BitmapFactory.DecodeByteArray导致生长堆（frag情况）

Question

我正在用Xamarin开发一个Android应用程序。我在从字流生成图像方面遇到了问题。BitmapFactory（似乎是最流行的解决方案）正在引发巨大的分配问题--Grow堆。

        ConnectToDb connect = new ConnectToDb ();
        byte[] arr = connect.SelectImgByte(3,"Thea");

        BitmapFactory.Options options=new BitmapFactory.Options();
        options.InJustDecodeBounds = true;
        bmp = BitmapFactory.DecodeByteArray (arr, 0, arr.Length/*,options*/);
        _imageView.SetImageBitmap (bmp);

Thread started: <Thread Pool> #6
[dalvikvm-heap] Grow heap (frag case) to 22.596MB for 1997584-byte allocation
[Choreographer] Skipped 129 frames!  The application may be doing too much work on its main thread.
[dalvikvm-heap] Grow heap (frag case) to 20.755MB for 1997584-byte allocation
[dalvikvm-heap] Grow heap (frag case) to 22.735MB for 1997584-byte allocation
[dalvikvm-heap] Grow heap (frag case) to 24.710MB for 1997584-byte allocation

Answer 1

我看到的第一件事是使用injustdecodebounds，它通常用于获取图像的宽度和高度，如下所示：

var options = new BitmapFactory.Options {
    InJustDecodeBounds = true,
};
using (var derp = BitmapFactory.DecodeResource(Resources, 
    Resource.Id.myimage, options)) { }

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

这通常用于获取图像的纵横比，然后再将其缩放，而不是将一个巨大的图像加载到内存中。

如果您看一下Xamarin文档中关于高效加载大位图的文档，您会发现他们正是为了这个目的才使用它的。

using(var bmp = DecodeBitmap(some args))
    imageView.SetImageBitmap(bmp);

using(var bmp = DecodeBitmap(some args)) {
    imageView.SetImageBitmap(bmp);
    bmp.Recycle();
}

因此，您需要执行类似的操作，可能还需要使用byte数组，因为它包含图像的所有像素。

因此，使用文档中的模式，您可以执行如下操作：

byte[] arr = connect.SelectImgByte(3,"Thea");

using(var bmp = DecodeSampledBitmapFromArray(arr, scaledWidth, scaledHeight)) {
    _imageView.SetImageBitmap(bmp);
    bmp.Recycle();
}

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

    if (height > reqHeight || width > reqWidth)
    {
        inSampleSize = width > height
                            ? height/reqHeight
                            : width/reqWidth;
    }

    return (int) inSampleSize;
}

public static Bitmap DecodeSampledBitmapFromArray(byte[] pixels, int reqWidth, int reqHeight)
{
    var options = new BitmapFactory.Options {
        InJustDecodeBounds = true,
    };
    using (var dispose = BitmapFactory.DecodeResource(arr, 0, arr.Length, options)) { }

    // Calculate inSampleSize
    options.InSampleSize = CalculateInSampleSize(options, reqWidth, reqHeight);

    // Decode bitmap with inSampleSize set
    options.InJustDecodeBounds = false;
    return BitmapFactory.DecodeResource(arr, 0, arr.Length, options);
}