不会相应地调用片段的生命周期回调(life cycle callbacks of fragments are not called accordingly)

我有一个主要的活动与视图寻呼机和三个动作标签“片段”。我为每个主要活动和三个片段实现了生命周期回调。并且在每个生命周期回调中我放置了一个日志语句，指示调用哪个生命周期回调以了解具有视图寻呼机的操作栏的行为。在运行时我发现了一种奇怪的行为，我无法理解或归因于任何原因。

第一个行为

when the App fisrt starts I receive: 02-08 15:16:14.771 32243-32243/com.example.com.vpager_00 W/MainActivity: onCreate() 02-08 15:16:14.901 32243-32243/com.example.com.vpager_00 W/MainActivity: onStart() 02-08 15:16:14.901 32243-32243/com.example.com.vpager_00 W/MainActivity: onResume() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_1: onAttach() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_1: onCreate() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_1: onCreateView() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_1: onStart() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_1: onResume() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_2: onAttach() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_2: onCreate() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_2: onCreateView() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_2: onStart() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_2: onResume()

我找不到任何理由为什么只有mainactivty，frag_1和frag_2的生命周期回调被称为但不是frag_3？任何解释？

第二种行为

occured when i touched tab3 "frag_3", i received: 02-08 15:16:36.031 32243-32243/com.example.com.vpager_00 D/ViewRootImpl: ViewPostImeInputStage ACTION_DOWN 02-08 15:16:36.121 32243-32243/com.example.com.vpager_00 W/Frag_3: onAttach() 02-08 15:16:36.121 32243-32243/com.example.com.vpager_00 W/Frag_3: onCreate() 02-08 15:16:36.121 32243-32243/com.example.com.vpager_00 W/Frag_3: onCreateView() 02-08 15:16:36.121 32243-32243/com.example.com.vpager_00 W/Frag_3: onStart() 02-08 15:16:36.121 32243-32243/com.example.com.vpager_00 W/Frag_3: onResume() 02-08 15:16:36.461 32243-32243/com.example.com.vpager_00 W/Frag_1: onPause() 02-08 15:16:36.461 32243-32243/com.example.com.vpager_00 W/Frag_1: onStop() 02-08 15:16:36.471 32243-32243/com.example.com.vpager_00 W/Frag_1: onDestroy()

我认为生命周期回调被称为这是可以理解的，但是为什么只有frag_1的生命周期回调也被称为，我认为，除了frag_3和frag_1的回调之外，aslo frag_2回调必须被调用，因为frag_2在其onResume中（）最近状态。

任何解释？

主要活动

public class MainActivity extends AppCompatActivity { private final String TAG = this.getClass().getSimpleName(); private Toolbar mTB = null; private TabLayout mTL = null; private ViewPager mVP = null; private VPagerAdapter mVPAdapter = null; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); Log.w(TAG, "onCreate()"); this.initViews(R.layout.act_main); this.initObjs(); } private void initObjs() { this.mVPAdapter = new VPagerAdapter(getSupportFragmentManager(), this.mTL.getTabCount()); this.mVP.setAdapter(this.mVPAdapter); this.mVP.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(this.mTL)); this.mTL.setOnTabSelectedListener(new TabLayout.OnTabSelectedListener() { @Override public void onTabSelected(TabLayout.Tab tab) { mVP.setCurrentItem(tab.getPosition()); } @Override public void onTabUnselected(TabLayout.Tab tab) { } @Override public void onTabReselected(TabLayout.Tab tab) { } }); } private void initViews(int rootView) { setContentView(rootView); this.mTB = (Toolbar) findViewById(R.id.toolbar); this.mTL = (TabLayout) findViewById(R.id.tab_layout); this.mVP = (ViewPager) findViewById(R.id.pager); setSupportActionBar(this.mTB); this.mTL.addTab(this.mTL.newTab().setText("Tab 1")); this.mTL.addTab(this.mTL.newTab().setText("Tab 2")); this.mTL.addTab(this.mTL.newTab().setText("Tab 3")); this.mTL.setTabGravity(TabLayout.GRAVITY_FILL); } @Override protected void onStart() { super.onStart(); Log.w(TAG, "onStart()"); } @Override protected void onResume() { super.onResume(); Log.w(TAG, "onResume()"); } @Override protected void onPause() { super.onPause(); Log.w(TAG, "onPause()"); } @Override protected void onStop() { super.onStop(); Log.w(TAG, "onStop()"); } @Override protected void onDestroy() { super.onDestroy(); Log.w(TAG, "onDestroy()"); }

}

frag_1

public class Frag_1 extends Fragment { private final String TAG = this.getClass().getSimpleName(); @Override public void onAttach(Context context) { super.onAttach(context); Log.w(TAG, "onAttach()"); } @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); Log.w(TAG, "onCreate()"); } @Nullable @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { Log.w(TAG, "onCreateView()"); return inflater.inflate(R.layout.frag_1, container, false); } @Override public void onStart() { super.onStart(); Log.w(TAG, "onStart()"); } @Override public void onResume() { super.onResume(); Log.w(TAG, "onResume()"); } @Override public void onPause() { super.onPause(); Log.w(TAG, "onPause()"); } @Override public void onStop() { super.onStop(); Log.w(TAG, "onStop()"); } @Override public void onDestroy() { super.onDestroy(); Log.w(TAG, "onDestroy()"); }

}

frag_2 ：

public class Frag_2 extends Fragment { private final String TAG = this.getClass().getSimpleName(); @Override public void onAttach(Context context) { super.onAttach(context); Log.w(TAG, "onAttach()"); } @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); Log.w(TAG, "onCreate()"); } @Nullable @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { Log.w(TAG, "onCreateView()"); return inflater.inflate(R.layout.frag_2, container, false); } @Override public void onStart() { super.onStart(); Log.w(TAG, "onStart()"); } @Override public void onResume() { super.onResume(); Log.w(TAG, "onResume()"); } @Override public void onPause() { super.onPause(); Log.w(TAG, "onPause()"); } @Override public void onStop() { super.onStop(); Log.w(TAG, "onStop()"); } @Override public void onDestroy() { super.onDestroy(); Log.w(TAG, "onDestroy()"); }

}

frag_3 ：

public class Frag_3 extends Fragment { private final String TAG = this.getClass().getSimpleName(); @Override public void onAttach(Context context) { super.onAttach(context); Log.w(TAG, "onAttach()"); } @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); Log.w(TAG, "onCreate()"); } @Nullable @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { Log.w(TAG, "onCreateView()"); return inflater.inflate(R.layout.frag_3, container, false); } @Override public void onStart() { super.onStart(); Log.w(TAG, "onStart()"); } @Override public void onResume() { super.onResume(); Log.w(TAG, "onResume()"); } @Override public void onPause() { super.onPause(); Log.w(TAG, "onPause()"); } @Override public void onStop() { super.onStop(); Log.w(TAG, "onStop()"); } @Override public void onDestroy() { super.onDestroy(); Log.w(TAG, "onDestroy()"); }

}

I have a main activity with view pager and three action tabs "fragment". and i implemented the life cycle callbacks for each of the main activity and the three fragments. and in each of the life cycle call back i placed a log statements indicates which life cycle callback being called to know how the action bars with view pager behaves. at run time i perceived a strange behaviour which i cant understand or attribute it to any reason.

first behaviour

when the App fisrt starts I receive: 02-08 15:16:14.771 32243-32243/com.example.com.vpager_00 W/MainActivity: onCreate() 02-08 15:16:14.901 32243-32243/com.example.com.vpager_00 W/MainActivity: onStart() 02-08 15:16:14.901 32243-32243/com.example.com.vpager_00 W/MainActivity: onResume() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_1: onAttach() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_1: onCreate() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_1: onCreateView() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_1: onStart() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_1: onResume() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_2: onAttach() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_2: onCreate() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_2: onCreateView() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_2: onStart() 02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_2: onResume()

and i cant find any reason why only the life cycle callbacks of the mainactivty, frag_1 and frag_2 were called BUT NOT frag_3? any explaination?

2nd behaviour

occured when i touched tab3 "frag_3", i received: 02-08 15:16:36.031 32243-32243/com.example.com.vpager_00 D/ViewRootImpl: ViewPostImeInputStage ACTION_DOWN 02-08 15:16:36.121 32243-32243/com.example.com.vpager_00 W/Frag_3: onAttach() 02-08 15:16:36.121 32243-32243/com.example.com.vpager_00 W/Frag_3: onCreate() 02-08 15:16:36.121 32243-32243/com.example.com.vpager_00 W/Frag_3: onCreateView() 02-08 15:16:36.121 32243-32243/com.example.com.vpager_00 W/Frag_3: onStart() 02-08 15:16:36.121 32243-32243/com.example.com.vpager_00 W/Frag_3: onResume() 02-08 15:16:36.461 32243-32243/com.example.com.vpager_00 W/Frag_1: onPause() 02-08 15:16:36.461 32243-32243/com.example.com.vpager_00 W/Frag_1: onStop() 02-08 15:16:36.471 32243-32243/com.example.com.vpager_00 W/Frag_1: onDestroy()

i think being the life cycle callbackes were called this is understandable, but why ONLY the life cycle callbacks of frag_1 also called, i think, beside the callbacks of frag_3 and frag_1, aslo frag_2 callbacks must have been called because frag_2 was in its onResume() state lately.

any explaination?

mainactivity

public class MainActivity extends AppCompatActivity { private final String TAG = this.getClass().getSimpleName(); private Toolbar mTB = null; private TabLayout mTL = null; private ViewPager mVP = null; private VPagerAdapter mVPAdapter = null; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); Log.w(TAG, "onCreate()"); this.initViews(R.layout.act_main); this.initObjs(); } private void initObjs() { this.mVPAdapter = new VPagerAdapter(getSupportFragmentManager(), this.mTL.getTabCount()); this.mVP.setAdapter(this.mVPAdapter); this.mVP.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(this.mTL)); this.mTL.setOnTabSelectedListener(new TabLayout.OnTabSelectedListener() { @Override public void onTabSelected(TabLayout.Tab tab) { mVP.setCurrentItem(tab.getPosition()); } @Override public void onTabUnselected(TabLayout.Tab tab) { } @Override public void onTabReselected(TabLayout.Tab tab) { } }); } private void initViews(int rootView) { setContentView(rootView); this.mTB = (Toolbar) findViewById(R.id.toolbar); this.mTL = (TabLayout) findViewById(R.id.tab_layout); this.mVP = (ViewPager) findViewById(R.id.pager); setSupportActionBar(this.mTB); this.mTL.addTab(this.mTL.newTab().setText("Tab 1")); this.mTL.addTab(this.mTL.newTab().setText("Tab 2")); this.mTL.addTab(this.mTL.newTab().setText("Tab 3")); this.mTL.setTabGravity(TabLayout.GRAVITY_FILL); } @Override protected void onStart() { super.onStart(); Log.w(TAG, "onStart()"); } @Override protected void onResume() { super.onResume(); Log.w(TAG, "onResume()"); } @Override protected void onPause() { super.onPause(); Log.w(TAG, "onPause()"); } @Override protected void onStop() { super.onStop(); Log.w(TAG, "onStop()"); } @Override protected void onDestroy() { super.onDestroy(); Log.w(TAG, "onDestroy()"); }

}

frag_1

public class Frag_1 extends Fragment { private final String TAG = this.getClass().getSimpleName(); @Override public void onAttach(Context context) { super.onAttach(context); Log.w(TAG, "onAttach()"); } @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); Log.w(TAG, "onCreate()"); } @Nullable @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { Log.w(TAG, "onCreateView()"); return inflater.inflate(R.layout.frag_1, container, false); } @Override public void onStart() { super.onStart(); Log.w(TAG, "onStart()"); } @Override public void onResume() { super.onResume(); Log.w(TAG, "onResume()"); } @Override public void onPause() { super.onPause(); Log.w(TAG, "onPause()"); } @Override public void onStop() { super.onStop(); Log.w(TAG, "onStop()"); } @Override public void onDestroy() { super.onDestroy(); Log.w(TAG, "onDestroy()"); }

}

frag_2:

public class Frag_2 extends Fragment { private final String TAG = this.getClass().getSimpleName(); @Override public void onAttach(Context context) { super.onAttach(context); Log.w(TAG, "onAttach()"); } @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); Log.w(TAG, "onCreate()"); } @Nullable @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { Log.w(TAG, "onCreateView()"); return inflater.inflate(R.layout.frag_2, container, false); } @Override public void onStart() { super.onStart(); Log.w(TAG, "onStart()"); } @Override public void onResume() { super.onResume(); Log.w(TAG, "onResume()"); } @Override public void onPause() { super.onPause(); Log.w(TAG, "onPause()"); } @Override public void onStop() { super.onStop(); Log.w(TAG, "onStop()"); } @Override public void onDestroy() { super.onDestroy(); Log.w(TAG, "onDestroy()"); }

}

frag_3:

public class Frag_3 extends Fragment { private final String TAG = this.getClass().getSimpleName(); @Override public void onAttach(Context context) { super.onAttach(context); Log.w(TAG, "onAttach()"); } @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); Log.w(TAG, "onCreate()"); } @Nullable @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { Log.w(TAG, "onCreateView()"); return inflater.inflate(R.layout.frag_3, container, false); } @Override public void onStart() { super.onStart(); Log.w(TAG, "onStart()"); } @Override public void onResume() { super.onResume(); Log.w(TAG, "onResume()"); } @Override public void onPause() { super.onPause(); Log.w(TAG, "onPause()"); } @Override public void onStop() { super.onStop(); Log.w(TAG, "onStop()"); } @Override public void onDestroy() { super.onDestroy(); Log.w(TAG, "onDestroy()"); }

}

最满意答案

在ViewPager中，提前创建了片段。

这意味着，您的Frag_2尚未可见（因为Frag_1覆盖整个屏幕），但它仍然会预先创建视图，因此滚动到Frag_2将是平滑的。

默认情况下，ViewPager会在每一侧预加载一个片段。因此，如果您将ViewPager的起始位置设置为Frag_2 ，它也会加载Frag_1和Frag_2 ，因为它们是邻居。

您会注意到当您滑动到Frag_2时将创建Frag_2

您可以通过调用ViewPager.setOffscreenPageLimit(int limit)来增加预加载片段的数量。

注意最小。 number为1，因此无法禁用。

In the ViewPager, fragments are being created ahead of time.

That means, that your Frag_2 is not visible yet (because Frag_1 covers the entire screen), but it still creates the view beforehand, so scrolling to Frag_2 will be smooth.

By default, the ViewPager preloads one fragment on each side. So if you would set the start position of the ViewPager to Frag_2, it would load Frag_1 and Frag_2 as well, because they are the neighbors.

You'll notice that Frag_3 will be created when you swipe to Frag_2

You can increase the number of preloaded fragments by calling ViewPager.setOffscreenPageLimit(int limit).

Note that the min. number is 1, so this can not be disabled.

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