWidth = (int)(width / 2);

        // Calculate a inSampleSize that is a power of 2 - the decoder will use a value that is a power of two anyway.
        while ((halfHeight / inSampleSize) > reqHeight && (halfWidth / inSampleSize) > reqWidth)
        {
            inSampleSize *= 2;
        }
    }

    return (int)inSampleSize;
}

异步加载图像

public async Task<Bitmap> LoadScaledDownBitmapForDisplayAsync(Resources res, BitmapFactory.Options options, int reqWidth, int reqHeight)
{
    // Calculate inSampleSize
    options.InSampleSize = CalculateInSampleSize(options, reqWidth, reqHeight);

    // Decode bitmap with inSampleSize set
    options.InJustDecodeBounds = false;

    return await BitmapFactory.DecodeResourceAsync(res, Resource.Drawable.someImage, options);
}

然后在OnCreate

中调用它加载Image

protected async override void OnCreate(Bundle bundle)
{
    base.OnCreate(bundle);
    SetContentView(Resource.Layout.Main);
    _imageView = FindViewById<ImageView>(Resource.Id.resized_imageview);

    BitmapFactory.Options options = await GetBitmapOptionsOfImageAsync();
    Bitmap bitmapToDisplay = await LoadScaledDownBitmapForDisplayAsync (Resources, 
                             options, 
                             150,  //for 150 X 150 resolution
                             150);
    _imageView.SetImageBitmap(bitmapToDisplay);
}

希望对您有所帮助。