Android绘制文字时垂直居中

canvas.drawText(String text, float x, float y, Paint paint);

是Android中绘制文本的方法，其中的x代表文字绘制时在X轴的起始点，而y是在Y轴绘制时，文字的 baseline，不是文字的中心点也不是文字的底部。

下面代码根据绘制的Y轴中心点centerY，算出了baseline，top，bottom，ascent和descent

    @Override
    protected void onDraw(Canvas canvas) {

        int top = mPaint.getFontMetricsInt().top;
        int bottom = mPaint.getFontMetricsInt().bottom;
        int ascent = mPaint.getFontMetricsInt().ascent;
        int descent = mPaint.getFontMetricsInt().descent;

        int baselineY =centerY + (bottom-top)/2 - bottom;
        top = baselineY + top;
        bottom = baselineY + bottom;
        ascent = baselineY + ascent;
        descent = baselineY + descent;

        mPaint.setColor(Color.GREEN);
        canvas.drawText(text, 100, baselineY, mPaint);
        canvas.drawLine(0, centerY, getMeasuredWidth(), centerY, mPaint);

        mPaint.setStrokeWidth(2);

        mPaint.setColor(Color.BLACK);
        canvas.drawLine(0, top, getMeasuredWidth(), top, mPaint);

        mPaint.setColor(Color.YELLOW);
        canvas.drawLine(0, ascent, getMeasuredWidth(), ascent, mPaint);

        mPaint.setColor(Color.RED);
        canvas.drawLine(0, baselineY, getMeasuredWidth(), baselineY, mPaint);

        mPaint.setColor(Color.YELLOW);
        canvas.drawLine(0, descent, getMeasuredWidth(), descent, mPaint);

        mPaint.setColor(Color.BLACK);
        canvas.drawLine(0, bottom, getMeasuredWidth(), bottom, mPaint);

    }