public class ExpandableTriangleView extends View {

    private Paint paint;
    private Path path;
    private boolean isExpanded;

    public ExpandableTriangleView(Context context) {
        super(context);
        paint = new Paint();
        paint.setStyle(Paint.Style.FILL);
        paint.setColor(Color.parseColor("#cccccc"));
        paint.setAntiAlias(true);
    }

    public void expand() {
        isExpanded = true;
        path = null;
        invalidate();
    }

    public void collapse() {
        isExpanded = false;
        path = null;
        invalidate();
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.drawPath(getTrianglePath(), paint);
    }

    private Path getTrianglePath() {
        if (path == null) {
            path = new Path();
            int width = getWidth();
            int height = getHeight();
            if (isExpanded) {
                path.moveTo(0, 0);
                path.lineTo(width, 0);
                path.lineTo(width / 2, height);
            } else {
                path.moveTo(0, 0);
                path.lineTo(0, height);
                path.lineTo(width, height / 2);
            }
        }
        return path;
    }
}