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

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);

        }

    }

 

}