我对使用ViewPager显示片段的应用程序有问题。一切正常，直到该应用程序进入后台并被操作系统杀死为止。似乎在还原后，我有2个IncidentScreenFragment处理事件，其中一个具有空的主持人（MVP），这会使我的应用程序崩溃。
我的HomeActivity如下：
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
presenter.onViewCreated()
initViews(savedInstanceState)
}
private fun initViews(savedInstanceState: Bundle?){
mapView.onCreate(savedInstanceState)
mapView.getMapAsync(this)
initFragment()
initMenu()
}
private fun initFragment(){
homeFragment = HomeScreenFragment.newInstance()
incidentFragment = IncidentScreenFragment.newInstance()
chatFragment = ChatFragment.newInstance()
weatherFragment = WeatherFragment.newInstance()
viewPager.adapter = ViewPagerAdapter(supportFragmentManager, this)
viewPager.offscreenPageLimit = 4
viewPager?.addOnPageChangeListener(object : ViewPager.OnPageChangeListener {
override fun onPageScrollStateChanged(state: Int) {}
override fun onPageScrolled(position: Int, positionOffset: Float, positionOffsetPixels: Int) {}
override fun onPageSelected(position: Int) {bottom_navigation.currentItem = position}
})
}
override fun getFragmentByPos(pos: Int): Fragment {
return when(pos){
0 -> homeFragment
1 -> incidentFragment
2 -> chatFragment
3 -> weatherFragment
else -> {
homeFragment
}
}
}
我的适配器：
class ViewPagerAdapter internal constructor(fm: FragmentManager, activity:infinite_software.intelligence_center.intelligencecenter.ui.home.FragmentManager) : FragmentPagerAdapter(fm) {
private val COUNT = 4
private val activity = activity
override fun getItem(position: Int): Fragment{
var fragment: Fragment? = null
when (position) {
0 -> fragment = activity.getFragmentByPos(0)
1 -> fragment = activity.getFragmentByPos(1)
2 -> fragment = activity.getFragmentByPos(2)
3 -> fragment = activity.getFragmentByPos(3)
}
return fragment!!
}
override fun destroyItem(container: ViewGroup, position: Int, `object`: Any) {
super.destroyItem(container, position, `object`)
}
override fun getCount(): Int {
return COUNT
}
override fun getPageTitle(position: Int): CharSequence? {
return "Section " + (position + 1)
}
}
每个Fragment都有一个返回新Fragment的静态方法：
companion object {
fun newInstance(): HomeScreenFragment {
return HomeScreenFragment()
}
}
当应用程序在后台被杀死时，我发现有2个对象（片段）可以监听事件，其中一个带有Presenter正确实例化，另一个没有实例化。
在我的抽象BaseFragment类下面：
abstract class BaseFragment<P : BasePresenter<BaseView>> : BaseView,Fragment() {
protected lateinit var presenter: P
override fun getContext(): Context {
return activity as Context
}
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
return super.onCreateView(inflater, container, savedInstanceState)
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
presenter = instantiatePresenter()
}
override fun showError(error: String) {
(activity as BaseActivity<BasePresenter<BaseView>>).showError(error)
}
override fun showError(errorResId: Int) {
(activity as BaseActivity<BasePresenter<BaseView>>).showError(errorResId)
}
abstract fun onBackPressed(): Boolean
/**
* Instantiates the presenter the Fragment is based on.
*/
protected abstract fun instantiatePresenter(): P
abstract val TAG: String
事件片段代码：
class IncidentScreenFragment: BaseFragment<IncidentScreenPresenter>(), BaseView, IncidentView, AlertFilterListener, AlertItemClickListener, IncidentDetailListener {
var rvAdapter : IncidentAdapter? = null
var state : Int = LIST_STATE
override fun instantiatePresenter(): IncidentScreenPresenter {
return IncidentScreenPresenter(this)
}
override val TAG: String
get() = "INCIDENT"
override fun getContext(): Context {
return activity as Context
}
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
super.onCreateView(inflater, container, savedInstanceState)
return inflater.inflate(R.layout.fragment_incident, container, false)
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
initViews()
presenter.onViewCreated()
initObserve()
}
private fun initViews(){
//Reclycler view
alertRV.layoutManager = LinearLayoutManager(context)
rvAdapter = IncidentAdapter(ArrayList(), context, this)
alertRV.adapter = rvAdapter
//Apply Listeners
headerBox.setFilterListener(this)
incidentDetailView.setListener(this)
}
override fun initObserve() {
//Init observe presenter model
val alertObserver = Observer<ArrayList<AlertModel>> { alerts ->
Timber.d("Data received from Presenter [$alerts]")
showAlertList(alerts)
}
presenter.filteredAlertList.observe(context as BaseActivity<BasePresenter<BaseView>>,alertObserver)
}
override fun updateThisFilters(boxState: Boolean, level: Int) {
presenter.updateFilterList(boxState,level)
}
fun showOnlyThisLevel(level:Int){
presenter.showOnlyThisLevel(level)
headerBox.disableBoxExcept(level)
}
fun showAlertList(list: ArrayList<AlertModel>){
rvAdapter?.updateData(list)
}
override fun onItemClick(model: AlertModel) {
presenter.loadAlertDetail(model)
}
override fun showAlertDetail(model: AlertModel) {
incidentDetailView.setUpFromModel(model)
WhiteWizard.slideLeftEffect(incidentDetailView,incidentListRootElement)
state = DETAIL_STATE
}
override fun onbackFromDetailPressed() {
WhiteWizard.slideRightEffect(incidentListRootElement,incidentDetailView)
state = LIST_STATE
}
override fun showLoader() {
loaderIncident.visibility = View.VISIBLE
}
override fun hideLoader() {
loaderIncident.visibility = View.INVISIBLE
}
override fun onBackPressed(): Boolean {
when(state){
LIST_STATE -> return false
DETAIL_STATE -> {
onbackFromDetailPressed()
return true
}
else -> return false
}
}
fun newInstance(): IncidentScreenFragment {
return IncidentScreenFragment()
}
}
当我单击主页上的按钮以显示片段内容时，我得到了：
Process: XXXXXX, PID: 3192
kotlin.UninitializedPropertyAccessException: lateinit property presenter has not been initialized
at infinite_software.intelligence_center.intelligencecenter.base.BaseFragment.getPresenter(BaseFragment.kt:11)
at XXXXXX.ui.home.incidentScreen.IncidentScreenFragment.showOnlyThisLevel(IncidentScreenFragment.kt:78)
at XXXXXX.ui.home.HomeActivity.filterDataWithSeverity(HomeActivity.kt:110)
at XXXXXX.ui.home.homeScreen.HomeScreenFragment.filterBy(HomeScreenFragment.kt:76)
at XXXXXX.ui.home.homeScreen.HomeScreenFragment$initViews$5.onClick(HomeScreenFragment.kt:56)
如果我尝试打印Fragment的ID，我会从方法调用showOnlyThisLevel（）和onBackPressed（）获得2个不同的ID。我想念什么？
答案 0 :(得分：3)
在仔细研究了证据之后，我必须得出结论，问题是源于FragmentPagerAdapter代表Android支持库作者的方法的错误命名，没有明确指出抽象方法getItem(int position)应该 返回一个片段的新实例 ，而不仅仅是“获取一个实例”。
当然，对于不正确的名称已经有7年的历史了，我们无能为力，但是至少我们可以在您的代码中解决由该问题引起的错误；）
事不宜迟，您造成NPE的原因是从未调用过onCreateView（演示者在其中实例化的地方）。
发生这种情况是因为您要在此处创建片段：
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
...
homeFragment = HomeScreenFragment.newInstance()
incidentFragment = IncidentScreenFragment.newInstance()
}
您从FragmentPagerAdapter的getItem(int position)内部返回此片段：
override fun getItem(position: Int): Fragment = when(position) {
...
1 -> activity.incidentFragment
...
}
所以我们对activity.incidentFragment的了解是，其中从未onCreateView()被调用。
这是由于它从未真正添加到FragmentManager并且从未在屏幕上显示。
这是因为活动 中的super.onCreate(savedInstanceState)使用其无参数构造函数通过反射重新创建所有片段，同时保留其标签（请参见findFragmentByTag） 。
因此，如您在this answer中所看到的，或者正如我在这里引用的那样：
// Do we already have this fragment?
String name = makeFragmentName(container.getId(), itemId);
Fragment fragment = mFragmentManager.findFragmentByTag(name);
if (fragment != null) {
if (DEBUG) Log.v(TAG, "Attaching item #" + itemId + ": f=" + fragment);
mCurTransaction.attach(fragment);
} else {
fragment = getItem(position);
if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment);
mCurTransaction.add(container.getId(), fragment,
makeFragmentName(container.getId(), itemId));
仅当FragmentPagerAdapter为该片段设置的片段标记未找到该片段时，才调用getItem(position)方法，该片段在内存不足的情况下杀死您的应用程序后会自动重新创建。
因此，永远不会使用您的新片段（您在Activity中手动创建的片段），因此它没有视图，也没有初始化，也没有添加到FragmentManager，它与ViewPager内的实例不同，并且调用时会崩溃。轰！
解决方案是在FragmentPagerAdapter的getItem(position)方法中实例化Fragment。要获取片段的实例，请使用this answer。