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;
}

/** 繪制FontMetrics對象的各種線 */
mPaint.reset();
mPaint.setColor(Color.WHITE);
mPaint.setTextSize(80);
// FontMetrics對象
FontMetrics fontMetrics = mPaint.getFontMetrics();
String text = "abcdefg";
// 計算每一個坐標
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;
// 繪制文本
canvas.drawText(text, baseX, baseY, mPaint);
// BaseLine描畫
mPaint.setColor(Color.RED);
canvas.drawLine(baseX, baseY, baseX + textWidth, baseY, mPaint);
mPaint.setTextSize(20);
canvas.drawText("base", baseX + textWidth, baseY, mPaint);
// Base描畫
canvas.drawCircle(baseX, baseY, 5, mPaint);
// TopLine描畫
mPaint.setColor(Color.LTGRAY);
canvas.drawLine(baseX, topY, baseX + textWidth, topY, mPaint);
canvas.drawText("top", baseX + textWidth, topY, mPaint);
// AscentLine描畫
mPaint.setColor(Color.GREEN);
canvas.drawLine(baseX, ascentY, baseX + textWidth, ascentY, mPaint);
canvas.drawText("ascent", baseX + textWidth, ascentY + 10, mPaint);
// DescentLine描畫
mPaint.setColor(Color.YELLOW);
canvas.drawLine(baseX, descentY, baseX + textWidth, descentY, mPaint);
canvas.drawText("descent", baseX + textWidth, descentY, mPaint);
// ButtomLine描畫
mPaint.setColor(Color.MAGENTA);
canvas.drawLine(baseX, bottomY, baseX + textWidth, bottomY, mPaint);
canvas.drawText("buttom", baseX + textWidth, bottomY + 10, mPaint);

/**
 * 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();