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

是Android中繪制文本的方法，其中的x代表文字繪制時在X軸的起始點，而y是在Y軸繪制時，文字的 baseline，不是文字的中心點也不是文字的底部。

下面代碼根據繪制的Y軸中心點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);

    }

Android繪制文字時垂直居中