неділя, 7 квітня 2013 р.

Android Bitmap and OutOfMemoryError

In this post I am going to collect tips and tricks to help developer avoid OutOfMemoryError.

In code below I use this function to suspend current thread for some time:

public static void sleep(long time) {
  try {
    Thread.sleep(time);
  } catch (Exception e) {}
}

First of all let's talk about bitmap loading. Imagine you must load whole image with width 10x for imageview control with width x. Control don't need full size original image for good quality. Also if you load it as is then bitmap allocate too much memory. (If image configuration is ARGB_8888 then 4 bytes were allocated for every pixel). If we load scaled image with width x this will be enough. In first step we read only image size without loading content:
BitmapFactory.Options options=new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeResource(context.getResources(),     R.drawable.sample_image, options);
Now options variable contain width and height of image. I scale image by control width. BitmapFactory.Options have inSampleSize property. This property used to return smaller image to save memory. The documentation mentions using values that are a power of 2. Result code looks like this:

int scale = 1;
BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeResource(context.getResources(), resouceId, options);
int bmpWidth = options.outWidth;
if (bmpWidth > destWidth) {
  do {
    bmpWidth /= 2;
    if (bmpWidth > destWidth) {
      scale *= 2;
    }
  } while (bmpWidth > destWidth);
}
options.inJustDecodeBounds = false;
options.inSampleSize = scale;
Bitmap image = BitmapFactory.decodeResource(context.getResources(), resouceId, options);

Here we multiply scale variable by 2 and divide image width while it greater then destWidth. We get image with minimal size and good quality for display in control with destWidth width.

Let's consider a simple situation. We need to load bitmap, process it and destroy:

Bitmap image = ... // Load image
// Process image content
image.recycle();
sleep(100);
image = null;
System.gc();
sleep(100);
recycle function free up memoty associated with image. sleep function here take time for GC.


If you display large image in activity and need to free resources after activity was closed:
@Override
public void onCreate(Bundle savedInstanceState) {
  ...
  imageView = (ImageView) findViewById(R.id.full_image_view);
  imageView.setImageBitmap(...);
  ...
}

@Override
protected void onDestroy() {
  ((BitmapDrawable)imageView.getDrawable()).getBitmap().recycle();
  sleep(100);
  super.onDestroy();
  System.gc();
  sleep(100);
}

Upd: If image quality is not important to you:
BitmapFactory.Options options = new BitmapFactory.Options();
options.inPreferredConfig = Bitmap.Config.RGB_565;
This value provide normal image quality and any pixel is stored in 2 bytes. If inPreferredConfig is null and decoder try to pick image with ARGB_8888 then one pixel is stored in 4 bytes in memory. For large images this will increase memory usage.

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