我正在开发中Xamarin的Andr​​oid应用程序。我有从字节流生成图像的问题。 BitmapFactory（这似乎是最流行的解决方案），是造成巨大的分配问题 - 成长堆。

I am developing an Android app in Xamarin. I am having problems with generating an image from a bytestream. BitmapFactory (which seems to be the most popular solution), is causing huge allocation problems - Grow Heap.

        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);

以上是在这里BitmapFactory.De codeByteArray的被调用的方法。它做工精细，显示图像。但它是缓慢的，并导致这些警告​​。

Above is the method where BitmapFactory.DecodeByteArray is being called. It works fine, the image is displayed. But it is slow and causes these "Warnings".

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

有关每次调用该方法，成长堆出现错误。正如你可以看到我已经加载的图像的ImageView的有4次。所以，我想知道如果任何人有同样的问题吗？我试过好几个小时来解决这个问题，通过寻找在这里（和其他地方），但我无法找到一个解决方案。请记住我写在Xamarin应用。（C＃语言 - 采用Android库）

For everytime the method is called, Grow Heap error appears. As you can see i have loaded an image to an imageview 4 times there. So, i was wondering if anyone had the same problem as me? I tried for many hours to solve the problem, by looking in here (and else where) but i could not find a solution. Keep in mind i am writing the application in Xamarin (c# language - using Android library).

对不起，糟糕的链接，但我没有足够的名气上传图片来呢：）

Sorry for the lousy links, but i don't have enough cred to upload images here yet :)

推荐答案

我看到的第一件事情就是你用 InJustDe codeBounds ，这通常是用来获取像这样图像的宽度和高度：

First thing I see is that you use InJustDecodeBounds, which usually is used to get the width and height of the image like so:

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;

此通常用于获得图像的高宽比，比例缩小，而不是加载巨大图像到存储器之前

This is generally used to get the aspect ratio of the image, before scaling it down, instead of loading a huge image into the memory.

如果你看看在有关装入大的位图的有效的Xamarin文档中​​，你会发现，他们使用它只是文档。

If you take a look at the docs about loading large bitmaps efficiently in the Xamarin documentation you will find that they use it for just that.

然后，他们要确保加载位图在使用语句，并将其分配给的ImageView 是这样的：

Then they make sure to load the Bitmap in a using statement and assign it to the ImageView like this:

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

在这里您不需要情况下，位图以后你可以叫回收站（）就可以了，这告诉Java Runtime，以摆脱它：

In cases where you don't need the Bitmap afterwards you can call Recycle() on it, which tells the Java Runtime to get rid of it:

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

所以，你需要做同样的事情，也可能与你的字节阵列，因为它包含了图像的所有像素。

So you need to do something similar, probably also with your byte array as it contains all the pixels for the image.

因此​​，使用中的文档模式，你可以做这样的事情：

So using the pattern in the docs you could do something like this:

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);
}