Implementation

# Activity declaration

public class MainActivity extends AppCompatActivity
        implements AppBarLayout.OnOffsetChangedListener
        
  //int the onCreate
  AppBarLayout.LayoutParams params = (AppBarLayout.LayoutParams) mCollapsingToolbar.getLayoutParams();
  mDefaultFlags = params.getScrollFlags();
  mSetSnapAfter0 = false; //set snap after event in the offsetchanged
  
  
#OnOffsetChangedListener implementation

@Override
public void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
    FragmentDescriptor fragments = FragmentDescriptor.from(sBackstackController.getHead());

    if (mSetSnapAfter0 && verticalOffset == 0) {
        AppBarLayout.LayoutParams params = (AppBarLayout.LayoutParams) mCollapsingToolbar.getLayoutParams();
        mSetSnapAfter0 = false;
        params.setScrollFlags(AppBarLayout.LayoutParams.SCROLL_FLAG_SNAP); // list other flags here by |
        mCollapsingToolbar.setLayoutParams(params);
    }
}


#The method to update the toolbar behaviour

//called whenever the toolbar behaviour must be changed : fragment transition, etc...
@UiThread
private void updateCollapsing() {
    AppBarLayout.LayoutParams params = (AppBarLayout.LayoutParams) mCollapsingToolbar.getLayoutParams();
    FragmentDescriptor frag = FragmentDescriptor.from(sBackstackController.getHead());
    ToolbarType type = frag.getToolbarType();

    mNewState = type;
    if (mSetCanExpandCollapse != null && mSetCanExpandCollapse) {
        params.setScrollFlags(mDefaultFlags); //set default
    } else if ((mSetCanExpandCollapse != null && !mSetCanExpandCollapse)
            || ToolbarType.EXPANDED.equals(type)) { //COLLAPSED FOR NOW
        if (!ToolbarType.COLLAPSED.equals(type)) {
            mAppBar.setExpanded(true, true); //set co
            mSetSnapAfter0 = true;
        } //managed right after
    } else {
        params.setScrollFlags(mDefaultFlags);
    }

    //force collapse mode since snap and collapse will not work
    if (ToolbarType.COLLAPSED.equals(type)) {
        mAppBar.setExpanded(false, true);
        mAppBar.setActivated(false);
        mCoordinatorLayout.setAllowForScroll(false);
        mCollapsingToolbar.setActivated(false);
        mCollapsingToolbar.setLayoutParams(params);
    } else {
        mAppBar.setActivated(true);
        mCollapsingToolbar.setActivated(true);
        mCoordinatorLayout.setAllowForScroll(true);
        mCollapsingToolbar.setLayoutParams(params);
    }
}
    
    
# Describe the Toolbar current mode

public enum ToolbarType {
    NONE,
    EXPANDABLE,
    EXPANDED,
    COLLAPSED
}


# The FragmentDescriptor enum implementation

public enum FragmentDescriptor {
    ....;
    
    
    
    FragmentDescriptor(boolean backable, //well add < in the main activity
                       ToolbarType toolbar_scrollable,
                       ...) {
                       }
                       
    
    public boolean isBackable() {
        return mBackable;
    }

    public ToolbarType getToolbarType() {
        return mToolbarScrollable;
    }
}