DecoratedDrawable.java

    abstract class DecoratedDrawable extends LevelListDrawable {
        public DecoratedDrawable(Context ctx, int resId) {
            TypedArray typedArray = ctx.obtainStyledAttributes(new int[] {resId});
            Drawable drawable = typedArray.getDrawable(0);
            typedArray.recycle();
            if (drawable == null) {
                drawable = ctx.getResources().getDrawable(resId);
            }
            init(drawable);
        }

        public DecoratedDrawable(Drawable drawable) {
            init(drawable);
        }

        private void init(Drawable drawable) {
            addLevel(0, 10000, drawable);
        };

        @Override
        protected boolean onLevelChange(int level) {
            invalidateSelf();
            return super.onLevelChange(level);
        }

        @Override
        public void draw(Canvas canvas) {
            onPreDraw(canvas);
            super.draw(canvas);
            onPostDraw(canvas);
        }

        protected void onPreDraw(Canvas canvas) {};
        protected void onPostDraw(Canvas canvas) {};
    }

    // sample implementation

    class ErrorDrawable extends DecoratedDrawable {
        Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);

        public ErrorDrawable(Context ctx, int resId) {
            super(ctx, resId);
        }

        @Override
        protected void onPreDraw(Canvas canvas) {
            int lvl = getLevel();
            paint.setColor(lvl < 5 ? (lvl < 3? 0xffff0000 : 0xffff8800) : 0xff00ff00);
            canvas.drawCircle(0, 0, getBounds().height() / 3f, paint);
        }
    }
    
    // sample usage

    final EditText editText = (EditText) findViewById(R.id.edit);
    final Drawable d = new ErrorDrawable(this, android.R.attr.editTextBackground);
    d.setLevel(editText.length());
    editText.setBackground(d);
    TextWatcher watcher = new TextWatcher() {
        @Override public void beforeTextChanged(CharSequence s, int start, int count, int after) {}
        @Override public void onTextChanged(CharSequence s, int start, int before, int count) {}
        @Override public void afterTextChanged(Editable s) {
            d.setLevel(s.length());
        }
    };
    editText.addTextChangedListener(watcher);