当 NestedScrollView 为空时，停止 AppBarLayout 滚出屏幕

Question

我有一个相当典型的列表功能，使用 CoordinatorLayout、AppBarLayout、SwipeRefreshLayout 和 RecyclerView -

当 RecyclerView 有足够的内容可以滚动时，页面看起来还不错。但是，当 RecyclerView 为空或没有足够的内容滚动时，行为是带有 app:layout_scrollFlags="scroll|enterAlwaysCollapsed" 的 AppBarLayout 子级将继续滚动 - 这看起来很奇怪。

有没有办法在 NestedScrollView 为空时阻止 AppBarLayout 子级滚动？

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <android.support.design.widget.CoordinatorLayout
        android:id="@+id/coordinatorLayout"
        android:background="@android:color/transparent"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <android.support.design.widget.AppBarLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:background="@android:color/transparent"
            android:elevation="4dp">

            <LinearLayout
                android:id="@+id/eventHeader"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:padding="16dp"
                android:background="@color/green"
                android:orientation="horizontal"
                app:layout_scrollFlags="scroll|enterAlwaysCollapsed">

                <View
                    android:layout_width="0dp"
                    android:layout_height="0dp"
                    android:layout_weight="1"/>

                <TextView
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:text="scroll|enterAlwaysCollapsed"
                    android:textColor="@color/white"
                    android:textSize="15sp"/>

                <View
                    android:layout_width="0dp"
                    android:layout_height="0dp"
                    android:layout_weight="1"/>

            </LinearLayout>

        </android.support.design.widget.AppBarLayout>

        <android.support.v4.widget.SwipeRefreshLayout
            android:id="@+id/swipeToRefresh"
            android:layout_width="match_parent"
            android:layout_gravity="fill_vertical"
            android:layout_height="match_parent"
            app:layout_behavior="@string/appbar_scrolling_view_behavior">

            <android.support.v7.widget.RecyclerView
                android:id="@+id/recyclerView"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:background="@android:color/transparent"
                android:dividerHeight="0dp"
                android:layout_gravity="fill_vertical"
                android:drawSelectorOnTop="true"
                android:listSelector="@drawable/selector_ripple_grey_transparent"
                android:scrollbars="vertical"/>

        </android.support.v4.widget.SwipeRefreshLayout>

    </android.support.design.widget.CoordinatorLayout>

    <TextView
        android:id="@+id/noData"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:padding="16dp"
        android:text="@string/no_data_available"
        android:textSize="17sp"/>

</FrameLayout>

Answer 1

Graeme 答案还可以，但我也在构造函数中添加了

public AppBarLayoutOnEmptyRecyclerViewScrollBehavior(@NonNull AppBarLayout appBarLayout, @NonNull RecyclerView recyclerView) {
    this.appBarLayout = checkNotNull(appBarLayout);
    this.recyclerView = checkNotNull(recyclerView);

    setDragCallback(new DragCallback() {
        @Override
        public boolean canDrag(@NonNull AppBarLayout appBarLayout) {
            return isRecylerViewScrollable(recyclerView);
        }
    });
}

所以当RecyclerView 为空时，我也会禁用来自AppBarLayout 的拖动

Answer 2

将数据加载到您的 RecyclerView 后，如果它是空的，只需手动禁用滚动标志：

AppBarLayout.LayoutParams toolbarLayoutParams = (AppBarLayout.LayoutParams) mEventHeader.getLayoutParams();
toolbarLayoutParams.setScrollFlags(0);
mEventHeader.setLayoutParams(toolbarLayoutParams);

如果不为空，则设置滚动标志：

AppBarLayout.LayoutParams toolbarLayoutParams = (AppBarLayout.LayoutParams) mEventHeader.getLayoutParams();
toolbarLayoutParams.setScrollFlags(AppBarLayout.LayoutParams.SCROLL_FLAG_SCROLL | AppBarLayout.LayoutParams.SCROLL_FLAG_ENTER_ALWAYS);
mEventHeader.setLayoutParams(toolbarLayoutParams);

Answer 3

（基于：Reference）

(1) 创建这个类。

public class AppBarLayoutBehaviorForEmptyRecyclerView extends AppBarLayout.Behavior
{
    private boolean canRecyclerViewBeScrolled = false;

    public AppBarLayoutBehaviorForEmptyRecyclerView()
    {
    }

    public AppBarLayoutBehaviorForEmptyRecyclerView(Context context, AttributeSet attrs)
    {
        super(context, attrs);
    }

    @Override
    public boolean onInterceptTouchEvent(CoordinatorLayout parent, AppBarLayout child, MotionEvent ev)
    {
        return canRecyclerViewBeScrolled && super.onInterceptTouchEvent(parent, child, ev);
    }

    @Override
    public boolean onStartNestedScroll(CoordinatorLayout parent, AppBarLayout child, View directTargetChild, View target, int nestedScrollAxes)
    {
        updateScrollable(target);
        return canRecyclerViewBeScrolled && super.onStartNestedScroll(parent, child, directTargetChild, target, nestedScrollAxes);
    }

    @Override
    public boolean onNestedFling(CoordinatorLayout coordinatorLayout, AppBarLayout child, View target, float velocityX, float velocityY, boolean consumed)
    {
        return canRecyclerViewBeScrolled && super.onNestedFling(coordinatorLayout, child, target, velocityX, velocityY, consumed);
    }

    private void updateScrollable(View targetChild)
    {
        if(targetChild instanceof RecyclerView)
        {
            RecyclerView.Adapter adapter = ((RecyclerView) targetChild).getAdapter();

            canRecyclerViewBeScrolled = adapter != null && adapter.getItemCount() > 0;
        }
        else
        {
            canRecyclerViewBeScrolled = true;
        }
    }
}

(2) 将以下属性添加到您的 AppBarLayout XML 元素：

app:layout_behavior="com.xxxx.xxxxxx.AppBarLayoutBehaviorForEmptyRecyclerView"

Answer 4

不确定这是多么优雅的解决方案，但我覆盖了 onStartNestedScroll() 事件，仅在 NestedScrollView 可滚动时触发（在本例中为 RecyclerView）

在 onCreate() 中：

CoordinatorLayout.LayoutParams layoutParams = (CoordinatorLayout.LayoutParams) mAppBarLayout.getLayoutParams();
layoutParams.setBehavior(new AppBarLayoutNoEmptyScrollBehavior(mAppBarLayout, mRecyclerView));

行为：

public class AppBarLayoutNoEmptyScrollBehavior extends AppBarLayout.Behavior {

    AppBarLayout mAppBarLayout;
    RecyclerView mRecyclerView;
    public AppBarLayoutNoEmptyScrollBehavior(AppBarLayout appBarLayout, RecyclerView recyclerView) {
        mAppBarLayout = appBarLayout;
        mRecyclerView = recyclerView;
    }

    public boolean isRecylerViewScrollable(RecyclerView recyclerView) {
        int recyclerViewHeight = recyclerView.getHeight(); // Height includes RecyclerView plus AppBarLayout at same level
        int appCompatHeight    = mAppBarLayout != null ? mAppBarLayout.getHeight() : 0;
        recyclerViewHeight -= appCompatHeight;

        return recyclerView.computeVerticalScrollRange() > recyclerViewHeight;
    }

    @Override
    public boolean onStartNestedScroll(CoordinatorLayout parent, AppBarLayout child, View directTargetChild, View target, int nestedScrollAxes) {
        if (isRecylerViewScrollable(mRecyclerView)) {
            return super.onStartNestedScroll(parent, child, directTargetChild, target, nestedScrollAxes);
        }
        return false;
    }

}

编辑

作为 RecyclerView 的已编辑解决方案将高度设为 visible RecyclerView 高度和 AppBarLayout 高度（即 CoordinatorLayout 高度）。

但是，如果您的滚动手势从可见的 AppBarLayout 区域开始，滚动将仍然发生，即使您将此 Behavior 添加到 AppBarLayout 也是如此。因此，此答案不能解决问题。