Android顶部标题栏随滑动时的渐变隐藏和渐变显示效果

compile 'com.bartoszlipinski.recyclerviewheader:library:1.2.1'
compile 'com.android.support:recyclerview-v7:25.3.1'

public class MainActivity extends AppCompatActivity {
  private TextView mTextView;
  private int mTextViewHeight;
  private RecyclerView mRecyclerView;
  private RecyclerViewHeader mRecyclerViewHeader;
  //头部图片(轮播图的高度)
  private int mRecyclerHeaderBannerHeight;
  //头部的高度
  private int mRecyclerHeaderHeight;
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mTextView = (TextView) findViewById(R.id.home_text);
    //recyclerView填充数据(忽略不计)
    initData();
    mRecyclerView = (RecyclerView) findViewById(R.id.recycler);
    mRecyclerView.setLayoutManager(new LinearLayoutManager(this));
    mRecyclerView.setAdapter(mAdapter);
    //获取到文本的高度
    mTextViewHeight = mTextView.getHeight();
    //轮播图片的高度--和xml图片的高度是一样的
    mRecyclerHeaderBannerHeight = (int) getResources().getDimension(R.dimen.home_page_banner_height);
    //RecyclerView每个Item之间的距离,和Adapter中设置的距离一样
    final int recyclerItemHeight = (int) getResources().getDimension(R.dimen.home_page_list_item_margin_top);
    //添加头部视图,其布局文件就忽略
    mRecyclerViewHeader = RecyclerViewHeader.fromXml(this, R.layout.list_item_prime_product_header);

    //设置其滑动事件
    mRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
      @Override
      public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
        //设置其透明度
        float alpha = 0;
        int scollYHeight = getScollYHeight(true, mRecyclerHeaderHeight);
        //起始截止变化高度,如可以变化高度为mRecyclerHeaderHeight
        int baseHeight = mRecyclerHeaderBannerHeight - recyclerItemHeight - mTextViewHeight;
        if(scollYHeight >= baseHeight) {
          //完全不透明
          alpha = 1;
        }else {
        //产生渐变效果
          alpha = scollYHeight / (baseHeight*1.0f);
        }
        mTextView.setAlpha(alpha);
        super.onScrolled(recyclerView, dx, dy);
      }
    });
    //将头部视图添加到RecyclerView中
    mRecyclerViewHeader.attachTo(mRecyclerView);
    //第一次进来其状态显示
    mRecyclerViewHeader.post(new Runnable() {
      @Override
      public void run() {
        mRecyclerHeaderHeight = mRecyclerViewHeader.getHeight();
        mTextViewHeight = mTextView.getHeight();
        mTextView.setVisibility(View.VISIBLE);
        mTextView.setAlpha(0);
      }
    });
  }

/**
   * 计算RecyclerView滑动的距离
   * @param hasHead 是否有头部
   * @param headerHeight RecyclerView的头部高度
   * @return 滑动的距离
   */
  private int getScollYHeight(boolean hasHead, int headerHeight) {
    LinearLayoutManager layoutManager = (LinearLayoutManager) mRecyclerView.getLayoutManager();
    //获取到第一个可见的position,其添加的头部不算其position当中
    int position = layoutManager.findFirstVisibleItemPosition();
    //通过position获取其管理器中的视图
    View firstVisiableChildView = layoutManager.findViewByPosition(position);
    //获取自身的高度
    int itemHeight = firstVisiableChildView.getHeight();
    //有头部
    if(hasHead) {
      return headerHeight + itemHeight*position - firstVisiableChildView.getTop();
    }else {
      return itemHeight*position - firstVisiableChildView.getTop();
    }
  }