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由于一些不相关的原因，我正在查看 ViewPager 的代码，我看到它覆盖了 ViewGroup 的 onMeasure。我一直在尝试学习如何正确实施 onMeasure，所以我决定仔细研究一下。以下是参考的方法-

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    // For simple implementation, our internal size is always 0.
    // We depend on the container to specify the layout size of
    // our view.  We can't really know what it is since we will be
    // adding and removing different arbitrary views and do not
    // want the layout to change as this happens.
    setMeasuredDimension(getDefaultSize(0, widthMeasureSpec),
            getDefaultSize(0, heightMeasureSpec));

    final int measuredWidth = getMeasuredWidth();
    final int maxGutterSize = measuredWidth / 10;
    mGutterSize = Math.min(maxGutterSize, mDefaultGutterSize);

    // Children are just made to fill our space.
    int childWidthSize = measuredWidth - getPaddingLeft() - getPaddingRight();
    int childHeightSize = getMeasuredHeight() - getPaddingTop() - getPaddingBottom();

    /*
     * Make sure all children have been properly measured. Decor views first.
     * Right now we cheat and make this less complicated by assuming decor
     * views won't intersect. We will pin to edges based on gravity.
     */
    int size = getChildCount();
    for (int i = 0; i < size; ++i) {
        final View child = getChildAt(i);
        if (child.getVisibility() != GONE) {
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            if (lp != null && lp.isDecor) {
                final int hgrav = lp.gravity & Gravity.HORIZONTAL_GRAVITY_MASK;
                final int vgrav = lp.gravity & Gravity.VERTICAL_GRAVITY_MASK;
                int widthMode = MeasureSpec.AT_MOST;
                int heightMode = MeasureSpec.AT_MOST;
                boolean consumeVertical = vgrav == Gravity.TOP || vgrav == Gravity.BOTTOM;
                boolean consumeHorizontal = hgrav == Gravity.LEFT || hgrav == Gravity.RIGHT;

                if (consumeVertical) {
                    widthMode = MeasureSpec.EXACTLY;
                } else if (consumeHorizontal) {
                    heightMode = MeasureSpec.EXACTLY;
                }

                int widthSize = childWidthSize;
                int heightSize = childHeightSize;
                if (lp.width != LayoutParams.WRAP_CONTENT) {
                    widthMode = MeasureSpec.EXACTLY;
                    if (lp.width != LayoutParams.MATCH_PARENT) {
                        widthSize = lp.width;
                    }
                }
                if (lp.height != LayoutParams.WRAP_CONTENT) {
                    heightMode = MeasureSpec.EXACTLY;
                    if (lp.height != LayoutParams.MATCH_PARENT) {
                        heightSize = lp.height;
                    }
                }
                final int widthSpec = MeasureSpec.makeMeasureSpec(widthSize, widthMode);
                final int heightSpec = MeasureSpec.makeMeasureSpec(heightSize, heightMode);
                child.measure(widthSpec, heightSpec);

                if (consumeVertical) {
                    childHeightSize -= child.getMeasuredHeight();
                } else if (consumeHorizontal) {
                    childWidthSize -= child.getMeasuredWidth();
                }
            }
        }
    }

    mChildWidthMeasureSpec = MeasureSpec.makeMeasureSpec(childWidthSize, MeasureSpec.EXACTLY);
    mChildHeightMeasureSpec = MeasureSpec.makeMeasureSpec(childHeightSize, MeasureSpec.EXACTLY);

    // Make sure we have created all fragments that we need to have shown.
    mInLayout = true;
    populate();
    mInLayout = false;

    // Page views next.
    size = getChildCount();
    for (int i = 0; i < size; ++i) {
        final View child = getChildAt(i);
        if (child.getVisibility() != GONE) {
            if (DEBUG) {
                Log.v(TAG, "Measuring #" + i + " " + child + ": " + mChildWidthMeasureSpec);
            }

            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            if (lp == null || !lp.isDecor) {
                final int widthSpec = MeasureSpec.makeMeasureSpec(
                        (int) (childWidthSize * lp.widthFactor), MeasureSpec.EXACTLY);
                child.measure(widthSpec, mChildHeightMeasureSpec);
            }
        }
    }
}

我无法理解处理 Decor 视图的重力背后的逻辑。据我了解，如果有两个子视图 - 一个具有左上角的重力，另一个具有右上角的重力，则代码将从childHeightSize中减去两个子视图的高度。这对我来说似乎是错误的，因为逻辑应该只减去具有最大高度的子视图的高度。

为什么要这样实现？

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