PercentImageView is an extension for ImageView that can control W & H by percentage.

README.md

PercentImageView

To implement it into xml

<PercentImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    app:piv_heightPercent="50"
    app:piv_widthPercent="50"/>

PercentImageView.java

public class PercentImageView extends AppCompatImageView {

    private int WIDTH_PERCENT = Integer.MIN_VALUE;
    private int HEIGHT_PERCENT = Integer.MIN_VALUE;

    public PercentImageView(Context context) {
        super(context);
        init(context, null);
    }

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

    public PercentImageView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init(context, attrs);
    }

    private void init(Context context, AttributeSet attrs) {
        if (attrs == null) return;
        TypedArray a = context.getTheme().obtainStyledAttributes(attrs,
                R.styleable.PercentImageView, 0, 0);
        try {
            WIDTH_PERCENT = a.getInteger(R.styleable.PercentImageView_piv_widthPercent, Integer.MIN_VALUE);
            HEIGHT_PERCENT = a.getInteger(R.styleable.PercentImageView_piv_heightPercent, Integer.MIN_VALUE);
        } finally {
            a.recycle();
        }
    }

    @Override
    protected void onAttachedToWindow() {
        super.onAttachedToWindow();
        ViewGroup.LayoutParams viewParams = getLayoutParams();
        DisplayMetrics metrics = getContext().getResources().getDisplayMetrics();
        int height = (int) (metrics.heightPixels * (HEIGHT_PERCENT / 100f));
        int width = (int) (metrics.widthPixels * (WIDTH_PERCENT / 100f));
        if (HEIGHT_PERCENT != Integer.MIN_VALUE)
            viewParams.height = height;
        if (WIDTH_PERCENT != Integer.MIN_VALUE)
            viewParams.width = width;
        setLayoutParams(viewParams);
    }
}

percentimageview_attrs.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="PercentImageView">
        <attr name="piv_heightPercent" format="integer" />
        <attr name="piv_widthPercent" format="integer" />
    </declare-styleable>
</resources>