Question

TextView 内部并不仅仅只有一个用来显示文本内容的 Layout，在设置了 hint 的时候，还需要有一个 mHintLayout 来处理 hint 的内容。如果设置了 Ellipsize 类型为 Marquee 时，还会有一个 mSavedMarqueeModeLayout 专门用来显示 marquee 效果。这些 Layout 都是通过内部的 makeNewLayout 方法来创建的：

protected void makeNewLayout(int wantWidth, int hintWidth
                 BoringLayout.Metrics boring,
                 BoringLayout.Metrics hintBoring,
                 int ellipsisWidth, boolean bringIntoView) {
    //如果当前有 marquee 动画，则先停止动画
    stopMarquee();

    mOldMaximum = mMaximum;
    mOldMaxMode = mMaxMode;

    mHighlightPathBogus = true;

    if (wantWidth < 0) {
      wantWidth = 0;
    }
    if (hintWidth < 0) {
      hintWidth = 0;
    }

    //文本对齐方式
    Layout.Alignment alignment = getLayoutAlignment();
    final boolean testDirChange = mSingleLine && mLayout != null &&
      (alignment == Layout.Alignment.ALIGN_NORMAL ||
       alignment == Layout.Alignment.ALIGN_OPPOSITE);
    int oldDir = 0;
    if (testDirChange) oldDir = mLayout.getParagraphDirection(0);
  
    //检测是否设置了 ellipsize
    boolean shouldEllipsize = mEllipsize != null && getKeyListener() == null;
    final boolean switchEllipsize = mEllipsize == TruncateAt.MARQUEE &&
        mMarqueeFadeMode != MARQUEE_FADE_NORMAL;
    TruncateAt effectiveEllipsize = mEllipsize;
    if (mEllipsize == TruncateAt.MARQUEE &&
        mMarqueeFadeMode == MARQUEE_FADE_SWITCH_SHOW_ELLIPSIS) {
      effectiveEllipsize = TruncateAt.END_SMALL;
    }
  
    //文本方向
    if (mTextDir == null) {
      mTextDir = getTextDirectionHeuristic();
    }

    //创建主 Layout
    mLayout = makeSingleLayout(wantWidth, boring, ellipsisWidth, alignment, shouldEllipsize,
        effectiveEllipsize, effectiveEllipsize == mEllipsize);
  
    //非常规的 Marquee 模式下，需要创建 mSavedMarqueeModeLayout 来保存 marquee 动画时所用的 Layout，并且在动画期间把它和 TextView 的主 Layout 对换
    if (switchEllipsize) {
      TruncateAt oppositeEllipsize = effectiveEllipsize == TruncateAt.MARQUEE ?
          TruncateAt.END : TruncateAt.MARQUEE;
      mSavedMarqueeModeLayout = makeSingleLayout(wantWidth, boring, ellipsisWidth, alignment,
          shouldEllipsize, oppositeEllipsize, effectiveEllipsize != mEllipsize);
    }

    shouldEllipsize = mEllipsize != null;
    mHintLayout = null;

    //判断是否需要创建 hintLayout
    if (mHint != null) {
      if (shouldEllipsize) hintWidth = wantWidth;

      if (hintBoring == UNKNOWN_BORING) {
        hintBoring = BoringLayout.isBoring(mHint, mTextPaint, mTextDir,
                           mHintBoring);
        if (hintBoring != null) {
          mHintBoring = hintBoring;
        }
      }

      //判断是否为 boring，如果是则创建 BoringLayout
      if (hintBoring != null) {
        if (hintBoring.width <= hintWidth &&
          (!shouldEllipsize || hintBoring.width <= ellipsisWidth)) {
          if (mSavedHintLayout != null) {
            mHintLayout = mSavedHintLayout.
                replaceOrMake(mHint, mTextPaint,
                hintWidth, alignment, mSpacingMult, mSpacingAdd,
                hintBoring, mIncludePad);
          } else {
            mHintLayout = BoringLayout.make(mHint, mTextPaint,
                hintWidth, alignment, mSpacingMult, mSpacingAdd,
                hintBoring, mIncludePad);
          }

          mSavedHintLayout = (BoringLayout) mHintLayout;
        } else if (shouldEllipsize && hintBoring.width <= hintWidth) {
          if (mSavedHintLayout != null) {
            mHintLayout = mSavedHintLayout.
                replaceOrMake(mHint, mTextPaint,
                hintWidth, alignment, mSpacingMult, mSpacingAdd,
                hintBoring, mIncludePad, mEllipsize,
                ellipsisWidth);
          } else {
            mHintLayout = BoringLayout.make(mHint, mTextPaint,
                hintWidth, alignment, mSpacingMult, mSpacingAdd,
                hintBoring, mIncludePad, mEllipsize,
                ellipsisWidth);
          }
        }
      }
      
      //不是 boring 的状态下，用 StaticLayout 来创建
      if (mHintLayout == null) {
        StaticLayout.Builder builder = StaticLayout.Builder.obtain(mHint, 0,
            mHint.length(), mTextPaint, hintWidth)
            .setAlignment(alignment)
            .setTextDirection(mTextDir)
            .setLineSpacing(mSpacingAdd, mSpacingMult)
            .setIncludePad(mIncludePad)
            .setBreakStrategy(mBreakStrategy)
            .setHyphenationFrequency(mHyphenationFrequency);
        if (shouldEllipsize) {
          builder.setEllipsize(mEllipsize)
              .setEllipsizedWidth(ellipsisWidth)
              .setMaxLines(mMaxMode == LINES ? mMaximum : Integer.MAX_VALUE);
        }
        mHintLayout = builder.build();
      }
    }

    if (bringIntoView || (testDirChange && oldDir != mLayout.getParagraphDirection(0))) {
      registerForPreDraw();
    }

    //判断是否需要开始 Marquee 动画
    if (mEllipsize == TextUtils.TruncateAt.MARQUEE) {
      if (!compressText(ellipsisWidth)) {
        final int height = mLayoutParams.height;
        if (height != LayoutParams.WRAP_CONTENT && height != LayoutParams.MATCH_PARENT) {
          startMarquee();
        } else {
          mRestartMarquee = true;
        }
      }
    }

    if (mEditor != null) mEditor.prepareCursorControllers();
  }

TextView 的布局创建过程涉及到一个 boring 的概念，boring 是指布局所用的文本里面不包含任何 Span，所有的文本方向都是从左到右的布局，并且仅需一行就能显示完全的布局。这种情况下，TextView 会使用 BoringLayout 类来创建相关的布局，以节省不必要的文本测量以及文本折行、Span 宽度、文本方向等的计算。下面我们来看一下 makeNewLayout 中使用频率比较高的 makeSingleLayout 的代码:

private Layout makeSingleLayout(int wantWidth, BoringLayout.Metrics boring, int ellipsisWidth,
      Layout.Alignment alignment, boolean shouldEllipsize, TruncateAt effectiveEllipsize,
      boolean useSaved) {
    Layout result = null;
    //判断是否 Spannable，如果是则用 DynamicLayout 类来创建布局，DynamicLayout 内部实际也是使用 StaticLayout 来做文本的测量绘制，并在 StaticLayout 的基础上增加了文本或者 Span 改变时的监听，及时对文本或者 Span 的变化做出反应。
    if (mText instanceof Spannable) {
      result = new DynamicLayout(mText, mTransformed, mTextPaint, wantWidth,
          alignment, mTextDir, mSpacingMult, mSpacingAdd, mIncludePad,
          mBreakStrategy, mHyphenationFrequency,
          getKeyListener() == null ? effectiveEllipsize : null, ellipsisWidth);
    } else {
      //如果 boring 是未知状态，则重新判断一次是否 boring
      if (boring == UNKNOWN_BORING) {
        boring = BoringLayout.isBoring(mTransformed, mTextPaint, mTextDir, mBoring);
        if (boring != null) {
          mBoring = boring;
        }
      }

      //根据 boring 的属性来创建对应的布局，如果有 mSavedLayout 则从 mSavedLayout 创建
      if (boring != null) {
        if (boring.width <= wantWidth &&
            (effectiveEllipsize == null || boring.width <= ellipsisWidth)) {
          if (useSaved && mSavedLayout != null) {
            //从之前保存的 Layout 中创建
            result = mSavedLayout.replaceOrMake(mTransformed, mTextPaint,
                wantWidth, alignment, mSpacingMult, mSpacingAdd,
                boring, mIncludePad);
          } else {
            //创建新的 Layout
            result = BoringLayout.make(mTransformed, mTextPaint,
                wantWidth, alignment, mSpacingMult, mSpacingAdd,
                boring, mIncludePad);
          }

          if (useSaved) {
            mSavedLayout = (BoringLayout) result;
          }
        } else if (shouldEllipsize && boring.width <= wantWidth) {
          if (useSaved && mSavedLayout != null) {
            result = mSavedLayout.replaceOrMake(mTransformed, mTextPaint,
                wantWidth, alignment, mSpacingMult, mSpacingAdd,
                boring, mIncludePad, effectiveEllipsize,
                ellipsisWidth);
          } else {
            result = BoringLayout.make(mTransformed, mTextPaint,
                wantWidth, alignment, mSpacingMult, mSpacingAdd,
                boring, mIncludePad, effectiveEllipsize,
                ellipsisWidth);
          }
        }
      }
    }
  
    //如果没有创建 BoringLayout, 则使用 StaticLayout 类来创建布局
    if (result == null) {
      StaticLayout.Builder builder = StaticLayout.Builder.obtain(mTransformed,
          0, mTransformed.length(), mTextPaint, wantWidth)
          .setAlignment(alignment)
          .setTextDirection(mTextDir)
          .setLineSpacing(mSpacingAdd, mSpacingMult)
          .setIncludePad(mIncludePad)
          .setBreakStrategy(mBreakStrategy)
          .setHyphenationFrequency(mHyphenationFrequency);
      if (shouldEllipsize) {
        builder.setEllipsize(effectiveEllipsize)
            .setEllipsizedWidth(ellipsisWidth)
            .setMaxLines(mMaxMode == LINES ? mMaximum : Integer.MAX_VALUE);
      }
      result = builder.build();
    }
    return result;
  }