背景

我们有一个相当复杂的布局，其中包含 CollapsingToolbarLayout，以及底部的 RecyclerView。

在某些情况下，我们通过在 RecyclerView 上调用 setNestedScrollingEnabled(boolean) 来暂时禁用 CollapsingToolbarLayout 的展开/折叠。

问题

这通常工作正常。

但是，在某些(很少见)情况下，RecyclerView 上的缓慢滚动会被半阻塞，这意味着它在向下滚动时会尝试向后滚动。就好像它有 2 个相互竞争的滚动(向上滚动和向下滚动):



触发它的代码如下:

res/layout/activity_scrolling.xml

<android.support.design.widget.CoordinatorLayout
    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:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:context="com.example.user.myapplication.ScrollingActivity">

    <android.support.design.widget.AppBarLayout
        android:id="@+id/app_bar"
        android:layout_width="match_parent"
        android:layout_height="@dimen/app_bar_height"
        android:fitsSystemWindows="true"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.design.widget.CollapsingToolbarLayout
            android:id="@+id/toolbar_layout"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:fitsSystemWindows="true"
            app:contentScrim="?attr/colorPrimary"
            app:layout_scrollFlags="scroll|exitUntilCollapsed|snap">

            <android.support.v7.widget.Toolbar
                android:id="@+id/toolbar"
                android:layout_width="match_parent"
                android:layout_height="?attr/actionBarSize"
                app:layout_collapseMode="pin"
                app:popupTheme="@style/AppTheme.PopupOverlay"/>

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

    <android.support.v7.widget.RecyclerView
        android:id="@+id/nestedView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="@string/appbar_scrolling_view_behavior"/>

    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        app:layout_anchor="@id/app_bar"
        app:layout_anchorGravity="bottom|end">

        <Button
            android:id="@+id/disableNestedScrollingButton"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="disable"/>

        <Button
            android:id="@+id/enableNestedScrollingButton"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="enable"
            />
    </LinearLayout>

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

public class ScrollingActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_scrolling);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        final RecyclerView nestedView = (RecyclerView) findViewById(R.id.nestedView);
        findViewById(R.id.disableNestedScrollingButton).setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(final View v) {
                nestedView.setNestedScrollingEnabled(false);
            }
        });
        findViewById(R.id.enableNestedScrollingButton).setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(final View v) {
                nestedView.setNestedScrollingEnabled(true);
            }
        });
        nestedView.setLayoutManager(new LinearLayoutManager(this));
        nestedView.setAdapter(new Adapter() {
            @Override
            public ViewHolder onCreateViewHolder(final ViewGroup parent, final int viewType) {
                return new ViewHolder(LayoutInflater.from(parent.getContext()).inflate(
                        android.R.layout.simple_list_item_1,
                        parent,
                        false)) {
                };
            }

            @Override
            public void onBindViewHolder(final ViewHolder holder, final int position) {
                ((TextView) holder.itemView.findViewById(android.R.id.text1)).setText("item " + position);
            }

            @Override
            public int getItemCount() {
                return 100;
            }
        });
    }

}

最佳答案

这是实现与 this answer 相同目标的另一种方法。 .虽然该答案使用了反射，但该答案没有，但推理保持不变。

为什么会这样？

问题是 RecyclerView有时对成员变量 mScrollOffset 使用过时的值. mScrollOffset仅在 RecyclerView 中的两个位置设置: dispatchNestedPreScroll和 dispatchNestedScroll .我们只关心dispatchNestedPreScroll .此方法由 RecyclerView#onTouchEvent 调用当它处理 MotionEvent.ACTION_MOVE事件。

以下来自 dispatchNestedPreScroll 的文档.

dispatchNestedPreScroll

boolean dispatchNestedPreScroll (int dx, int dy, int[] consumed, int[] offsetInWindow)

Dispatch one step of a nested scroll in progress before this view consumes any portion of it.

Nested pre-scroll events are to nested scroll events what touch intercept is to touch. dispatchNestedPreScroll offers an opportunity for the parent view in a nested scrolling operation to consume some or all of the scroll operation before the child view consumes it.

...

offsetInWindow int: Optional. If not null, on return this will contain the offset in local view coordinates of this view from before this operation to after it completes. View implementations may use this to adjust expected input coordinate tracking.

mLastTouchY = y - mScrollOffset[1];

public class ScrollingActivity extends AppCompatActivity
        implements MyRecyclerView.OnClampPrescrollOffsetListener {

    private CollapsingToolbarLayout mCollapsingToolbarLayout;
    private AppBarLayout mAppBarLayout;
    private MyRecyclerView mNestedView;
    // This variable will be true when the app bar is completely open or completely collapsed.
    private boolean mAppBarIdle = true;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_scrolling);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        mNestedView = (MyRecyclerView) findViewById(R.id.nestedView);
        mAppBarLayout = (AppBarLayout) findViewById(R.id.app_bar);
        mCollapsingToolbarLayout = (CollapsingToolbarLayout) findViewById(R.id.toolbar_layout);

        // Set the listener for the patch code.
        mNestedView.setOnClampPrescrollOffsetListener(this);

        // Listener to determine when the app bar is collapsed or fully open (idle).
        mAppBarLayout.addOnOffsetChangedListener(new AppBarLayout.OnOffsetChangedListener() {
            @Override
            public final void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
                mAppBarIdle = verticalOffset == 0
                        || verticalOffset <= appBarLayout.getTotalScrollRange();
            }
        });
        findViewById(R.id.disableNestedScrollingButton).setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(final View v) {
                // If the AppBar is fully expanded or fully collapsed (idle), then disable
                // expansion and apply the patch; otherwise, set a flag to disable the expansion
                // and apply the patch when the AppBar is idle.
                setExpandEnabled(false);

            }
        });
        findViewById(R.id.enableNestedScrollingButton).setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(final View v) {
                setExpandEnabled(true);
            }
        });
        mNestedView.setLayoutManager(new LinearLayoutManager(this));
        mNestedView.setAdapter(new Adapter() {
            @Override
            public ViewHolder onCreateViewHolder(final ViewGroup parent, final int viewType) {
                return new ViewHolder(LayoutInflater.from(parent.getContext()).inflate(
                        android.R.layout.simple_list_item_1,
                        parent,
                        false)) {
                };
            }

            @Override
            public void onBindViewHolder(final ViewHolder holder, final int position) {
                ((TextView) holder.itemView.findViewById(android.R.id.text1)).setText("item " + position);
            }

            @Override
            public int getItemCount() {
                return 100;
            }
        });
    }

    private void setExpandEnabled(boolean enabled) {
        mNestedView.setNestedScrollingEnabled(enabled);
    }

    // Return "true" when the app bar is idle and nested scrolling is disabled. This is a signal
    // to the custom RecyclerView to clamp the y prescroll offset to zero.
    @Override
    public boolean clampPrescrollOffsetListener() {
        return mAppBarIdle && !mNestedView.isNestedScrollingEnabled();
    }

    private static final String TAG = "ScrollingActivity";
}

public class MyRecyclerView extends RecyclerView {
    private OnClampPrescrollOffsetListener mPatchListener;

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

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

    public MyRecyclerView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }

    // Just a call to super plus code to force offsetInWindow[1] to zero if the patchlistener
    // instructs it.
    @Override
    public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow) {
        boolean returnValue;
        int currentOffset;
        returnValue = super.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow);
        currentOffset = offsetInWindow[1];
        Log.d(TAG, "<<<<dispatchNestedPreScroll: " + currentOffset);
        if (mPatchListener.clampPrescrollOffsetListener() && offsetInWindow[1] != 0) {
            Log.d(TAG, "<<<<dispatchNestedPreScroll: " + currentOffset + " -> 0");
            offsetInWindow[1] = 0;
        }
        return returnValue;
    }

    public void setOnClampPrescrollOffsetListener(OnClampPrescrollOffsetListener patchListener) {
        mPatchListener = patchListener;
    }

    public interface OnClampPrescrollOffsetListener {
        boolean clampPrescrollOffsetListener();
    }

    private static final String TAG = "MyRecyclerView";
}

关于android - 使用 setNestedScrollingEnabled(false) 时如何避免阻止滚动本身？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/44484950/