ViewPager 片段随着时间的推移而被破坏?

ViewPager Fragments getting destroyed over time?(ViewPager 片段随着时间的推移而被破坏?)

问题描述

我正在使用 Android 的 support.v4 包开发一个包含多个 Fragment 的 ViewPager.

I am using Android's support.v4 package to develop a ViewPager containing multiple Fragments.

我试图在用户更改页面时隐藏视图.因此，我实现了一个 OnPageChangeListener 来监听我的适配器，并在发生页面更改时在我的 Fragment 中调用 update() 方法.但是我得到了一个 NullPointerException，我不知道为什么.

I am trying to hide the Views when the user changes page. So, I implemented an OnPageChangeListener to listen to my adapter, and call an update() method in my Fragments when when a page change occurs. But I'm getting a NullPointerException, and I cannot figure out why.

代码如下:

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，但它们在那里.bindViews(v) 方法记录如果 charts == null，并且总是返回 false.当我页面时，updateOnPage 被调用，charts == null 返回 true.为什么会发生这种情况，我该如何解决?

wWhen I copy/pasted I removed the SurfaceHolder.Callback methods, but they are there. The bindViews(v) method logs if charts == null, and that always returns false. When I page, updateOnPage gets called, and charts == null returns true. Why does this happen and how do I get around it?

这是第 108 行:

charts.setVisibility(View.GONE);

推荐答案

在初始化你的ViewPager时添加以下内容，

Add the following when you initialize your ViewPager,

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，当另一个页面当前处于焦点时，它将在内存中保留额外的屏幕外页面).这将强制您的 ViewPager 保留所有 Fragment，即使它们不在焦点上.

In your case, this will set the number of off-screen pages to 1 (i.e. it will keep the additional off-screen page in memory when the other page is currently in focus). This will force your ViewPager to keep all of the Fragments even when they are not in focus.

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