Android Canvas的drawText()与文字居中方案详解

// 第一类
public void drawText (String text, float x, float y, Paint paint)
public void drawText (String text, int start, int end, float x, float y, Paint paint)
public void drawText (CharSequence text, int start, int end, float x, float y, Paint paint)
public void drawText (char[] text, int index, int count, float x, float y, Paint paint)

// 第二类
public void drawPosText (String text, float[] pos, Paint paint)
public void drawPosText (char[] text, int index, int count, float[] pos, Paint paint)

// 第三类
public void drawTextOnPath (String text, Path path, float hOffset, float vOffset, Paint paint)
public void drawTextOnPath (char[] text, int index, int count, Path path, float hOffset, float vOffset, Paint paint)

Paint paint=new Paint();
 paint.setStyle(Paint.Style.FILL);
 paint.setStrokeWidth(12);
 paint.setTextSize(100);

 String text="测试：my text";
 canvas.drawText(text, 200, 400, paint);

 //画两条线标记位置
 paint.setStrokeWidth(4);
 paint.setColor(Color.RED);
 canvas.drawLine(0, 400, 2000, 400, paint);
 paint.setColor(Color.BLUE);
 canvas.drawLine(200, 0, 200, 2000, paint);

Paint.FontMetrics fontMetrics=paint.getFontMetrics();
 fontMetrics.top
 fontMetrics.ascent
 fontMetrics.descent
 fontMetrics.bottom

//矩形背景
 Paint bgRect=new Paint();
 bgRect.setStyle(Paint.Style.FILL);
 bgRect.setColor(Color.YELLOW);
 RectF rectF=new RectF(200, 200, 800, 600);
 canvas.drawRect(rectF, bgRect);

 Paint textPaint=new Paint();
 textPaint.setStyle(Paint.Style.FILL);
 textPaint.setStrokeWidth(8);
 textPaint.setTextSize(50);
 textPaint.setTextAlign(Paint.Align.CENTER);

 String text="测试：my text";
 //计算baseline
 Paint.FontMetrics fontMetrics=textPaint.getFontMetrics();
 float distance=(fontMetrics.bottom - fontMetrics.top)/2 - fontMetrics.bottom;
 float baseline=rectF.centerY()+distance;
 canvas.drawText(text, rectF.centerX(), baseline, textPaint);

距离 = 文字高度的一半 - 基线到文字底部的距离（也就是bottom）
 = (fontMetrics.bottom - fontMetrics.top)/2 - fontMetrics.bottom