canvas 绘制文本居中

//已知 rect
        Paint.FontMetricsInt fontMetricsInt = mTextPaint.getFontMetricsInt();
        float baseLine = (mTextBgRect.top + mTextBgRect.bottom - fontMetricsInt.bottom - fontMetricsInt.top) / 2;
        mTextPaint.setTextAlign(Paint.Align.CENTER);
        canvas.drawText(tip,mTextBgRect.centerX(),baseLine,mTextPaint);