Canvas 绘制文本时，使用FontMetrics对象，计算位置的坐标。
1 public static class FontMetrics {
2 /**
3 * The maximum distance above the baseline for the tallest glyph in
4 * the font at a given text size.
5 */
6 public float top;
7 /**
8 * The recommended distance above the baseline for singled spaced text.
9 */
10 public float ascent;
11 /**
12 * The recommended distance below the baseline for singled spaced text.
13 */
14 public float descent;
15 /**
16 * The maximum distance below the baseline for the lowest glyph in
17 * the font at a given text size.
18 */
19 public float bottom;
20 /**
21 * The recommended additional space to add between lines of text.
22 */
23 public float leading;
24 }
它的各基准线可以参考下图：
1 /** 绘制FontMetrics对象的各种线 */
2 mPaint.reset();
3 mPaint.setColor(Color.WHITE);
4 mPaint.setTextSize(80);
5 // FontMetrics对象
6 FontMetrics fontMetrics = mPaint.getFontMetrics();
7 String text = "abcdefg";
8 // 计算每一个坐标
9 float textWidth = mPaint.measureText(text);
10 float baseX = 30;
11 float baseY = 700;
12 float topY = baseY + fontMetrics.top;
13 float ascentY = baseY + fontMetrics.ascent;
14 float descentY = baseY + fontMetrics.descent;
15 float bottomY = baseY + fontMetrics.bottom;
16 // 绘制文本
17 canvas.drawText(text, baseX, baseY, mPaint);
18 // BaseLine描画
19 mPaint.setColor(Color.RED);
20 canvas.drawLine(baseX, baseY, baseX + textWidth, baseY, mPaint);
21 mPaint.setTextSize(20);
22 canvas.drawText("base", baseX + textWidth, baseY, mPaint);
23 // Base描画
24 canvas.drawCircle(baseX, baseY, 5, mPaint);
25 // TopLine描画
26 mPaint.setColor(Color.LTGRAY);
27 canvas.drawLine(baseX, topY, baseX + textWidth, topY, mPaint);
28 canvas.drawText("top", baseX + textWidth, topY, mPaint);
29 // AscentLine描画
30 mPaint.setColor(Color.GREEN);
31 canvas.drawLine(baseX, ascentY, baseX + textWidth, ascentY, mPaint);
32 canvas.drawText("ascent", baseX + textWidth, ascentY + 10, mPaint);
33 // DescentLine描画
34 mPaint.setColor(Color.YELLOW);
35 canvas.drawLine(baseX, descentY, baseX + textWidth, descentY, mPaint);
36 canvas.drawText("descent", baseX + textWidth, descentY, mPaint);
37 // ButtomLine描画
38 mPaint.setColor(Color.MAGENTA);
39 canvas.drawLine(baseX, bottomY, baseX + textWidth, bottomY, mPaint);
40 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
1 /**
2 * Return the distance above (negative) the baseline (ascent) based on the
3 * current typeface and text size.
4 *
5 * @return the distance above (negative) the baseline (ascent) based on the
6 * current typeface and text size.
7 */
8 public native float ascent();
9
10 /**
11 * Return the distance below (positive) the baseline (descent) based on the
12 * current typeface and text size.
13 *
14 * @return the distance below (positive) the baseline (descent) based on
15 * the current typeface and text size.
16 */
17 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)