Logo-hosting CollapsingToolbarLayout

attrs.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="CollapsingLogoToolbarLayout">
        <attr name="logoViewId" format="reference" />
        <attr name="collapsedViewOffset" format="dimension" />
        <attr name="expandedViewOffset" format="dimension" />
    </declare-styleable>
</resources>

CollapsingLogoToolbarLayout.java

package com.actinarium.random.common;

import android.content.Context;
import android.content.res.TypedArray;
import android.support.design.widget.AppBarLayout;
import android.support.design.widget.CollapsingToolbarLayout;
import android.support.v4.view.ViewCompat;
import android.support.v4.view.WindowInsetsCompat;
import android.util.AttributeSet;
import android.view.View;
import android.view.ViewParent;
import com.actinarium.random.R;

import java.lang.reflect.Field;

/**
 * <p></p>
 *
 * @author Paul Danyliuk
 * @version $Id$
 */
public class CollapsingLogoToolbarLayout extends CollapsingToolbarLayout {

    private int mLogoViewId;
    private int mExpandedViewOffset;
    private int mCollapsedViewOffset;

    private View mLogoView;
    private WindowInsetsCompat mLastInsets;
    private AppBarLayout.OnOffsetChangedListener mLogoOffsetChangeListener;

    public CollapsingLogoToolbarLayout(Context context) {
        this(context, null);
    }

    public CollapsingLogoToolbarLayout(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public CollapsingLogoToolbarLayout(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);

        TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.CollapsingLogoToolbarLayout);
        mLogoViewId = a.getResourceId(R.styleable.CollapsingLogoToolbarLayout_logoViewId, -1);
        mExpandedViewOffset = a.getDimensionPixelOffset(R.styleable.CollapsingLogoToolbarLayout_expandedViewOffset, 0);
        mCollapsedViewOffset = a.getDimensionPixelOffset(R.styleable.CollapsingLogoToolbarLayout_collapsedViewOffset, 0);
        a.recycle();
    }

    @Override
    protected void onAttachedToWindow() {
        ViewParent parent = getParent();
        if(parent instanceof AppBarLayout) {
            if(mLogoOffsetChangeListener == null) {
                mLogoOffsetChangeListener = new LogoOffsetUpdateListener();
            }
            ((AppBarLayout)parent).addOnOffsetChangedListener(this.mLogoOffsetChangeListener);
        }

        super.onAttachedToWindow();
    }

    @Override
    protected void onDetachedFromWindow() {
        ViewParent parent = this.getParent();
        if(this.mLogoOffsetChangeListener != null && parent instanceof AppBarLayout) {
            ((AppBarLayout)parent).removeOnOffsetChangedListener(this.mLogoOffsetChangeListener);
        }

        super.onDetachedFromWindow();
    }

    /**
     * This method takes care of updating logo view based on expansion fraction
     * @param fraction Expansion fraction: 0 if action bar space is fully expanded; 1 if collapsed
     */
    protected void onExpansionFractionUpdate(float fraction) {
        ensureLogoView();
        mLogoView.setTranslationY(mExpandedViewOffset + (mCollapsedViewOffset - mExpandedViewOffset) * fraction);
    }

    private void ensureLogoView() {
        if (mLogoView == null) {
            mLogoView = findViewById(mLogoViewId);
        }
    }

    private void ensureWindowInset() {
        if (mLastInsets == null) {
            // Reflection...
            //noinspection TryWithIdenticalCatches
            try {
                Field field = CollapsingToolbarLayout.class.getDeclaredField("mLastInsets");
                field.setAccessible(true);
                this.mLastInsets = (WindowInsetsCompat) field.get(this);
            } catch (NoSuchFieldException ignored) {
            } catch (IllegalAccessException ignored) {
            }
        }
    }

    private class LogoOffsetUpdateListener implements AppBarLayout.OnOffsetChangedListener {
        private LogoOffsetUpdateListener() {
        }

        public void onOffsetChanged(AppBarLayout layout, int verticalOffset) {
            ensureWindowInset();
            int insetTop = CollapsingLogoToolbarLayout.this.mLastInsets != null ? CollapsingLogoToolbarLayout.this.mLastInsets.getSystemWindowInsetTop() : 0;
            int expandRange = CollapsingLogoToolbarLayout.this.getHeight() - ViewCompat.getMinimumHeight(CollapsingLogoToolbarLayout.this) - insetTop;
            CollapsingLogoToolbarLayout.this.onExpansionFractionUpdate((float)Math.abs(verticalOffset) / (float)expandRange);
        }
    }
}