自定义 CoordinatorLayout.Behavior 和 RecyclerView 滚动问题

Question

我有一个CoordinatorLayout 和两个孩子，一个View 充当标题，一个RecyclerView：

<android.support.design.widget.CoordinatorLayout
    android:id="@+id/coordinator"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <LinearLayout
        android:id="@+id/view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        android:gravity="center_horizontal"
        app:layout_behavior="some.package.AlphaBehavior">

        <ImageView
            android:id="@+id/header_iv"
            style="@style/some_style"/>

        <TextView
            android:id="@+id/header_retails_tv"
            style="@style/some_style_tv"
            android:text="@string/some_text"/>

        </LinearLayout>

        <android.support.v7.widget.RecyclerView
            android:id="@+id/recyclerview"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:clipToPadding="false" />

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

我将填充动态设置为RecyclerView，并使用标题大小将clipToPadding 设置为false，因此RecyclerView 显示在标题下方，当用户向上滚动时，@987654330 @ 显示在标题视图上方。

我制作了一个自定义CoordinatorLayout.Behavior，以便在用户向上滚动list 时实现淡出视图，并在标题必须再次可见时淡入，AlphaBehavior：

public class AlphaBehavior extends CoordinatorLayout.Behavior {

    private float alpha                 = 1.0f;
    private float scrolly               = 0.f;
    private int headerSize = 0;

    private Animation defaultFadeInAnimation;

    public AlphaBehavior(Context context, AttributeSet attrs) {
        super(context, attrs);


        defaultFadeInAnimation = AnimationUtils.loadAnimation(context, android.R.anim.fade_in);
    }

    public void setHeaderSize(int headerSize) {
        this.headerSize = headerSize;
    }

    @Override
    public boolean layoutDependsOn(CoordinatorLayout parent, View child, View dependency) {
        return dependency instanceof RecyclerView;
    }


    @Override
    public void onNestedScroll(CoordinatorLayout coordinatorLayout, View child, View target, int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed) {

        scrolly += dyConsumed;

        Log.d(Constants.TAG, dyConsumed + "/" + dyUnconsumed + "/" + scrolly);

        float totalScrollY = ((RecyclerView)target).computeVerticalScrollOffset();

        Log.d(Constants.TAG, "totalScrollY:" + totalScrollY);

        alpha = (headerSize - totalScrollY) / headerSize;

        if (alpha < 0.f) alpha = 0.f;
        if (alpha > 1.0f) alpha = 1.f;

        if (dyConsumed < 0 && totalScrollY > headerSize) {
            alpha = 0.f;
        }

        Log.d(Constants.TAG, "alpha:" + alpha);

        child.setAlpha(alpha);
    }

    @Override
    public void onStopNestedScroll(CoordinatorLayout coordinatorLayout, View child, View target) {

        int pos = ((LinearLayoutManager)((RecyclerView)target).getLayoutManager()).findFirstCompletelyVisibleItemPosition();

        Log.d(Constants.TAG, "pos:" + pos);

        if (pos == 0 && child.getAlpha() == 0.f) {
            child.startAnimation(defaultFadeInAnimation);
        }
    }



    // overriding this in case we don't the other events are not called
    @Override
    public boolean onStartNestedScroll(CoordinatorLayout coordinatorLayout, View child, View directTargetChild, View target, int nestedScrollAxes) {
        return nestedScrollAxes == ViewCompat.SCROLL_AXIS_VERTICAL;
    }
}

但我面临一个问题：如果用户滚动速度非常快，则行为事件无法正确调用。 scrollY 成员与总滚动关系不好，totalScrollY 成员（通过计算 RecyclerView 的滚动获得）不正确。即使我试图在onStopNestedScroll 事件中找到firstCompletelyVisibleItem，但是当recyclerView 到达列表的开头时，它会返回位置2 或3。

Answer 1

最后，我使用OnScrollListener 而不是CoordinatorLayout.Behavior 解决了这个问题，它的工作原理非常棒。我放了代码，也许对某人有用：

用于隐藏视图的自定义 onScrollListener：

public class HideViewOnScrollListener extends RecyclerView.OnScrollListener {

    private float alpha = 1.f;
    private float scrolly = 0.f;

    private int heightViewToHide;
    private final View viewToHide;

    public HideViewOnScrollListener(View viewToHide) {
        this.viewToHide = viewToHide;

        heightViewToHide = viewToHide.getHeight();
        if (heightViewToHide == 0) {

            ViewTreeObserver viewTreeObserver = viewToHide.getViewTreeObserver();
            viewTreeObserver.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
                @Override
                public void onGlobalLayout() {

                    heightViewToHide = viewToHide.getHeight();

                    if (heightViewToHide > 0)
                        viewToHide.getViewTreeObserver().removeOnGlobalLayoutListener(this);
                }
            });
        }

    }

    @Override
    public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
        super.onScrolled(recyclerView, dx, dy);

        scrolly += dy;

        alpha = (heightViewToHide - scrolly) / heightViewToHide;

        if (alpha < 0.f) alpha = 0.f;
        if (alpha > 1.0f) alpha = 1.f;

        if (dy < 0 && scrolly > heightViewToHide) {
            alpha = 0.f;
        }

        viewToHide.setAlpha(alpha);
    }
}

您可以通过这种方式添加到 RecyclerView：

recyclerView.addOnScrollListener(new HideViewOnScrollListener(viewToHide));