@Override

    protected void onDraw(Canvas canvas) {

        super.onDraw(canvas);

            Paint mTextPaint = new Paint(Paint.ANTI_ALIAS_FLAG);

            // Measure a text

            Rect textBounds = new Rect();

            mTextPaint.getTextBounds(String.valueOf(mLetter), 0, 1, textBounds);

            float textWidth = mTextPaint.measureText(String.valueOf(mLetter));

            float textHeight = textBounds.height();

            FontMetrics fontMetrics = mTextPaint.getFontMetrics();

            // 计算文字高度

            float fontHeight = fontMetrics.bottom - fontMetrics.top;

            // 计算文字baseline

            float textBaseY = getHeight() - (getHeight() - fontHeight) / 2

                    - fontMetrics.bottom;

            // Draw the text

            canvas.drawText(String.valueOf(mLetter), getWidth() / 2f

                    - textWidth / 2f, textBaseY, mTextPaint);

    }