RightAlignedHorizontalScrollView

attrs.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="RightAlignedHorizontalScrollView">
        <attr name="rahsv_gravityRight" format="boolean" />
        <attr name="rahsv_autoScroll" format="boolean" />
    </declare-styleable>
</resources>

RightAlignedHorizontalScrollView.java

public class RightAlignedHorizontalScrollView extends HorizontalScrollView {
    private static final boolean DEFAULT_GRAVITY_RIGHT = true;
    private static final boolean DEFAULT_AUTOSCROLL = true;

    private boolean autoScroll;
    private boolean gravityRight;

    public RightAlignedHorizontalScrollView(Context context) {
        this(context, null);
    }

    public RightAlignedHorizontalScrollView(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

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

    @TargetApi(Build.VERSION_CODES.LOLLIPOP)
    public RightAlignedHorizontalScrollView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
        init(context, attrs);
    }

    private void init(final Context context, final AttributeSet attrs) {
        final TypedArray typedArray = context.obtainStyledAttributes(attrs, R.styleable.RightAlignedHorizontalScrollView);
        gravityRight = typedArray.getBoolean(R.styleable.RightAlignedHorizontalScrollView_rahsv_gravityRight, DEFAULT_GRAVITY_RIGHT);
        autoScroll = typedArray.getBoolean(R.styleable.RightAlignedHorizontalScrollView_rahsv_autoScroll, DEFAULT_AUTOSCROLL);
        typedArray.recycle();
    }

    View getChildView() {
        return getChildAt(0);
    }

    private int getScrollRange() {
        int scrollRange = 0;
        if (getChildCount() > 0) {
            View childView = getChildView();
            scrollRange = Math.max(0, childView.getWidth() - (getWidth() - getPaddingLeft() - getPaddingRight()));
        }
        return scrollRange;
    }

    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        boolean canScroll = true;

        FrameLayout.LayoutParams layoutParams = (LayoutParams) getChildView().getLayoutParams();
        int verticalGravity = layoutParams.gravity & Gravity.VERTICAL_GRAVITY_MASK;

        int childWidth = getChildView().getWidth();
        super.onLayout(changed, left, top, right, bottom);

        int delta = getChildView().getWidth() - childWidth;

        if (gravityRight) {
            if (getScrollRange() > 0) {
                canScroll = true;
                layoutParams.gravity = Gravity.START | verticalGravity;
                getChildView().setLayoutParams(layoutParams);
                super.onLayout(changed, left, top, right, bottom);

            } else if (getScrollRange() == 0) {
                canScroll = false;
                layoutParams.gravity = Gravity.END | verticalGravity;
                getChildView().setLayoutParams(layoutParams);
                super.onLayout(changed, left, top, right, bottom);
            }
        }

        if (autoScroll && gravityRight && canScroll && delta > 0) {
            scrollBy(delta, 0);
        }
    }

    public boolean isAutoScroll() {
        return autoScroll;
    }

    public void setAutoScroll(boolean autoScroll) {
        this.autoScroll = autoScroll;
        invalidate();
    }

    public boolean isGravityRight() {
        return gravityRight;
    }

    public void setGravityRight(boolean gravityRight) {
        this.gravityRight = gravityRight;
        invalidate();
    }
}