ViewPager片段随着时间的流逝而被破坏？

Question

我正在使用Android的support.v4软件包来开发包含多个Fragment的ViewPager 。

我正在尝试在用户更改页面时隐藏视图。因此，我实现了一个OnPageChangeListener来监听适配器，并update()在Fragment发生页面更改时在s中调用一个方法。但是我得到了NullPointerException，我不知道为什么。

这是代码：

public class MyActivity extends FragmentActivity implements
        OnPageChangeListener {

    private static final int NUM_ITEMS = 2;
    private ViewPager mPager;
    private static SpaceAdapter mAdapter;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.fragment_pager);

        mAdapter = new SpaceAdapter(getSupportFragmentManager());
        mPager = (ViewPager) findViewById(R.id.pager);
        mPager.setAdapter(mAdapter);

        mPager.setOnPageChangeListener(this);
}

    public static class SpaceAdapter extends FragmentPagerAdapter {

        public SpaceAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public int getCount() {
            return NUM_ITEMS;
        }

        @Override
        public Fragment getItem(int position) {
            switch (position) {
            case 0:
            return new MyFragment();
            case 1:
            return new MyFragment();
            default:
            return new MyFragment();
            }
        }
    }

    @Override
    public void onPageScrollStateChanged(int arg0) {
        Log.i("pagination", "scroll state changed: " + arg0);
}

    @Override
    public void onPageScrolled(int arg0, float arg1, int arg2) {
        // Log.i("pagination", "page scroll: " + arg0 + " with: " + arg1 + ", " + arg2);
        if (mAdapter.getItem(arg0) instanceof IUpdateOnPage) {
            ((IUpdateOnPage) mAdapter.getItem(arg0)).updateOnPage();
        }
    }

    @Override
    public void onPageSelected(int arg0) {
        Log.i("pagination", "page selected: " + arg0);
    }
}

和

public class MyFragment extends Fragment implements
        SurfaceHolder.Callback, IUpdateOnPage {

    private SurfaceView charts;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.crew, null);
        bindViews(v);
        return v;
    }

    private void bindViews(View v) {
        charts = (SurfaceView) v.findViewById(R.id.civ_charts);

        charts.getHolder().addCallback(this);
        Log.i("wtf", "huh " + String.valueOf(charts == null)); // logs false
    }

    @Override
    public void updateOnPage() {
        if (charts != null) {
            Log.i("crew surface", "hidden");
            charts.setVisibility(View.GONE);
        } else {
            Log.i("crew surface", "charts is null");  //THIS HERE gets logged
        }
    }
}

w当我复制/粘贴时，我删除了SurfaceHolder.Callback方法，但是它们在那里。该bindViews(v)方法记录if charts == null，并且始终返回false。当我寻呼时，updateOnPage被调用并charts == null返回true。为什么会发生这种情况，我该如何解决？

这是第108行：

charts.setVisibility(View.GONE);

Answer 1

初始化时ViewPager，添加以下内容：

mPager.setOffscreenPageLimit(NUM_ITEMS-1);

在您的情况下，这会将屏幕外页面的数量设置为1（即，当其他页面当前处于焦点状态时，它将把其他屏幕外页面保留在内存中）。这将迫使您ViewPager保留所有Fragments，即使它们不在焦点上。