我使用Android的 support.v4 包来开发一个包含多个ViewPager 片段秒。
我试图隐藏视图，当用户更改页面。所以，我实现了一个 OnPageChangeListener 来听我的适配器，在我的片段 S当一个网页发生变化的时候。但我发现了一个 NullPointerException异常，我想不通这是为什么。
下面是code：
公共类MyActivity扩展FragmentActivity工具
OnPageChangeListener {
私有静态最终诠释NUM_ITEMS = 2;
私人ViewPager mPager;
私有静态SpaceAdapter mAdapter;
/ **第一次创建活动时调用。 * /
@覆盖
公共无效的onCreate（包savedInstanceState）{
super.onCreate（savedInstanceState）;
的setContentView（R.layout.fragment_pager）;
mAdapter =新SpaceAdapter（getSupportFragmentManager（））;
mPager =（ViewPager）findViewById（R.id.pager）;
mPager.setAdapter（mAdapter）;
mPager.setOnPageChangeListener（本）;
}
公共静态类SpaceAdapter扩展FragmentPagerAdapter {
公共SpaceAdapter（FragmentManager FM）{
超（FM）;
}
@覆盖
公众诠释getCount将（）{
返回NUM_ITEMS;
}
@覆盖
公共片段的getItem（INT位置）{
开关（位置）{
情况下0：
返回新MyFragment（）;
情况1：
返回新MyFragment（）;
默认：
返回新MyFragment（）;
}
}
}
@覆盖
公共无效onPageScrollStateChanged（INT为arg0）{
Log.i（分页，滚动状态改为：+为arg0）;
}
@覆盖
公共无效onPageScrolled（INT为arg0，浮动ARG1，ARG2 INT）{
// Log.i（分页，页面滚动：+为arg0 +与+ ARG1 +，+ ARG2）;
如果（mAdapter.getItem（arg0中）的instanceof IUpdateOnPage）{
（（IUpdateOnPage）mAdapter.getItem（arg0中））updateOnPage（）。
}
}
@覆盖
公共无效onPageSelected（INT为arg0）{
（选定的页面：分页，+为arg0）Log.i;
}
}
和
公共类MyFragment扩展片段工具
SurfaceHolder.Callback，IUpdateOnPage {
私人SurfaceView图;
@覆盖
公共无效的onCreate（包savedInstanceState）{
super.onCreate（savedInstanceState）;
}
@覆盖
公共查看onCreateView（LayoutInflater充气，容器的ViewGroup，
捆绑savedInstanceState）{
视图V = inflater.inflate（R.layout.crew，NULL）;
bindViews（五）;
返回伏;
}
私人无效bindViews（视图v）{
图表=（SurfaceView）v.findViewById（R.id.civ_charts）;
。charts.getHolder（）的addCallback（本）;
Log.i（跆拳道，呵呵+将String.valueOf（图== NULL））; //日志假
}
@覆盖
公共无效updateOnPage（）{
如果（图！= NULL）{
Log.i（剧组面，隐藏）;
charts.setVisibility（View.GONE）;
} 其他 {
Log.i（剧组面，图表为空）; //这个在这里得到记录
}
}
}
w当我复制/粘贴我删除了 SurfaceHolder.Callback 方法，但它们的存在。该 bindViews（V）方法日志，如果图表== NULL ，并且始终返回false。当我页， updateOnPage 被调用，而图表== NULL 返回true。为什么会这样？我如何围绕它得到什么呢？
这是行108：
charts.setVisibility（View.GONE）;
解决方案
添加以下时，初始化 ViewPager ，
mPager.setOffscreenPageLimit（NUM_ITEMS-1）;
在你的情况，这将设置的屏幕外的页面1（即它将保持额外的离屏页面的内存时，另一个页面当前处于焦点的）的数量。这将迫使你的 ViewPager 保留所有的片段第即使他们没有焦点。
I am using Android's support.v4 package to develop a ViewPager containing multiple Fragments.
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.
Here is the code:
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);
}
}
and
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
}
}
}
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?
and this is line 108:
charts.setVisibility(View.GONE);
解决方案
Add the following when you initialize your ViewPager,
mPager.setOffscreenPageLimit(NUM_ITEMS-1);
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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