CoordiantorLayout下的FloatingActionButton不是动画

Question

我正在尝试为FloatingActionButton设置动画（在CoordiantorLayout下）。当用户向下滚动页面时，浮动操作按钮应该消失。一旦页面滚动到顶部，它就会重新出现。但是它没有像我预期的那样工作我在哪里搞砸了？

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/DrawerLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@android:color/white"
    android:elevation="7dp"
    tools:context=".MainActivity">

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

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical">

            <include
                android:id="@+id/tool_bar"
                layout="@layout/tool_bar"></include>

            <android.support.v7.widget.RecyclerView
                android:id="@+id/tripslist"
                android:layout_width="match_parent"
                android:layout_height="match_parent" />
        </LinearLayout>

        <android.support.design.widget.FloatingActionButton
            android:id="@+id/createfab"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="bottom|end"
            android:layout_margin="16dp"
            android:src="@drawable/ic_plus"
            app:backgroundTint="@color/colorPrimary"
            app:layout_behavior="mogo.trasome.views.ScrollAwareFABBehavior" />

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


    <android.support.v7.widget.RecyclerView
        android:id="@+id/RecyclerView"
        android:layout_width="320dp"
        android:layout_height="match_parent"
        android:layout_gravity="left"
        android:background="#ffffff"
        android:scrollbars="vertical" />


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

这是我的行为类，

import android.content.Context;
import android.support.design.widget.CoordinatorLayout;
import android.support.design.widget.FloatingActionButton;
import android.support.v4.view.ViewCompat;
import android.util.AttributeSet;
import android.view.View;

public class ScrollAwareFABBehavior extends FloatingActionButton.Behavior {

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

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

        if (child.getVisibility() == View.VISIBLE && dyConsumed > 0) {
            child.hide();
        } else if (child.getVisibility() == View.GONE && dyConsumed < 0) {
            child.show();
        }
    }

    @Override
    public boolean onStartNestedScroll(CoordinatorLayout coordinatorLayout, FloatingActionButton child, View directTargetChild, View target, int nestedScrollAxes) {
        return nestedScrollAxes == ViewCompat.SCROLL_AXIS_VERTICAL;
    }
}