An extension of Android AppBarLayout, that allows to receive state changes (collapsed, expanded and idle). This is a simplified version of ControllableAppBarLayout (https://gist.github.com/skimarxall/863585dcd7abde8f4153) - only implementing a state change listener.

StateAwareAppBarLayout.java

package io.github.ktchernov.play_aroundapp.views;

/**
 * Copyright 2017 Konstantin Tchernov
 *
 * Adapted from: ControllableAppBarLayout by Bartosz Lipinski and
 * Marcel Pintó Biescas
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import android.content.Context;
import android.support.design.widget.AppBarLayout;
import android.support.design.widget.CoordinatorLayout;
import android.util.AttributeSet;

public class StateAwareAppBarLayout extends AppBarLayout
        implements AppBarLayout.OnOffsetChangedListener {
    private AppBarLayout.Behavior mBehavior;
    private State state;
    private OnStateChangeListener onStateChangeListener;

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

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

    @Override
    protected void onAttachedToWindow() {
        super.onAttachedToWindow();

        addOnOffsetChangedListener(this);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        if (mBehavior == null) {
            mBehavior =
                    (Behavior) ((CoordinatorLayout.LayoutParams) getLayoutParams()).getBehavior();
        }
    }

    @Override
    public void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
        if (verticalOffset == 0) {
            if (onStateChangeListener != null && state != State.EXPANDED) {
                onStateChangeListener.onStateChange(State.EXPANDED);
            }
            state = State.EXPANDED;
        } else if (Math.abs(verticalOffset) >= appBarLayout.getTotalScrollRange()) {
            if (onStateChangeListener != null && state != State.COLLAPSED) {
                onStateChangeListener.onStateChange(State.COLLAPSED);
            }
            state = State.COLLAPSED;
        } else {
            if (onStateChangeListener != null && state != State.IDLE) {
                onStateChangeListener.onStateChange(State.IDLE);
            }
            state = State.IDLE;
        }
    }

    public void setOnStateChangeListener(OnStateChangeListener listener) {
        this.onStateChangeListener = listener;
    }

    public State getState() {
        return state;
    }

    public interface OnStateChangeListener {
        void onStateChange(State toolbarChange);
    }

    public enum State {
        COLLAPSED,
        EXPANDED,
        IDLE
    }
}