上一篇文章，學習了ViewGroup和View的measure流程。文章最後講到，本文將會學習ViewGroup和普通View的onMeasure方法的工作。

       因為ViewGroup是抽象類，它並沒有實現onMeasure方法，而是選擇讓它的子類實現。之前學習DecorView的measure過程時，說到DecorView的父類繼承自FrameLayout，FrameLayout又繼承自ViewGroup，並且重寫了onMeasure方法。那我們就來看一下FrameLayout的onMeasure方法。

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int count = getChildCount();

      //...

        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (mMeasureAllChildren || child.getVisibility() != GONE) {
                measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                maxWidth = Math.max(maxWidth,
                        child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
                maxHeight = Math.max(maxHeight,
                        child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
                childState = combineMeasuredStates(childState, child.getMeasuredState());
                if (measureMatchParentChildren) {
                    if (lp.width == LayoutParams.MATCH_PARENT ||
                            lp.height == LayoutParams.MATCH_PARENT) {
                        mMatchParentChildren.add(child);
                    }
                }
            }
        }

        // Account for padding too
        maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
        maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

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

      //...

        count = mMatchParentChildren.size();
        if (count > 1) {
            for (int i = 0; i < count; i++) {
                final View child = mMatchParentChildren.get(i);
                final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

                final int childWidthMeasureSpec;
                if (lp.width == LayoutParams.MATCH_PARENT) {
                    final int width = Math.max(0, getMeasuredWidth()
                            - getPaddingLeftWithForeground() - getPaddingRightWithForeground()
                            - lp.leftMargin - lp.rightMargin);
                    childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
                            width, MeasureSpec.EXACTLY);
                } else {
                    childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                            getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
                            lp.leftMargin + lp.rightMargin,
                            lp.width);
                }

                final int childHeightMeasureSpec;
                if (lp.height == LayoutParams.MATCH_PARENT) {
                    final int height = Math.max(0, getMeasuredHeight()
                            - getPaddingTopWithForeground() - getPaddingBottomWithForeground()
                            - lp.topMargin - lp.bottomMargin);
                    childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                            height, MeasureSpec.EXACTLY);
                } else {
                    childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
                            getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
                            lp.topMargin + lp.bottomMargin,
                            lp.height);
                }

                child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
            }
        }
    }
 

         從程式碼可以看到，child.measure方法的呼叫引數childWidthMeasureSpec和childHeightMeasureSpec的生成，是與父容器的MeasureSpec值和child的LayoutParams有關的。很明顯子元素的測量值，是去除了子元素本身的margin和padding值的。檢視LinearLayout和RelativeLayout的原始碼，可以發現它們的onMeasure方法中，呼叫child.measure方法的呼叫引數也是去除了子元素本身的margin和padding值的。由此，我們可以知道，在自定義ViewGroup的時候，我們需要在重寫的onMeasure方法中處理子元素的margin和padding值。注意到上述程式碼中子元素的childWidthMeasureSpec值可以是呼叫了getChildMeasureSpec獲得的（childHeightMeasureSpec同理）。下面我們看一下它的原始碼：

 //VIewGroup.java 6602行
  public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec);
        int specSize = MeasureSpec.getSize(spec);

        int size = Math.max(0, specSize - padding);

        int resultSize = 0;
        int resultMode = 0;

        switch (specMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size. So be it.
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent has imposed a maximum size on us
        case MeasureSpec.AT_MOST:
            if (childDimension >= 0) {
                // Child wants a specific size... so be it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size, but our size is not fixed.
                // Constrain child to not be bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent asked to see how big we want to be
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                // Child wants a specific size... let him have it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size... find out how big it should
                // be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size.... find out how
                // big it should be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        //noinspection ResourceType
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
  }

        從上面的getChildMeasureSpec方法原始碼（這裡我們依然不分析UNSPECIFIED模式），結合上面FrameLayout（或者LinearLayout/RelativeLayout）的onMeasure方法，可以知道，getChildMeasureSpec方法的三個引數是：父容器的測量規格值、子元素的LayoutParams中的margin和子元素的padding值的和、子元素的LayoutParams中的width（或者LayoutParams中的height）。

        從原始碼可以總結出子元素MeasureSpec建立的規律，如下表。表格中，第一行表示父容器的測量模式，第一列表示子元素在父容器的約束下，子元素LayoutParams的值：

        其中：parentSize是值父容器的測量值的大小減去子元素margin和padding佔用的值。childSize就是子元素的LayoutParams中的width或者LayoutParams中的height的值。

下面我們看一下，ViewGroup給我們提供的一些關於Measure過程的方法。

//ViewGroup.java  6522行
 protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
     final int size = mChildrenCount;
     final View[] children = mChildren;
     for (int i = 0; i < size; ++i) {
          final View child = children[i];
          if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
                measureChild(child, widthMeasureSpec, heightMeasureSpec);
          }
     }
 }

 //ViewGroup.java  6542行 
  protected void measureChild(View child, int parentWidthMeasureSpec,
            int parentHeightMeasureSpec) {
      final LayoutParams lp = child.getLayoutParams();

      final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight, lp.width);
      final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom, lp.height);

      child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
  }

        ViewGroup能夠擁有子元素，所以它的measure過程除了測量自己的大小以外，還需要遍歷去呼叫所有子元素的measure方法。遍歷子元素經行測量這個過程可以通過呼叫measureChildren方法實現，也可以自己在自定義的ViewGroup的onMeasure方法中實現。 

        measureChildren方法很簡單，就是遍歷ViewGroup的子元素，再根據子元素的可見狀態呼叫measureChild方法。在measureChild方法中建立子元素的MeasureSpec，然後呼叫子元素的measure方法，如果子元素是ViewGroup那麼重複這個過程。如果子元素是View，那就回到了View的measure過程了。

 //VIewGroup.java  6568行
  protected void measureChildWithMargins(View child,
      int parentWidthMeasureSpec, int widthUsed,
      int parentHeightMeasureSpec, int heightUsed) {
      final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

      final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
                        + widthUsed, lp.width);
      final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
                        + heightUsed, lp.height);

      child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
  }

        自定義ViewGroup的時候，可以自己實現對子元素的margin和padding的處理，也可以呼叫在這個方法。

       普通View的onMeasure方法相對ViewGroup簡單些，根據父容器剩餘空間，處理普通View的margin和padding，如果有額外的需求，加上這些需求的處理計算出普通View的寬高，呼叫setMeasuredDimension設定普通View全域性寬高值即可。