当 recyclerview 为 scrolled/fling 时展开 appbarlayout 到顶部

Question

我实现了一个带有 recyclerview 的 collapsingtoolbar 布局，如所附的示例代码所示。我的问题是，当我向下滑动列表时，它并没有一直到顶部。

发生的情况是，滚动恰好在 AppBarLayout 应该结束的位置停止。

我想要的效果是在向下滑动列表时，列表将一直到顶部并且 reveal/expand AppBarLayout

我的 minSdk 是 14。非常感谢任何帮助或建议。

<?xml version="1.0" encoding="utf-8"?>

<android.support.design.widget.AppBarLayout>

    <android.support.design.widget.CollapsingToolbarLayout
        app:layout_scrollFlags="scroll|exitUntilCollapsed">

        <LinearLayout
            app:layout_collapseMode="parallax">

            //some elements

        </LinearLayout>

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

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

<android.support.v7.widget.RecyclerView
    app:layout_behavior="@string/appbar_scrolling_view_behavior"/> //value android.support.design.widget.AppBarLayout$ScrollingViewBehavior

<android.support.v7.widget.Toolbar
    app:popupTheme="@style/AppTheme.PopupOverlay"
    app:layout_collapseMode="parallax" />

Answer 1

您可以使用 setExpanded() 方法完全展开或折叠应用栏。一种实现可能涉及覆盖 Activity class 中的 dispatchTouchEvent() 和 auto-collapsing/expanding 你的应用栏，这取决于它是否折叠超过了中点：

@Override
public boolean dispatchTouchEvent(MotionEvent event) {
    if (event.getAction() == MotionEvent.ACTION_UP) {
        float per = Math.abs(mAppBarLayout.getY()) / mAppBarLayout.getTotalScrollRange();
        boolean setExpanded = (per <= 0.5F);
        mAppBarLayout.setExpanded(setExpanded, true);
    }
    return super.dispatchTouchEvent(event);
}

关于自动滚动到 fling 的最后位置，我在 GitHub 上放了一些代码，展示了如何 programmatically smooth scroll to a specific location that may help. Calling a scroll to list.size() - 1 on a fling for instance could replicate the behaviour. Parts of this code by the way are adapted from the StylingAndroid and Novoda 博客：

public class RecyclerLayoutManager extends LinearLayoutManager {

    private AppBarManager mAppBarManager;
    private int visibleHeightForRecyclerView;

    public RecyclerLayoutManager(Context context) {
        super(context);
    }

    @Override
    public void smoothScrollToPosition(RecyclerView recyclerView, RecyclerView.State state, int position) {
        View firstVisibleChild = recyclerView.getChildAt(0);
        final int childHeight = firstVisibleChild.getHeight();
        int distanceInPixels = ((findFirstVisibleItemPosition() - position) * childHeight);
        if (distanceInPixels == 0) {
            distanceInPixels = (int) Math.abs(firstVisibleChild.getY());
        }
        //Called Once
        if (visibleHeightForRecyclerView == 0) {
            visibleHeightForRecyclerView = mAppBarManager.getVisibleHeightForRecyclerViewInPx();
        }
        //Subtract one as adapter position 0 based
        final int visibleChildCount = visibleHeightForRecyclerView/childHeight - 1;

        if (position <= visibleChildCount) {
            //Scroll to the very top and expand the app bar
            position = 0;
            mAppBarManager.expandAppBar();
        } else {
            mAppBarManager.collapseAppBar();
        }

        SmoothScroller smoothScroller = new SmoothScroller(recyclerView.getContext(), Math.abs(distanceInPixels), 1000);
        smoothScroller.setTargetPosition(position);
        startSmoothScroll(smoothScroller);
    }

    public void setAppBarManager(AppBarManager appBarManager) {
        mAppBarManager = appBarManager;
    }

    private class SmoothScroller extends LinearSmoothScroller {
        private static final int TARGET_SEEK_SCROLL_DISTANCE_PX = 10000;
        private final float distanceInPixels;
        private final float duration;

        public SmoothScroller(Context context, int distanceInPixels, int duration) {
            super(context);
            this.distanceInPixels = distanceInPixels;
            float millisecondsPerPx = calculateSpeedPerPixel(context.getResources().getDisplayMetrics());
            this.duration = distanceInPixels < TARGET_SEEK_SCROLL_DISTANCE_PX ?
                    (int) (Math.abs(distanceInPixels) * millisecondsPerPx) : duration;
        }

        @Override
        public PointF computeScrollVectorForPosition(int targetPosition) {
            return RecyclerLayoutManager.this
                    .computeScrollVectorForPosition(targetPosition);
        }

        @Override
        protected int calculateTimeForScrolling(int dx) {
            float proportion = (float) dx / distanceInPixels;
            return (int) (duration * proportion);
        }
    }
}

编辑：

上述代码片段中的

AppBarManager指的是一个interface，用于与Activity中的AppBarLayout通信。 Collapse/expand 应用栏方法就是这样做的，带有动画。最后一种方法用于计算屏幕上可见的 RecyclerView 行数：

AppBarManager.java

public interface AppBarManager {

    void collapseAppBar();
    void expandAppBar();
    int getVisibleHeightForRecyclerViewInPx();

}

MainActivity.java

public class MainActivity extends AppCompatActivity implements AppBarManager{

@Override
public void collapseAppBar() {
    mAppBarLayout.setExpanded(false, true);
}

@Override
public void expandAppBar() {
    mAppBarLayout.setExpanded(true, true);
}

@Override
public int getVisibleHeightForRecyclerViewInPx() {

    if (mRecyclerFragment == null) mRecyclerFragment =
            (RecyclerFragment) getSupportFragmentManager().findFragmentByTag(RecyclerFragment.TAG);

    int windowHeight, appBarHeight, headerViewHeight;
    windowHeight = getWindow().getDecorView().getHeight();
    appBarHeight = mAppBarLayout.getHeight();
    headerViewHeight = mRecyclerFragment.getHeaderView().getHeight();
    return windowHeight - (appBarHeight + headerViewHeight);
}

Answer 2

我有类似的问题，当 RecyclerView 跳到顶部时，我使用了一个简单的技巧来扩展 AppBarLayout（你需要有支持库 >= 23.x.x）

mRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
            @Override
            public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
                super.onScrollStateChanged(recyclerView, newState);
                if (newState == RecyclerView.SCROLL_STATE_IDLE) {
                    int firstVisiblePosition = linearLayoutManager.findFirstCompletelyVisibleItemPosition();
                    if (firstVisiblePosition == 0) {
                        mAppBarLayout.setExpanded(true, true);
                    }
                }
            }
});

当 recyclerview 为 scrolled/fling 时展开 appbarlayout 到顶部

Expand appbarlayout when recyclerview is scrolled/fling to top

android

android-recyclerview

android-collapsingtoolbarlayout