Android View 相关源码分析之四 LinearLayout源码分析

(Updated: )
android
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LinearLayout 源码分析

measure过程

主要过程

  • 根据布局方向选择measure过程分支
  • 初始化相关变量
  • 对View进行第一次测量
  • mTotalLength的再次测量
  • 二次测量部分View和对为测量的子View进行测量
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protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    //判断布局方向
    if (mOrientation == VERTICAL) {
        measureVertical(widthMeasureSpec, heightMeasureSpec);
    } else {
        measureHorizontal(widthMeasureSpec, heightMeasureSpec);
    }
}

measureVertical和measureHorizontal只是布局方向上的区别 以下主要分析measureVertical方法

初始化相关变量

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//mTotalLength是记录内部使用的高度也就是子View的高度和 而不是LinearLayout的高度
mTotalLength = 0;
//子视图的最大宽度(不包括layout_weight>0的子View)
int maxWidth = 0;
int childState = 0;
int alternativeMaxWidth = 0;
//子视图的最大宽度(仅包含layout_weight>0的子View)
int weightedMaxWidth = 0;
//子视图是否均为fillParent 用于判断是否需要重新计算
boolean allFillParent = true;
//权重值的总和
float totalWeight = 0;
//子View的数量(统一级别下)
final int count = getVirtualChildCount();
//高度宽度模式
final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
//子View的宽度是否需要由父View决定
boolean matchWidth = false;
boolean skippedMeasure = false;
//第几个子View的baseLine作为LinearLayout的基准线
final int baselineChildIndex = mBaselineAlignedChildIndex;  
//mUseLargestChild为是否使用最大子元素的尺寸作为标准再次测量
final boolean useLargestChild = mUseLargestChild;
//子View中最高高度
int largestChildHeight = Integer.MIN_VALUE;

第一次测量

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// See how tall everyone is. Also remember max width.
for (int i = 0; i < count; ++i) {
    final View child = getVirtualChildAt(i);

    // 测量为null的子视图的高度
    // measureNullChild() 暂时返回 0 便于扩展
    if (child == null) {
        mTotalLength += measureNullChild(i);
        continue;
    }
    //Visibility为Gone的时候跳过该View
    // getChildrenSkipCount()方法同样返回0 便于扩展
    if (child.getVisibility() == View.GONE) {
       i += getChildrenSkipCount(child, i);
       continue;
    }
    //根据showDivider的值(通过hasDividerBeforeChildAt()) 来决定当前子View是否需要添加分割线的高度
    if (hasDividerBeforeChildAt(i)) {
        mTotalLength += mDividerHeight;
    }

    //会将子view的LayoutParams强转为父View的LayoutParams类型
    LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams)child.getLayoutParams();

    totalWeight += lp.weight;

    if (heightMode == MeasureSpec.EXACTLY && lp.height == 0 && lp.weight > 0) {
        // 满足该条件的话 不需要现在计算该子视图的高度 测量工作会在之后进行
        // 若子View的height=0 且weight> 0 则说明该View希望使用的是LinearLayout的剩余空间
        // LinearLayout是EXACTLY模式的说明LinearLayout高度已经确定 不需要依赖子View的测量结果来计算自己 就无需测量该子View

        final int totalLength = mTotalLength;
        mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
        skippedMeasure = true;
    } else {
        //测量子View

        int oldHeight = Integer.MIN_VALUE;

        //当前View的height=0 且weight> 0 则说明该LinearLayout的高度需要靠子View测量(不需要的在上面分支处理了)
        //将子View的高度设为-1 防止子View高度为0
        if (lp.height == 0 && lp.weight > 0) {
            oldHeight = 0;
            lp.height = LayoutParams.WRAP_CONTENT;
        }

        //调用子View的measureChildWithMargins() 对子View进行测量
        //第四个参数表示当前已使用的宽度  因为是竖直模式 所以为0
        //最后一个参数表示已使用的高度 如果之前的子View或者当前的View有weight属性 则当前子视图使用 LinearLayout 的所有高度 已使用的高度为0
        measureChildBeforeLayout(child, i, widthMeasureSpec, 0, heightMeasureSpec,
               totalWeight == 0 ? mTotalLength : 0);

        if (oldHeight != Integer.MIN_VALUE) {
           //测量完成后 重置子View高度
           lp.height = oldHeight;
        }

        final int childHeight = child.getMeasuredHeight();
        final int totalLength = mTotalLength;
        // 比较child测量前后总高度 取较大值
        ///getNextLocationOffset() 返回0 便于扩展
        mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
        // 设置最高子视图大小
        if (useLargestChild) {
            largestChildHeight = Math.max(childHeight, largestChildHeight);
        }
    }

     // mBaselineChildTop 表示指定的 baseline 的子视图的顶部高度
    if ((baselineChildIndex >= 0) && (baselineChildIndex == i + 1)) {
       mBaselineChildTop = mTotalLength;
    }

    // 设置为 baseline 的子视图的前面不允许设置 weiget 属性
    if (i < baselineChildIndex && lp.weight > 0) {
        throw new RuntimeException("A child of LinearLayout with index "
                + "less than mBaselineAlignedChildIndex has weight > 0, which "
                + "won't work.  Either remove the weight, or don't set "
                + "mBaselineAlignedChildIndex.");
    }

    // 宽度测量相关

    boolean matchWidthLocally = false;

    //当LinearLayout非EXACTLY模式 并且自View为MATCH_PARENT时
    //设置matchWidth和matchWidthLocally为true
    //该子View占据LinearLayout水平方向上所有空间

    if (widthMode != MeasureSpec.EXACTLY && lp.width == LayoutParams.MATCH_PARENT) {
        matchWidth = true;
        matchWidthLocally = true;
    }

    final int margin = lp.leftMargin + lp.rightMargin;
    final int measuredWidth = child.getMeasuredWidth() + margin;

    //对一堆变量赋值
    maxWidth = Math.max(maxWidth, measuredWidth);
    childState = combineMeasuredStates(childState, child.getMeasuredState());

    allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;
    if (lp.weight > 0) {
        weightedMaxWidth = Math.max(weightedMaxWidth,
                matchWidthLocally ? margin : measuredWidth);
    } else {
        alternativeMaxWidth = Math.max(alternativeMaxWidth,
                matchWidthLocally ? margin : measuredWidth);
    }

    i += getChildrenSkipCount(child, i);
}

二次测量mTotalLength

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//根据hasDividerBeforeChildAt得到showDivider的值是否为end 来判断是否需要加上divider的高度
if (mTotalLength > 0 && hasDividerBeforeChildAt(count))
    mTotalLength += mDividerHeight;
}
//如果高度测量模式为AT_MOST或者UNSPECIFIED 则进行二次测量 且设置了measureWithLargestChild
if (useLargestChild && (heightMode == MeasureSpec.AT_MOST ||
    heightMode == MeasureSpec.UNSPECIFIED)) {
    mTotalLength = 0;
    for (int i = 0; i < count; ++i) {
        final View child = getVirtualChildAt(i);
        if (child == null) {
            mTotalLength += measureNullChild(i);
            continue;
        }
        if (child.getVisibility() == GONE) {
            i += getChildrenSkipCount(child, i);
            continue;
        }
        final LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams)
                child.getLayoutParams();
        // 计算所有子View的高度之和
        final int totalLength = mTotalLength;
        mTotalLength = Math.max(totalLength, totalLength + largestChildHeight +
                lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
    }
}

就是需要useLargestChild

而 mUseLargestChild = a.getBoolean(R.styleable.LinearLayout_measureWithLargestChild, false);

就是说仅在LinearLayout的measureWithLargestChild属性设置为True时(默认为false)才可能出现某个child被二次测量

实例如下

LinearLayout二次测量

二次测量部分View和对为测量的子View进行测量

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//加上padding的值
mTotalLength += mPaddingTop + mPaddingBottom;
int heightSize = mTotalLength;
//minHeight和当前使用的高度比较取较大值
heightSize = Math.max(heightSize, getSuggestedMinimumHeight());

//根据heightMeasureSpec协助计算heightSizeAndState的大小
//resolveSizeAndState方法之后会分析
int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);
heightSize = heightSizeAndState & MEASURED_SIZE_MASK;

// Either expand children with weight to take up available space or
// shrink them if they extend beyond our current bounds. If we skipped
// measurement on any children, we need to measure them now.



//delta为额外的空间 及LinearLayout中未被分配的空间(可以为负)
int delta = heightSize - mTotalLength;
if (skippedMeasure || delta != 0 && totalWeight > 0.0f) {
    //skippedMeasure为第一次测量下对跳过测量的子View设置的
    //weightSum为权重和 如果设置了总权重则使用我们所设置的  如果没有则使用子View的weight和
    float weightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;

    mTotalLength = 0;
    //测量什么的
    for (int i = 0; i < count; ++i) {
        final View child = getVirtualChildAt(i);

        if (child.getVisibility() == View.GONE) {
            continue;
        }

        LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();

        float childExtra = lp.weight;
        if (childExtra > 0) {
            // Child said it could absorb extra space -- give him his share
            //计算weight属性分配的大小
            int share = (int) (childExtra * delta / weightSum);
            //权重和减去已经分配权重
            weightSum -= childExtra;
            //剩余高度减去分配的高度
            delta -= share;

            final int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                    mPaddingLeft + mPaddingRight +
                            lp.leftMargin + lp.rightMargin, lp.width);

            // TODO: Use a field like lp.isMeasured to figure out if this
            // child has been previously measured


            if ((lp.height != 0) || (heightMode != MeasureSpec.EXACTLY)) {
                //子视图已经被测量过
                //非EXACTLY view需要加上share
                int childHeight = child.getMeasuredHeight() + share;
                if (childHeight < 0) {
                    childHeight = 0;
                }
                //重新测量View
                child.measure(childWidthMeasureSpec,
                        MeasureSpec.makeMeasureSpec(childHeight, MeasureSpec.EXACTLY));
            } else {
                //如果当前是EXACTLY模式 说明没有被测量 需要进行测量
                //子视图首次被测量
                //EXACTLY模式下 将weight占比的高度分配给子View    
                child.measure(childWidthMeasureSpec,
                        MeasureSpec.makeMeasureSpec(share > 0 ? share : 0,
                                MeasureSpec.EXACTLY));
            }

            // Child may now not fit in vertical dimension.
            childState = combineMeasuredStates(childState, child.getMeasuredState()
                    & (MEASURED_STATE_MASK>>MEASURED_HEIGHT_STATE_SHIFT));
        }

       //处理子视图宽度
        final int margin =  lp.leftMargin + lp.rightMargin;
        final int measuredWidth = child.getMeasuredWidth() + margin;
        maxWidth = Math.max(maxWidth, measuredWidth);

        boolean matchWidthLocally = widthMode != MeasureSpec.EXACTLY &&
                lp.width == LayoutParams.MATCH_PARENT;

        alternativeMaxWidth = Math.max(alternativeMaxWidth,
                matchWidthLocally ? margin : measuredWidth);

        allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;

        final int totalLength = mTotalLength;
        mTotalLength = Math.max(totalLength, totalLength + child.getMeasuredHeight() +
                lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
    }

    // Add in our padding
    mTotalLength += mPaddingTop + mPaddingBottom;
    // TODO: Should we recompute the heightSpec based on the new total length?
} else {
    alternativeMaxWidth = Math.max(alternativeMaxWidth,
                                   weightedMaxWidth);


    // We have no limit, so make all weighted views as tall as the largest child.
    // Children will have already been measured once.
    if (useLargestChild && heightMode != MeasureSpec.EXACTLY) {
        for (int i = 0; i < count; i++) {
            final View child = getVirtualChildAt(i);

            if (child == null || child.getVisibility() == View.GONE) {
                continue;
            }

            final LinearLayout.LayoutParams lp =
                    (LinearLayout.LayoutParams) child.getLayoutParams();

            float childExtra = lp.weight;
            if (childExtra > 0) {
               //使用最大子视图高度测量
                child.measure(
                        MeasureSpec.makeMeasureSpec(child.getMeasuredWidth(),
                                MeasureSpec.EXACTLY),
                        MeasureSpec.makeMeasureSpec(largestChildHeight,
                                MeasureSpec.EXACTLY));
            }
        }
    }
}

if (!allFillParent && widthMode != MeasureSpec.EXACTLY) {
    maxWidth = alternativeMaxWidth;
}

maxWidth += mPaddingLeft + mPaddingRight;

// Check against our minimum width
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
        heightSizeAndState);

if (matchWidth) {
    forceUniformWidth(count, heightMeasureSpec);
}

resolveSizeAndState方法 定义在View中

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/**
   * Utility to reconcile a desired size and state, with constraints imposed
   * by a MeasureSpec. Will take the desired size, unless a different size
   * is imposed by the constraints. The returned value is a compound integer,
   * with the resolved size in the {@link #MEASURED_SIZE_MASK} bits and
   * optionally the bit {@link #MEASURED_STATE_TOO_SMALL} set if the
   * resulting size is smaller than the size the view wants to be.
   *
   * @param size How big the view wants to be.
   * @param measureSpec Constraints imposed by the parent.
   * @param childMeasuredState Size information bit mask for the view's
   *                           children.
   * @return Size information bit mask as defined by
   *         {@link #MEASURED_SIZE_MASK} and
   *         {@link #MEASURED_STATE_TOO_SMALL}.
   */
  public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {
      final int specMode = MeasureSpec.getMode(measureSpec);
      final int specSize = MeasureSpec.getSize(measureSpec);
      final int result;
      switch (specMode) {
          case MeasureSpec.AT_MOST:
              if (specSize < size) {
                  result = specSize | MEASURED_STATE_TOO_SMALL;
              } else {
                  result = size;
              }
              break;
          case MeasureSpec.EXACTLY:
              result = specSize;
              break;
          case MeasureSpec.UNSPECIFIED:
          default:
              result = size;
      }
      return result | (childMeasuredState & MEASURED_STATE_MASK);
  }
delta为负的相关解析

相关代码及效果如下

负delta.png

根据之前的measure流程分析一下

  • 相关变量初始化
  • 第一次测量 两个子TextView都会被测量 TextView1.height = TextView1.height = 500dp 则mToatalLength为1000dp
  • mToatalLength再次测量跳过
  • 计算delta delta = heightSize - mTotalLength 根据resolveSizeAndState方法 父LinearLayout是EXACTLY模式 所以最终heightSize为500dp delta = -500dp
  • 根据weight分配剩余空间 TextView1.height = 500 + 1 / 5 * (- 500) = 400 dp
    TextView2.height = 500 + 4 / 5 * (- 500) = 100 dp

layout过程

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protected void onLayout(boolean changed, int l, int t, int r, int b) {
  if (mOrientation == VERTICAL) {
      layoutVertical(l, t, r, b);
  } else {
      layoutHorizontal(l, t, r, b);
  }
}

我们可以看出 同样是分成水平和竖直两个方向的 同样分析竖直 方向下的layout过程

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/**
    * Position the children during a layout pass if the orientation of this
    * LinearLayout is set to {@link #VERTICAL}.
    *
    * @see #getOrientation()
    * @see #setOrientation(int)
    * @see #onLayout(boolean, int, int, int, int)
    * @param left
    * @param top
    * @param right
    * @param bottom
    */
   void layoutVertical(int left, int top, int right, int bottom) {
       final int paddingLeft = mPaddingLeft;

       int childTop;
       int childLeft;

       //父View默认子View的宽度
       final int width = right - left;
       //子View的右侧默认位置
       int childRight = width - mPaddingRight;

       // 子View的可用空间大小
       int childSpace = width - paddingLeft - mPaddingRight;

       //子View的个数
       final int count = getVirtualChildCount();

       final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
       final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;

       //根据LinearLayout设置的对其方式 设置第一个子View的Top值
       switch (majorGravity) {
          case Gravity.BOTTOM:
              // mTotalLength contains the padding already
              childTop = mPaddingTop + bottom - top - mTotalLength;
              break;

              // mTotalLength contains the padding already
          case Gravity.CENTER_VERTICAL:
              childTop = mPaddingTop + (bottom - top - mTotalLength) / 2;
              break;

          case Gravity.TOP:
          default:
              childTop = mPaddingTop;
              break;
       }

       //遍历各个子View
       for (int i = 0; i < count; i++) {
           final View child = getVirtualChildAt(i);
           if (child == null) {
               childTop += measureNullChild(i);
           } else if (child.getVisibility() != GONE) {
                //LinearLayout中子View的宽和高有measure过程决定
               final int childWidth = child.getMeasuredWidth();
               final int childHeight = child.getMeasuredHeight();
               //获取子View的LayoutParams
               final LinearLayout.LayoutParams lp =
                       (LinearLayout.LayoutParams) child.getLayoutParams();

               int gravity = lp.gravity;
               if (gravity < 0) {
                   gravity = minorGravity;
               }
               final int layoutDirection = getLayoutDirection();
               final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
               //根据子View的对其方式设置Left值
               switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
                   case Gravity.CENTER_HORIZONTAL:
                       childLeft = paddingLeft + ((childSpace - childWidth) / 2)
                               + lp.leftMargin - lp.rightMargin;
                       break;

                   case Gravity.RIGHT:
                       childLeft = childRight - childWidth - lp.rightMargin;
                       break;

                   case Gravity.LEFT:
                   default:
                       childLeft = paddingLeft + lp.leftMargin;
                       break;
               }
               //如果有分割线 添加分割线的高度
               if (hasDividerBeforeChildAt(i)) {
                   childTop += mDividerHeight;
               }
               //子View的top修改
               childTop += lp.topMargin;
               //用setChildFrame()方法设置子控件控件的在父控件上的坐标轴
               setChildFrame(child, childLeft, childTop + getLocationOffset(child),
                       childWidth, childHeight);
               childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);

               i += getChildrenSkipCount(child, i);
           }
       }
   }

draw 源码分析

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protected void onDraw(Canvas canvas) {
  if (mDivider == null) {
      return;
  }

  if (mOrientation == VERTICAL) {
      drawDividersVertical(canvas);
  } else {
      drawDividersHorizontal(canvas);
  }
}

同样主要分析垂直方向的处理

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void drawDividersVertical(Canvas canvas) {
    final int count = getVirtualChildCount();
    //根据计算好的坐标绘制对应的子View
    for (int i = 0; i < count; i++) {
        final View child = getVirtualChildAt(i);

        if (child != null && child.getVisibility() != GONE) {
            if (hasDividerBeforeChildAt(i)) {
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                final int top = child.getTop() - lp.topMargin - mDividerHeight;
                drawHorizontalDivider(canvas, top);
            }
        }
    }
    //绘制分割线
    if (hasDividerBeforeChildAt(count)) {
        final View child = getLastNonGoneChild();
        int bottom = 0;
        if (child == null) {
            bottom = getHeight() - getPaddingBottom() - mDividerHeight;
        } else {
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            bottom = child.getBottom() + lp.bottomMargin;
        }
        drawHorizontalDivider(canvas, bottom);
    }
}

void drawHorizontalDivider(Canvas canvas, int top) {
  mDivider.setBounds(getPaddingLeft() + mDividerPadding, top,
          getWidth() - getPaddingRight() - mDividerPadding, top + mDividerHeight);
  mDivider.draw(canvas);
}