FontMetrics ----- 绘制文本，获取文本高度

Canvas 绘制文本时，使用FontMetrics对象，计算位置的坐标。

 public static class FontMetrics {
     /**
      * The maximum distance above the baseline for the tallest glyph in
      * the font at a given text size.
      */
     public float   top;
     /**
      * The recommended distance above the baseline for singled spaced text.
      */
     public float   ascent;
     /**
      * The recommended distance below the baseline for singled spaced text.
      */
     public float   descent;
     /**
      * The maximum distance below the baseline for the lowest glyph in
      * the font at a given text size.
      */
     public float   bottom;
     /**
      * The recommended additional space to add between lines of text.
      */
     public float   leading;
 }

它的各基准线可以参考下图：

 /** 绘制FontMetrics对象的各种线 */
 mPaint.reset();
 mPaint.setColor(Color.WHITE);
 mPaint.setTextSize(80);
 // FontMetrics对象
 FontMetrics fontMetrics = mPaint.getFontMetrics();
 String text = "abcdefg";
 // 计算每一个坐标
 float textWidth = mPaint.measureText(text);
 float baseX = 30;
 float baseY = 700;
 float topY = baseY + fontMetrics.top;
 float ascentY = baseY + fontMetrics.ascent;
 float descentY = baseY + fontMetrics.descent;
 float bottomY = baseY + fontMetrics.bottom;
 // 绘制文本
 canvas.drawText(text, baseX, baseY, mPaint);
 // BaseLine描画
 mPaint.setColor(Color.RED);
 canvas.drawLine(baseX, baseY, baseX + textWidth, baseY, mPaint);
 mPaint.setTextSize(20);
 canvas.drawText("base", baseX + textWidth, baseY, mPaint);
 // Base描画
 canvas.drawCircle(baseX, baseY, 5, mPaint);
 // TopLine描画
 mPaint.setColor(Color.LTGRAY);
 canvas.drawLine(baseX, topY, baseX + textWidth, topY, mPaint);
 canvas.drawText("top", baseX + textWidth, topY, mPaint);
 // AscentLine描画
 mPaint.setColor(Color.GREEN);
 canvas.drawLine(baseX, ascentY, baseX + textWidth, ascentY, mPaint);
 canvas.drawText("ascent", baseX + textWidth, ascentY + 10, mPaint);
 // DescentLine描画
 mPaint.setColor(Color.YELLOW);
 canvas.drawLine(baseX, descentY, baseX + textWidth, descentY, mPaint);
 canvas.drawText("descent", baseX + textWidth, descentY, mPaint);
 // ButtomLine描画
 mPaint.setColor(Color.MAGENTA);
 canvas.drawLine(baseX, bottomY, baseX + textWidth, bottomY, mPaint);
 canvas.drawText("buttom", baseX + textWidth, bottomY + 10, mPaint);

相信通过以上程序，能够很好的理解topLine，buttomLine，baseLine，ascentLine，descentLine。

另外：Paint类有两个方法

 /**
  * Return the distance above (negative) the baseline (ascent) based on the
  * current typeface and text size.
  *
  * @return the distance above (negative) the baseline (ascent) based on the
  *         current typeface and text size.
  */
 public native float ascent();

 /**
  * Return the distance below (positive) the baseline (descent) based on the
  * current typeface and text size.
  *
  * @return the distance below (positive) the baseline (descent) based on
  *         the current typeface and text size.
  */
 public native float descent();

ascent()：the distance above the baseline（baseline以上的height）

descent()：the distance below the baseline（baseline以下的height）

所以ascent() + descent() 可以看成文字的height。

到此为止，怎么获取文字的height和width都已经揭晓了：

获取height ： mPaint.ascent() + mPaint.descent()

获取width ： mPaint.measureText(text)