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)