How to expand AppBarLayout when scrolling down reaches at top of the RecyclerView

Question

I am working on one android app in which I am using CoordinatorLayout,AppBarLayout and CollapsingToolbarLayout to use the advance coll

Answer 1

I've just put together a sample app that seems to demonstrate the behaviour you're after. The AppBar will automatically expand whenever the RecyclerView beneath it is scrolled to the top, rather than stopping and waiting for another swipe to open the AppBar.

The most relevant components reside in a custom RecyclerView class. I've added a few explanatory comments:

public class ScrollFeedbackRecyclerView extends RecyclerView {

    private WeakReference<Callbacks> mCallbacks;

    public ScrollFeedbackRecyclerView(Context context) {
        super(context);
        attachCallbacks(context);
    }

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

    /*If the first completely visible item in the RecyclerView is at
    index 0, then we're at the top of the list, so we want the AppBar to expand 
    **if the AppBar is also collapsed** (otherwise the AppBar will constantly 
    attempt to expand).
    */
    @Override
    public void onScrolled(int dx, int dy) {
        if(((LinearLayoutManager)getLayoutManager()).findFirstCompletelyVisibleItemPosition() == 0) {
            Log.e(getClass().getSimpleName(), "index 0 visible");
            if(mCallbacks.get().isAppBarCollapsed()) {
                mCallbacks.get().setExpanded(true);
            }
        }
        super.onScrolled(dx, dy);
    }

    /* the findFirstCompletelyVisibleItem() method is only available with 
    LinearLayoutManager and its subclasses, so test for it when setting the 
    LayoutManager
    */
    @Override
    public void setLayoutManager(LayoutManager layout) {
        if(!(layout instanceof LinearLayoutManager)) {
            throw new IllegalArgumentException(layout.toString() + " must be of type LinearLayoutManager");
        }
        super.setLayoutManager(layout);
    }

    private void attachCallbacks(Context context) {

        try {
            mCallbacks = new WeakReference<>((Callbacks)context);
        } catch (ClassCastException e) {
            throw new ClassCastException(context.toString() + " must implement " +
                    "ScrollFeedbackRecyclerView.Callbacks");
        }

    }

    /* Necessary to interact with the AppBarLayout in the hosting Activity
    */
    interface Callbacks {

        boolean isAppBarCollapsed();
        void setExpanded(boolean expanded);

    }
}

MainActivity.java

public class MainActivity extends AppCompatActivity implements ScrollFeedbackRecyclerView.Callbacks{

    private AppBarLayout mAppBarLayout;
    private Toolbar mToolbar;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ScrollFeedbackRecyclerView mRecyclerView = (ScrollFeedbackRecyclerView) findViewById(R.id.rv_container);
        RecyclerViewAdapter mAdapter = new RecyclerViewAdapter();

        mRecyclerView.setAdapter(mAdapter);
        mRecyclerView.setLayoutManager(new LinearLayoutManager(this));

        mAppBarLayout = (AppBarLayout) findViewById(R.id.app_bar);
        mToolbar = (Toolbar) findViewById(R.id.toolbar);

    }

    /* When collapsed, calling getY() on the AppBar will return a negative number. 
    Adding this number to getHeight() will return the same value as the toolbar's 
    height if the AppBar is fully collapsed.
    */
    @Override
    public boolean isAppBarCollapsed() {
        final int appBarVisibleHeight = (int) (mAppBarLayout.getY() + mAppBarLayout.getHeight());
        final int toolbarHeight = mToolbar.getHeight();
        return (appBarVisibleHeight == toolbarHeight);
    }

    @Override
    public void setExpanded(boolean expanded) {
        mAppBarLayout.setExpanded(expanded, true);
    }
}