Android -- Toolbar跟随ListView滑动隐藏和显现

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"  
    xmlns:tools="http://schemas.android.com/tools"  
    android:layout_width="match_parent"  
    android:layout_height="match_parent">  
    <ListView  
        android:id="@+id/main_list_view"  
        android:layout_width="fill_parent"  
        android:layout_height="fill_parent"  
        />  
    <android.support.v7.widget.Toolbar  
        android:id="@+id/main_bar"  
        android:layout_width="fill_parent"  
        android:layout_height="wrap_content"  
        android:background="@color/std_color_A"  
        />  
</RelativeLayout>

public class MainActivity extends BaseActivity{  
  
    private static final String TAG = MainActivity.class.getSimpleName();  
  
    private Toolbar mMainToolbar = null;  
    private ListView mMainListView = null;  
  
    private float mStartY = 0, mLastY = 0, mLastDeltaY;  
  
    @Override  
    protected void onCreate(Bundle savedInstanceState) {  
        super.onCreate(savedInstanceState);  
        setContentView(R.layout.activity_main);  
  
        mMainToolbar = (Toolbar)this.findViewById(R.id.main_bar);  
        this.setSupportActionBar(mMainToolbar);  
  
        mMainListView = (ListView)this.findViewById(R.id.main_list_view);  
        final View header = LayoutInflater.from(this).inflate(R.layout.layout_header, null);//这个header高度要与toolbar相同  
        mMainListView.addHeaderView(header);  
        mMainListView.setAdapter(new AudioAdapter(this));  
    
        mMainListView.setOnTouchListener(new View.OnTouchListener() {  
            @Override  
            public boolean onTouch(View v, MotionEvent event) {  
                final float y = event.getY();  
                float translationY = mMainToolbar.getTranslationY();  
                switch (event.getAction()) {  
                    case MotionEvent.ACTION_DOWN:  
                        mStartY = y;  
                        mLastY = mStartY;  
                        break;  
                    case MotionEvent.ACTION_MOVE:  
                        float mDeltaY = y - mLastY;  
  
                        float newTansY = translationY + mDeltaY;  
                        if (newTansY <= 0 && newTansY >= -mMainToolbar.getHeight()) {  
                            mMainToolbar.setTranslationY(newTansY);  
                        }  
                        mLastY = y;  
                        mLastDeltaY = mDeltaY;  
                        break;  
                    case MotionEvent.ACTION_UP:  
                        ObjectAnimator animator = null;  
                        Log.d(TAG, "mLastDeltaY=" + mLastDeltaY);  
                        if (mLastDeltaY < 0 && mMainListView.getFirstVisiblePosition() > 1) {  
                            Log.v(TAG, "listView.first=" + mMainListView.getFirstVisiblePosition());  
                            animator = ObjectAnimator.ofFloat(mMainToolbar, "translationY", mMainToolbar.getTranslationY(), -mMainToolbar.getHeight());  
                        } else {  
                            animator = ObjectAnimator.ofFloat(mMainToolbar, "translationY", mMainToolbar.getTranslationY(), 0);  
                        }  
                        animator.setDuration(100);  
                        animator.start();  
                        animator.setInterpolator(AnimationUtils.loadInterpolator(MainActivity.this, android.R.interpolator.linear));  
                        break;  
                }  
                return false;  
            }  
        });  
    }  
}