Android: How to stretch an image to the screen width while maintaining aspect ratio?

Question

I accomplished this with a custom view. Set layout_width="fill_parent" and layout_height="wrap_content", and point it to the appropriate drawable:

public class Banner extends View {

  private final Drawable logo;

  public Banner(Context context) {
    super(context);
    logo = context.getResources().getDrawable(R.drawable.banner);
    setBackgroundDrawable(logo);
  }

  public Banner(Context context, AttributeSet attrs) {
    super(context, attrs);
    logo = context.getResources().getDrawable(R.drawable.banner);
    setBackgroundDrawable(logo);
  }

  public Banner(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
    logo = context.getResources().getDrawable(R.drawable.banner);
    setBackgroundDrawable(logo);
  }

  @Override protected void onMeasure(int widthMeasureSpec,
      int heightMeasureSpec) {
    int width = MeasureSpec.getSize(widthMeasureSpec);
    int height = width * logo.getIntrinsicHeight() / logo.getIntrinsicWidth();
    setMeasuredDimension(width, height);
  }
}

In the end, I generated the dimensions manually, which works great:

DisplayMetrics dm = new DisplayMetrics();
context.getWindowManager().getDefaultDisplay().getMetrics(dm);
int width = dm.widthPixels;
int height = width * mainImage.getHeight() / mainImage.getWidth(); //mainImage is the Bitmap I'm drawing
addView(mainImageView,new LinearLayout.LayoutParams( 
        width, height));

I just read the source code for ImageView and it is basically impossible without using the subclassing solutions in this thread. In ImageView.onMeasure we get to these lines:

        // Get the max possible width given our constraints
        widthSize = resolveAdjustedSize(w + pleft + pright, mMaxWidth, widthMeasureSpec);

        // Get the max possible height given our constraints
        heightSize = resolveAdjustedSize(h + ptop + pbottom, mMaxHeight, heightMeasureSpec);

Where h and w are the dimensions of the image, and p* is the padding.

And then:

private int resolveAdjustedSize(int desiredSize, int maxSize,
                               int measureSpec) {
    ...
    switch (specMode) {
        case MeasureSpec.UNSPECIFIED:
            /* Parent says we can be as big as we want. Just don't be larger
               than max size imposed on ourselves.
            */
            result = Math.min(desiredSize, maxSize);

So if you have a layout_height="wrap_content" it will set widthSize = w + pleft + pright, or in other words, the maximum width is equal to the image width.

This means that unless you set an exact size, images are NEVER enlarged. I consider this to be a bug, but good luck getting Google to take notice or fix it. Edit: Eating my own words, I submitted a bug report and they say it has been fixed in a future release!

Another solution

Here is another subclassed workaround, but you should (in theory, I haven't really tested it much!) be able to use it anywhere you ImageView. To use it set layout_width="match_parent", and layout_height="wrap_content". It is quite a lot more general than the accepted solution too. E.g. you can do fit-to-height as well as fit-to-width.

import android.content.Context;
import android.util.AttributeSet;
import android.widget.ImageView;

// This works around the issue described here: http://stackoverflow.com/a/12675430/265521
public class StretchyImageView extends ImageView
{

    public StretchyImageView(Context context)
    {
        super(context);
    }

    public StretchyImageView(Context context, AttributeSet attrs)
    {
        super(context, attrs);
    }

    public StretchyImageView(Context context, AttributeSet attrs, int defStyle)
    {
        super(context, attrs, defStyle);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)
    {
        // Call super() so that resolveUri() is called.
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);

        // If there's no drawable we can just use the result from super.
        if (getDrawable() == null)
            return;

        final int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
        final int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);

        int w = getDrawable().getIntrinsicWidth();
        int h = getDrawable().getIntrinsicHeight();
        if (w <= 0)
            w = 1;
        if (h <= 0)
            h = 1;

        // Desired aspect ratio of the view's contents (not including padding)
        float desiredAspect = (float) w / (float) h;

        // We are allowed to change the view's width
        boolean resizeWidth = widthSpecMode != MeasureSpec.EXACTLY;

        // We are allowed to change the view's height
        boolean resizeHeight = heightSpecMode != MeasureSpec.EXACTLY;

        int pleft = getPaddingLeft();
        int pright = getPaddingRight();
        int ptop = getPaddingTop();
        int pbottom = getPaddingBottom();

        // Get the sizes that ImageView decided on.
        int widthSize = getMeasuredWidth();
        int heightSize = getMeasuredHeight();

        if (resizeWidth && !resizeHeight)
        {
            // Resize the width to the height, maintaining aspect ratio.
            int newWidth = (int) (desiredAspect * (heightSize - ptop - pbottom)) + pleft + pright;
            setMeasuredDimension(newWidth, heightSize);
        }
        else if (resizeHeight && !resizeWidth)
        {
            int newHeight = (int) ((widthSize - pleft - pright) / desiredAspect) + ptop + pbottom;
            setMeasuredDimension(widthSize, newHeight);
        }
    }
}

Setting adjustViewBounds to true and using a LinearLayout view group worked very well for me. No need to subclass or ask for device metrics:

//NOTE: "this" is a subclass of LinearLayout
ImageView splashImageView = new ImageView(context);
splashImageView.setImageResource(R.drawable.splash);
splashImageView.setAdjustViewBounds(true);
addView(splashImageView);

I've been struggling with this problem in one form or another for AGES, thank you, Thank You, THANK YOU.... :)

I just wanted to point out that you can get a generalizable solution from what Bob Lee's done by just extending View and overriding onMeasure. That way you can use this with any drawable you want, and it won't break if there's no image:

    public class CardImageView extends View {
        public CardImageView(Context context, AttributeSet attrs, int defStyle) {
            super(context, attrs, defStyle);
        }

        public CardImageView(Context context, AttributeSet attrs) {
            super(context, attrs);
        }

        public CardImageView(Context context) {
            super(context);
        }

        @Override
        protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
            Drawable bg = getBackground();
            if (bg != null) {
                int width = MeasureSpec.getSize(widthMeasureSpec);
                int height = width * bg.getIntrinsicHeight() / bg.getIntrinsicWidth();
                setMeasuredDimension(width,height);
            }
            else {
                super.onMeasure(widthMeasureSpec, heightMeasureSpec);
            }
        }
    }