ScrollView 內嵌 ListView 只能看到第一個Item

解決方案：

給ScrollView 設置 fillViewport 爲 true,ListView 就不會只顯示第一個了。

<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fillViewport="true"
    tools:context="com.example.fragment.ScrollAndListFragment">

    <ListView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:text="@string/hello_blank_fragment" />

</ScrollView>

原因：

ScrollView 設置 fillViewport 爲true，有什麼作用呢？

看下ScrollView 使用 fillViewport 的地方：

    @Override
    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);
            }
        }
    }

可以看到，在onMeasure 裏面，如果使用了fillViewport，那麼就會把ScrollView 的高度設置給裏面的子View.
這樣ListView 的高度就和ScrollView是一樣高的。ListView 就可以顯示多個Item

如果沒有設置fillViewport，那麼默認的ScrollView 傳遞給子View 的高度測量模式是UNSPECIFIED,可以看到ScrollView 重寫了measureChildWithMargins方法，指定了子View 的測量模式爲UNSPECIFIED

    @Override
    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 usedTotal = mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin +
                heightUsed;
        final int childHeightMeasureSpec = MeasureSpec.makeSafeMeasureSpec(
                Math.max(0, MeasureSpec.getSize(parentHeightMeasureSpec) - usedTotal),
                MeasureSpec.UNSPECIFIED);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

ListView 的 onMeasure 方法裏面：我們看到，如果高度是UNSPECIFIED，那麼ListView 的高度就是第一個Item 的高度，所以會會出現ScrollView 嵌套ListView 出現只能看到第一個Item 的問題。

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        // Sets up mListPadding
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);

        final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        int heightSize = MeasureSpec.getSize(heightMeasureSpec);

        int childWidth = 0;
        int childHeight = 0;
        int childState = 0;

        mItemCount = mAdapter == null ? 0 : mAdapter.getCount();
        if (mItemCount > 0 && (widthMode == MeasureSpec.UNSPECIFIED
                || heightMode == MeasureSpec.UNSPECIFIED)) {
            final View child = obtainView(0, mIsScrap);

            // Lay out child directly against the parent measure spec so that
            // we can obtain exected minimum width and height.
            measureScrapChild(child, 0, widthMeasureSpec, heightSize);

            childWidth = child.getMeasuredWidth();
            childHeight = child.getMeasuredHeight();
            childState = combineMeasuredStates(childState, child.getMeasuredState());

            if (recycleOnMeasure() && mRecycler.shouldRecycleViewType(
                    ((LayoutParams) child.getLayoutParams()).viewType)) {
                mRecycler.addScrapView(child, 0);
            }
        }

        if (widthMode == MeasureSpec.UNSPECIFIED) {
            widthSize = mListPadding.left + mListPadding.right + childWidth +
                    getVerticalScrollbarWidth();
        } else {
            widthSize |= (childState & MEASURED_STATE_MASK);
        }

        if (heightMode == MeasureSpec.UNSPECIFIED) {
            heightSize = mListPadding.top + mListPadding.bottom + childHeight +
                    getVerticalFadingEdgeLength() * 2;
        }

        if (heightMode == MeasureSpec.AT_MOST) {
            // TODO: after first layout we should maybe start at the first visible position, not 0
            heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);
        }

        setMeasuredDimension(widthSize, heightSize);

        mWidthMeasureSpec = widthMeasureSpec;
    }