FontMetrics类源码：
/**
* Class that describes the various metrics for a font at a given text size.
* Remember, Y values increase going down, so those values will be positive,
* and values that measure distances going up will be negative. This class
* is returned by getFontMetrics().
*/
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;
}
FontMetrics类是Paint的内部类，主要定义了绘制字体相关的度量（参数）：
11.gif
22.png
然而top和bottom都要比ascent和descent要更远离baseline，这种情况的原因是为了兼容诸如拉丁字符等字符。
其他的就是TextPaint中的各种方法用于设置绘制字体的各种属性。
坑：在drawtext时发现，颜色设置Paint.setColor如果是0x000000这样的rgb就不显示，必须设置成0x000000ff这样argb才可以。要么就是Color.argb(int,int,int,int)来取得颜色.
