Question

Span 有很多种，这里拿最简单的 CharacterStyle 来举例说明我们设置的 Span 最终是如何影响到文字绘制的。前面的代码中出现的 UnderlineSpan 就是 CharacterStyle 的子类之一，可以在 官网 上看到其他的子类。CharacterStyle 有一个抽象方法是 updateDrawState，下面是 UnderlineSpan 的简易版：

public class UnderlineSpan extends CharacterStyle 
  implements UpdateAppearance, ParcelableSpan {
  ...

  @Override
  public void updateDrawState(TextPaint ds) {
    ds.setUnderlineText(true);
  }
}

从这个实现来看，在文字绘制的时候会将绘制文字的 TextPaint 传进来，在这里更新其设置后就能实现对应的文字效果。从 TextView 源码分析 中可以看出在对 Span 的处理上分为两步：

TextLine tl = TextLine.obtain();

tl.set(paint, buf, start, end, dir, directions, hasTabOrEmoji, tabStops);
tl.draw(canvas, x, ltop, lbaseline, lbottom);

通过 TextLine.obtain() 从 shared pool 中获取一个 TextLine（这么做的原因是这个对象本身比较大而且使用次数多，为了避免多次创建导致的频繁 GC），然后通过 TextLine.set 对 TextLine 进行初始化，最后 draw 方法绘制出文字。从源码可以看出在 set 的时候保存了 Span：

void set(TextPaint paint, CharSequence text, int start, int limit, int dir,
      Directions directions, boolean hasTabs, TabStops tabStops) {
  mPaint = paint;
  mText = text;
  mStart = start;
  mLen = limit - start;
  mDir = dir;
  mDirections = directions;
  if (mDirections == null) {
    throw new IllegalArgumentException("Directions cannot be null");
  }
  mHasTabs = hasTabs;
  mSpanned = null;
  boolean hasReplacement = false;
  if (text instanceof Spanned) {
    mSpanned = (Spanned) text;   // 保存 span
    mReplacementSpanSpanSet.init(mSpanned, start, limit);
    hasReplacement = mReplacementSpanSpanSet.numberOfSpans > 0;
  }
  ...
}

然后是 draw 方法：

void draw(Canvas c, float x, int top, int y, int bottom) {
  if (!mHasTabs) {
    if (mDirections == Layout.DIRS_ALL_LEFT_TO_RIGHT) {
      drawRun(c, 0, mLen, false, x, top, y, bottom, false);
      return;
    }
    if (mDirections == Layout.DIRS_ALL_RIGHT_TO_LEFT) {
      drawRun(c, 0, mLen, true, x, top, y, bottom, false);
      return;
    }
  }
  ...
}

 private float drawRun(Canvas c, int start,
    int limit, boolean runIsRtl, float x, int top, int y, int bottom,
    boolean needWidth) {
  if ((mDir == Layout.DIR_LEFT_TO_RIGHT) == runIsRtl) {
    float w = -measureRun(start, limit, limit, runIsRtl, null);
    handleRun(start, limit, limit, runIsRtl, c, x + w, top,
        y, bottom, null, false);
    return w;
  }
  return handleRun(start, limit, limit, runIsRtl, c, x, top,
      y, bottom, null, needWidth);
}

private float handleRun(int start, int measureLimit,
      int limit, boolean runIsRtl, Canvas c, float x, int top, int y,
      int bottom, FontMetricsInt fmi, boolean needWidth) {
  ...
  // 解析 Span
  mCharacterStyleSpanSet.init(mSpanned, mStart + start, mStart + limit);
  ...
  for (int j = i, jnext; j < mlimit; j = jnext) {
    jnext = mCharacterStyleSpanSet.getNextTransition(mStart + j, mStart + inext) -
        mStart;
    int offset = Math.min(jnext, mlimit);
    wp.set(mPaint);
    // 遍历 Span
    for (int k = 0; k < mCharacterStyleSpanSet.numberOfSpans; k++) {
      // Intentionally using >= and <= as explained above
      if ((mCharacterStyleSpanSet.spanStarts[k] >= mStart + offset) ||
          (mCharacterStyleSpanSet.spanEnds[k] <= mStart + j)) continue;
      CharacterStyle span = mCharacterStyleSpanSet.spans[k];
      span.updateDrawState(wp);  // updateDrawState!!!
    }
    // Only draw hyphen on last run in line
    if (jnext < mLen) {
      wp.setHyphenEdit(0);
    }
    x += handleText(wp, j, jnext, i, inext, runIsRtl, c, x,
        top, y, bottom, fmi, needWidth || jnext < measureLimit, offset);
  }
  ...
}

几经周折到了 handleRun 这里，首先是 CharacterStyleSpanSet 的初始化，初始化的时候会将 mSpanned 中所带的 Span 都解析出来具体的过程可以看 SpanSet 的源码，然后使用 updateDrawState 更新 TextPaint 加入想要的效果，最后通过 handleText 绘制。