Android布局：滑动FragmentContainerView时隐藏Toolbar的实现方案

方案一：让Fragment内的可滑动视图主动分发滚动事件 #

CoordinatorLayout的AppBarLayout滚动逻辑依赖子视图传递的滚动事件，FragmentContainerView本身不会触发滚动事件，因此需要让其内部的可滑动视图主动将事件传递给CoordinatorLayout：

• WebView场景：
  自定义WebView实现NestedScrollingChild接口，通过NestedScrollingChildHelper分发滚动事件：

  public class NestedScrollWebView extends WebView implements NestedScrollingChild {
      private final NestedScrollingChildHelper mChildHelper;
      private int mLastY;
  
      public NestedScrollWebView(Context context) {
          this(context, null);
      }
  
      public NestedScrollWebView(Context context, AttributeSet attrs) {
          this(context, attrs, android.R.attr.webViewStyle);
      }
  
      public NestedScrollWebView(Context context, AttributeSet attrs, int defStyleAttr) {
          super(context, attrs, defStyleAttr);
          mChildHelper = new NestedScrollingChildHelper(this);
          setNestedScrollingEnabled(true);
      }
  
      @Override
      public void setNestedScrollingEnabled(boolean enabled) {
          mChildHelper.setNestedScrollingEnabled(enabled);
      }
  
      @Override
      public boolean isNestedScrollingEnabled() {
          return mChildHelper.isNestedScrollingEnabled();
      }
  
      @Override
      public boolean startNestedScroll(int axes) {
          return mChildHelper.startNestedScroll(axes);
      }
  
      @Override
      public void stopNestedScroll() {
          mChildHelper.stopNestedScroll();
      }
  
      @Override
      public boolean hasNestedScrollingParent() {
          return mChildHelper.hasNestedScrollingParent();
      }
  
      @Override
      public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow) {
          return mChildHelper.dispatchNestedScroll(dxConsumed, dyConsumed, dxUnconsumed, dyUnconsumed, offsetInWindow);
      }
  
      @Override
      public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow) {
          return mChildHelper.dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow);
      }
  
      @Override
      protected void onScrollChanged(int l, int t, int oldl, int oldt) {
          super.onScrollChanged(l, t, oldl, oldt);
          int dy = t - mLastY;
          startNestedScroll(ViewCompat.SCROLL_AXIS_VERTICAL);
          dispatchNestedPreScroll(0, dy, null, null);
          mLastY = t;
      }
  }
  

  在Fragment布局中使用该自定义WebView，即可让CoordinatorLayout接收到滚动事件，驱动Toolbar隐藏/显示。

• RecyclerView/ListView场景：
  这类视图默认已实现NestedScrollingChild接口，只需确保布局中android:nestedScrollingEnabled="true"（默认开启），滚动事件会自动传递给CoordinatorLayout。

方案二：给FragmentContainerView自定义Behavior #

如果不想修改Fragment内部视图，可给FragmentContainerView编写自定义Behavior，监听其内部子视图的滚动事件并主动通知AppBarLayout：

public class FragmentContainerScrollBehavior extends AppBarLayout.ScrollingViewBehavior {
    private View mChildView;

    public FragmentContainerScrollBehavior() {
    }

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

    @Override
    public boolean onLayoutChild(CoordinatorLayout parent, View child, int layoutDirection) {
        boolean result = super.onLayoutChild(parent, child, layoutDirection);
        if (child instanceof FragmentContainerView) {
            mChildView = ((FragmentContainerView) child).getChildAt(0);
            if (mChildView != null) {
                setupScrollListener();
            }
        }
        // 监听Fragment切换，更新子视图引用
        parent.addOnLayoutChangeListener((v, left, top, right, bottom, oldLeft, oldTop, oldRight, oldBottom) -> {
            if (child instanceof FragmentContainerView) {
                mChildView = ((FragmentContainerView) child).getChildAt(0);
                if (mChildView != null) {
                    setupScrollListener();
                }
            }
        });
        return result;
    }

    private void setupScrollListener() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            mChildView.setOnScrollChangeListener((v, scrollX, scrollY, oldScrollX, oldScrollY) -> {
                int dy = scrollY - oldScrollY;
                dispatchNestedScrollEvent(v, dy);
            });
        } else {
            if (mChildView instanceof WebView) {
                ((WebView) mChildView).setOnScrollChangeListener((View v, int scrollX, int scrollY, int oldScrollX, int oldScrollY) -> {
                    int dy = scrollY - oldScrollY;
                    dispatchNestedScrollEvent(v, dy);
                });
            } else if (mChildView instanceof RecyclerView) {
                ((RecyclerView) mChildView).addOnScrollListener(new RecyclerView.OnScrollListener() {
                    @Override
                    public void onScrolled(@NonNull RecyclerView recyclerView, int dx, int dy) {
                        super.onScrolled(recyclerView, dx, dy);
                        dispatchNestedScrollEvent(recyclerView, dy);
                    }
                });
            }
        }
    }

    private void dispatchNestedScrollEvent(View v, int dy) {
        if (v instanceof NestedScrollingChild) {
            ((NestedScrollingChild) v).startNestedScroll(ViewCompat.SCROLL_AXIS_VERTICAL);
            ((NestedScrollingChild) v).dispatchNestedPreScroll(0, dy, null, null);
        }
    }
}

在布局中给FragmentContainerView设置该Behavior：

app:layout_behavior="com.abc.myApp.features.main.FragmentContainerScrollBehavior"

方案三：修复NestedScrollView嵌套FragmentContainerView的显示问题 #

之前嵌套后视图不显示，核心原因是NestedScrollView未开启fillViewport属性，导致内容高度被压缩为0，按以下步骤修复：

1. 修改布局，将FragmentContainerView嵌套进NestedScrollView：

<androidx.core.widget.NestedScrollView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fillViewport="true"
    app:layout_behavior="@string/appbar_scrolling_view_behavior">

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/nav_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:navGraph="@navigation/nav_graph" />

</androidx.core.widget.NestedScrollView>

2. 确保Fragment的根布局高度设置为match_parent或wrap_content，避免内容被截断。
3. 对于WebView场景，需开启自适应设置：

webView.getSettings().setLoadWithOverviewMode(true);
webView.getSettings().setUseWideViewPort(true);

注意：此方案可能导致内部可滚动视图与NestedScrollView出现双重滚动冲突，需根据实际场景禁用内部视图的滚动（如WebView设置setNestedScrollingEnabled(false)）。

内容的提问来源于stack exchange，提问作者Franklin84