ScrollView的fillViewPort属性

ScrollView嵌套Relative时候会发生问题，RelativeLayout不会充满ScrollView，即使设置match_parent属性也不行

这个时候就需要fillViewPort属性登场了，在ScrollView的源码的onMeasure中，专门针对这个属性做了特殊的处理

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);

    if (!mFillViewport) {
        return;
    }

    final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
    if (heightMode == MeasureSpec.UNSPECIFIED) {
        return;
    }

    if (getChildCount() > 0) {
        final View child = getChildAt(0);
        final int widthPadding;
        final int heightPadding;
        final int targetSdkVersion = getContext().getApplicationInfo().targetSdkVersion;
        final FrameLayout.LayoutParams lp = (LayoutParams) child.getLayoutParams();
        if (targetSdkVersion >= VERSION_CODES.M) {
            widthPadding = mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin;
            heightPadding = mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin;
        } else {
            widthPadding = mPaddingLeft + mPaddingRight;
            heightPadding = mPaddingTop + mPaddingBottom;
        }

        final int desiredHeight = getMeasuredHeight() - heightPadding;
        if (child.getMeasuredHeight() < desiredHeight) {
            final int childWidthMeasureSpec = getChildMeasureSpec(
                    widthMeasureSpec, widthPadding, lp.width);
            final int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                    desiredHeight, MeasureSpec.EXACTLY);
            child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        }
    }
}

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