从第3个片段选项卡转换到第2个/第1个片段选项卡，打破了应用(Transition from 3rd fragment tab to 2nd/1st fragment tab , breaks the app)
我可以在3个选项卡之间进行转换，每个类都在扩展Fragment。
我的片段A包含地图，片段B和C是空白的，带有一些默认背景。 当我运行我的应用程序时，片段A加载地图，并在转换为片段b和c背景颜色变化表明成功。 但在转换回片段时，应用程序崩溃了。
片段A：
package com.example.hspot2; import android.os.Bundle; import android.support.v4.app.Fragment; import android.support.v4.app.FragmentActivity; import android.support.v4.app.FragmentManager; import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; import com.google.android.gms.common.GooglePlayServicesNotAvailableException; import com.google.android.gms.maps.CameraUpdateFactory; import com.google.android.gms.maps.GoogleMap; import com.google.android.gms.maps.MapFragment; import com.google.android.gms.maps.MapView; import com.google.android.gms.maps.MapsInitializer; import com.google.android.gms.maps.SupportMapFragment; import com.google.android.gms.maps.model.BitmapDescriptorFactory; import com.google.android.gms.maps.model.LatLng; import com.google.android.gms.maps.model.Marker; import com.google.android.gms.maps.model.MarkerOptions; public class FragmentA extends Fragment{ static final LatLng HAMBURG = new LatLng(53.558, 9.927); GoogleMap map; MapView mapView; SupportMapFragment fragment; @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View v = inflater.inflate(R.layout.fragment_a, container, false); map = ((SupportMapFragment) getFragmentManager().findFragmentById(R.id.map)).getMap(); return v; } }片段B：
package com.example.hspot2; import android.os.Bundle; import android.support.v4.app.Fragment; import android.support.v4.app.FragmentActivity; import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; public class FragmentB extends Fragment { @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { return inflater.inflate(R.layout.fragment_b, container, false); } }片段c：
package com.example.hspot2; import android.os.Bundle; import android.support.v4.app.Fragment; import android.support.v4.app.FragmentActivity; import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; public class FragmentC extends Fragment { public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { return inflater.inflate(R.layout.fragment_c, container, false); } }mainactivity.java：
package com.example.hspot2; import java.util.Locale; import android.app.ActionBar; import android.app.FragmentTransaction; import android.os.Bundle; import android.support.v4.app.Fragment; import android.support.v4.app.FragmentActivity; import android.support.v4.app.FragmentManager; import android.support.v4.app.FragmentPagerAdapter; import android.support.v4.app.FragmentStatePagerAdapter; import android.support.v4.app.NavUtils; import android.support.v4.view.ViewPager; import android.view.Gravity; import android.view.LayoutInflater; import android.view.Menu; import android.view.MenuItem; import android.view.View; import android.view.ViewGroup; import android.widget.TextView; public class MainActivity extends FragmentActivity implements ActionBar.TabListener { SectionsPagerAdapter mSectionsPagerAdapter; ViewPager mViewPager; ActionBar actionBar; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); mViewPager = (ViewPager) findViewById(R.id.pager); actionBar = getActionBar(); actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS); mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager()); mViewPager.setAdapter(mSectionsPagerAdapter); for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) { actionBar.addTab(actionBar.newTab() .setText(mSectionsPagerAdapter.getPageTitle(i)) .setTabListener(this)); } // When swiping between different sections, select the corresponding // tab. We can also use ActionBar.Tab#select() to do this if we have // a reference to the Tab. mViewPager .setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() { @Override public void onPageSelected(int position) { actionBar.setSelectedNavigationItem(position); } }); } @Override public boolean onCreateOptionsMenu(Menu menu) { // Inflate the menu; this adds items to the action bar if it is present. getMenuInflater().inflate(R.menu.main, menu); return true; } @Override public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) { // When the given tab is selected, switch to the corresponding page in // the ViewPager. mViewPager.setCurrentItem(tab.getPosition()); } @Override public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) { } @Override public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) { } /** * A {@link FragmentPagerAdapter} that returns a fragment corresponding to * one of the sections/tabs/pages. */ public class SectionsPagerAdapter extends FragmentStatePagerAdapter { public SectionsPagerAdapter(FragmentManager fm) { super(fm); } @Override public Fragment getItem(int i) { Fragment fragment = null; if(i==0){ fragment = new FragmentA(); } if(i==1){ fragment = new FragmentB(); } if(i==2){ fragment = new FragmentC(); } return fragment; } @Override public int getCount() { return 3; } @Override public CharSequence getPageTitle(int position) { Locale l = Locale.getDefault(); switch (position) { case 0: return getString(R.string.title_section1).toUpperCase(l); case 1: return getString(R.string.title_section2).toUpperCase(l); case 2: return getString(R.string.title_section3).toUpperCase(l); } return null; } } }I am able to do transition between 3 tabs where each class is extending Fragment.
My fragment A holds the map , fragment B and C are blank with some default background. When I run my application , Fragment A loads with the map and upon transition to fragment b and c background color changes indicating a success . But on transition back to fragment a crashes the app .
Fragment A:
package com.example.hspot2; import android.os.Bundle; import android.support.v4.app.Fragment; import android.support.v4.app.FragmentActivity; import android.support.v4.app.FragmentManager; import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; import com.google.android.gms.common.GooglePlayServicesNotAvailableException; import com.google.android.gms.maps.CameraUpdateFactory; import com.google.android.gms.maps.GoogleMap; import com.google.android.gms.maps.MapFragment; import com.google.android.gms.maps.MapView; import com.google.android.gms.maps.MapsInitializer; import com.google.android.gms.maps.SupportMapFragment; import com.google.android.gms.maps.model.BitmapDescriptorFactory; import com.google.android.gms.maps.model.LatLng; import com.google.android.gms.maps.model.Marker; import com.google.android.gms.maps.model.MarkerOptions; public class FragmentA extends Fragment{ static final LatLng HAMBURG = new LatLng(53.558, 9.927); GoogleMap map; MapView mapView; SupportMapFragment fragment; @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View v = inflater.inflate(R.layout.fragment_a, container, false); map = ((SupportMapFragment) getFragmentManager().findFragmentById(R.id.map)).getMap(); return v; } }fragment B :
package com.example.hspot2; import android.os.Bundle; import android.support.v4.app.Fragment; import android.support.v4.app.FragmentActivity; import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; public class FragmentB extends Fragment { @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { return inflater.inflate(R.layout.fragment_b, container, false); } }fragment c:
package com.example.hspot2; import android.os.Bundle; import android.support.v4.app.Fragment; import android.support.v4.app.FragmentActivity; import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; public class FragmentC extends Fragment { public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { return inflater.inflate(R.layout.fragment_c, container, false); } }mainactivity.java :
package com.example.hspot2; import java.util.Locale; import android.app.ActionBar; import android.app.FragmentTransaction; import android.os.Bundle; import android.support.v4.app.Fragment; import android.support.v4.app.FragmentActivity; import android.support.v4.app.FragmentManager; import android.support.v4.app.FragmentPagerAdapter; import android.support.v4.app.FragmentStatePagerAdapter; import android.support.v4.app.NavUtils; import android.support.v4.view.ViewPager; import android.view.Gravity; import android.view.LayoutInflater; import android.view.Menu; import android.view.MenuItem; import android.view.View; import android.view.ViewGroup; import android.widget.TextView; public class MainActivity extends FragmentActivity implements ActionBar.TabListener { SectionsPagerAdapter mSectionsPagerAdapter; ViewPager mViewPager; ActionBar actionBar; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); mViewPager = (ViewPager) findViewById(R.id.pager); actionBar = getActionBar(); actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS); mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager()); mViewPager.setAdapter(mSectionsPagerAdapter); for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) { actionBar.addTab(actionBar.newTab() .setText(mSectionsPagerAdapter.getPageTitle(i)) .setTabListener(this)); } // When swiping between different sections, select the corresponding // tab. We can also use ActionBar.Tab#select() to do this if we have // a reference to the Tab. mViewPager .setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() { @Override public void onPageSelected(int position) { actionBar.setSelectedNavigationItem(position); } }); } @Override public boolean onCreateOptionsMenu(Menu menu) { // Inflate the menu; this adds items to the action bar if it is present. getMenuInflater().inflate(R.menu.main, menu); return true; } @Override public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) { // When the given tab is selected, switch to the corresponding page in // the ViewPager. mViewPager.setCurrentItem(tab.getPosition()); } @Override public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) { } @Override public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) { } /** * A {@link FragmentPagerAdapter} that returns a fragment corresponding to * one of the sections/tabs/pages. */ public class SectionsPagerAdapter extends FragmentStatePagerAdapter { public SectionsPagerAdapter(FragmentManager fm) { super(fm); } @Override public Fragment getItem(int i) { Fragment fragment = null; if(i==0){ fragment = new FragmentA(); } if(i==1){ fragment = new FragmentB(); } if(i==2){ fragment = new FragmentC(); } return fragment; } @Override public int getCount() { return 3; } @Override public CharSequence getPageTitle(int position) { Locale l = Locale.getDefault(); switch (position) { case 0: return getString(R.string.title_section1).toUpperCase(l); case 1: return getString(R.string.title_section2).toUpperCase(l); case 2: return getString(R.string.title_section3).toUpperCase(l); } return null; } } }最满意答案
选择选项卡时，ViewPager会将关联的片段和片段靠近它（tab-1和tab + 1）加载以进行缓存。 因此，从第三个选项卡传递到第二个/一个选项卡意味着ViewPAger始终加载两个片段（1和2）。 因为2是一个void Fragment我建议专注于第一个Fragment，我想你的findFragmentById（R.id.map））返回null，所以getMap抛出异常。 你必须以不同的方式识别你的碎片
try : mViewPager.setOffScreenPageLimit(4); // since 3 is the toal no. of swipe tabs above
Don't know if this is really efficient way to solve this problem. Any comment and discussion ?
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