LinearLayout测量过程疑问

疑问：

在LinearLayout的测量方法measureHorizontal里，有个地方存在疑问，widthMode和isExactly赋初值之后，在后面的for循环整个流程里，其值并没有变化，按这个逻辑，如果下面第一份if语句能进去，第二个if则应该一直为true，else语句是永远不会执行的，但为何要这么写呢？

void measureHorizontal(int widthMeasureSpec, int heightMeasureSpec) {

final int widthMode = MeasureSpec.getMode(widthMeasureSpec);

final int heightMode = MeasureSpec.getMode(heightMeasureSpec);

…

final boolean isExactly =
widthMode == MeasureSpec.EXACTLY;

…

for (int i = 0; i < count; ++i) {

…

if (widthMode == MeasureSpec.EXACTLY && lp.width == 0 && lp.weight > 0) {

// Optimization: don't bother measuring children who are going to use

// leftover space. These views will get measured again down below if

// there is any leftover space.

if (isExactly) {

mTotalLength += lp.leftMargin + lp.rightMargin;

} else {

final int totalLength = mTotalLength;

mTotalLength = Math.max(totalLength, totalLength +

lp.leftMargin + lp.rightMargin);

}