LinearLayout.onMeasure-設定LinearLayout的高度

    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);
    }

1. 判斷allFillParent以及widthMode，如果滿足的話，那麼maxWidth則設為alternativeMaxWidth
2. maxWidth再加上左右的間距
3. 比較和背景圖的大小
4. 通過resolveSizeAndState獲取widthMeasureSpec，通過setMeasuredDimension設定LinearLayout的寬高