仿百度手机助手标题栏透明度随ListView或ScrollView滚动改变的实现方法

浏览数：123 / 时间：2015年06月11日

有时做项目遇到ListView或ScrollView上方加图片和标题栏布局，在滚动时要求改变标题栏的透明度，即上滑标题栏出现，下拉标题栏变透明，类似百度手机助手6.0装机必备界面效果
要实现这种标题栏透明度随滚动渐变的动画很简单，我们只需要实现onScroll方法就可以，完成对HeaderView的位置检测，然后通过HeaderView的高度和显示的高度来计算比例，设置TitleBar的背景图片的透明度即可，此种实现方法与FadingActionBar是不同的。详细实现源码如下：
public class MainActivity extends Activity implements OnScrollListener {
    private XListView listView;
    // 标题栏的布局
    private RelativeLayout title;
    // ListView的头布局
    private View headerView;
    // 头布局的高度
    private int headerHeight;
    private LayoutInflater inflater;
    private Handler handler = new Handler();
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        title = (RelativeLayout) findViewById(R.id.title);
        listView = (XListView) findViewById(R.id.list);
 
        title.getBackground().setAlpha(0);
        inflater = LayoutInflater.from(this);
        headerView = inflater.inflate(R.layout.header, null);
        // 添加头布局
        listView.addHeaderView(headerView);
        listView.setAdapter(new MyAdapter());
        // 设置滚动监听
        listView.setOnScrollListener(this);
        // 设置可以刷新与加载更多
        listView.setPullLoadEnable(true);
        listView.setPullRefreshEnable(true);
        listView.setXListViewListener(new IXListViewListener() {
 
            @Override
            public void onRefresh() {
                // 单纯的模拟刷新过程
                handler.postDelayed(new Runnable() {
 
                    @Override
                    public void run() {
                        listView.stopRefresh();
                    }
                }, 500);
            }
 
            @Override
            public void onLoadMore() {
                // 单纯的模拟加载过程
                handler.postDelayed(new Runnable() {
 
                    @Override
                    public void run() {
                        listView.stopLoadMore();
                    }
                }, 500);
            }
        });
 
    }
 
    // 自定义适配器
    private class MyAdapter extends BaseAdapter {
 
        // 默认显示10个item
        @Override
        public int getCount() {
            return 10;
        }
        @Override
        public Object getItem(int position) {
            return position;
        }
 
        @Override
        public long getItemId(int position) {
            return position;
        }
        @Override
       public View getView(int position, View convertView, ViewGroup parent) {
            if (convertView == null) {
                convertView = inflater.inflate(R.layout.item_list, null);
            }
 
            return convertView;
        }
 
    }
    @Override
    public void onScrollStateChanged(AbsListView view, int scrollState) {
 
    }
 
    // 最重要的方法，标题栏的透明度变化在这个方法实现
    @Override
    public void onScroll(AbsListView listView, int firstVisibleItem,
            int visibleItemCount, int totalItemCount) {
        // 判断当前最上面显示的是不是头布局，因为Xlistview有刷新控件，所以头布局的位置是1，即第二个
        if (firstVisibleItem == 1) {
            // 获取头布局
            View view = listView.getChildAt(0);
            if (view != null) {
                // 获取头布局现在的最上部的位置的相反数
                int top = -view.getTop();
                // 获取头布局的高度
                headerHeight = view.getHeight();
                // 满足这个条件的时候，是头布局在XListview的最上面第一个控件的时候，只有这个时候，我们才调整透明度
                if (top <= headerHeight && top >= 0) {
                    // 获取当前位置占头布局高度的百分比
                    float f = (float) top / (float) headerHeight;
                    title.getBackground().setAlpha((int) (f * 255));
                    // 通知标题栏刷新显示
                    title.invalidate();
                }
            }
        } else if (firstVisibleItem > 1) {
            title.getBackground().setAlpha(255);
        } else {
            title.getBackground().setAlpha(0);
        }
    }
 
}