dx during scrolling with BottomSheetBehavior is zero

问题

I have my custom BottomSheetBehavior:

public class CustomBehavior extends BottomSheetBehavior {

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

    @Override
    public void onNestedPreScroll(CoordinatorLayout coordinatorLayout, View 
    child, View target, int dx, int dy, int[] consumed) {
        super.onNestedPreScroll(coordinatorLayout, child, target, dx, dy, 
        consumed);
    }
}

And I use it in my xml code.

<android.support.design.widget.CoordinatorLayout
    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.v4.widget.NestedScrollView
        android:id="@+id/bottom_sheet"
        android:layout_width="match_parent"
        android:layout_height="350dp" 
        android:clipToPadding="true"
        android:background="@android:color/holo_orange_light"
        app:layout_behavior=
            "com.hackspace.bottomsheetproblem.CustomBehavior">

        <TextView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:text="text"/>

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

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

This behavior works, all nice. But in callback onNestedPreScroll dx is always zero. Log something like this:

dx: 0, dy: -1
dx: 0, dy: -70
dx: 0, dy: -108
dx: 0, dy: -85
dx: 0, dy: -44
dx: 0, dy: -16
dx: 0, dy: 16
dx: 0, dy: 44
dx: 0, dy: 62
dx: 0, dy: 84
dx: 0, dy: 58
dx: 0, dy: 54
dx: 0, dy: 43
dx: 0, dy: 17
dx: 0, dy: -20
dx: 0, dy: -42

Actually, my goal is to make some changes in detection swipes in default BottomSheetBehavior. I need my custom behavior react only on vertical (or almost vertical) swipes, not even on almost horizontal.

来源：https://stackoverflow.com/questions/46844954/dx-during-scrolling-with-bottomsheetbehavior-is-zero