ViewPager 的 SetCurrentItem 仅在 OnTabSelected 中不起作用
SetCurrentItem for ViewPager is not working in OnTabSelected only
一个小问题找了好久也没搞清楚。如果能帮我解决问题，那就太好了。
我有一个 ActionbarActivity，我实际上想要四个选项卡，所以我只使用了 viewpager 和 actionbar 选项卡。一切正常但是当我使用
viewPager.setCurrentItem() 在我的 OnTabSelected() 方法中，它给出空点异常，即使它在 OnTabReselect() 或 onTabUnselect() 中工作正常
这是我的主要代码 activity
包裹 co.howlabs.cafe.lahore;
import android.os.Bundle;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.support.v4.view.ViewPager;
import android.support.v7.app.ActionBar;
import android.support.v7.app.ActionBar.Tab;
import android.support.v7.app.ActionBar.TabListener;
import android.support.v7.app.ActionBarActivity;
import android.util.Log;
public class MainActivity extends ActionBarActivity implements TabListener {
ViewPager viewpager;
TabPagerAdapter TabAdapter;
ActionBar actionbar;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// actionBar.setDisplayShowTitleEnabled(false);
// actionBar.setDisplayShowHomeEnabled(false);
setContentView(R.layout.activity_main);
actionbar = getSupportActionBar();
actionbar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
ActionBar.Tab tab1 = actionbar.newTab();
tab1.setText("Promotions");
tab1.setTabListener(this);
ActionBar.Tab tab2 = actionbar.newTab();
tab2.setText("All Cafes");
tab2.setTabListener(this);
ActionBar.Tab tab3 = actionbar.newTab();
tab3.setText("Categories");
tab3.setTabListener(this);
ActionBar.Tab tab4 = actionbar.newTab();
tab4.setText("Recommended");
tab4.setTabListener(this);
actionbar.addTab(tab1);
actionbar.addTab(tab2);
actionbar.addTab(tab3);
actionbar.addTab(tab4);
viewpager = (ViewPager)findViewById(R.id.pager);
FragmentManager fragmentmanager = getSupportFragmentManager();
viewpager.setAdapter(new TabPagerAdapter(fragmentmanager));
viewpager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
@Override
public void onPageSelected(int arg0) {
actionbar.setSelectedNavigationItem(arg0);
}
@Override
public void onPageScrolled(int arg0, float arg1, int arg2) {
// TODO Auto-generated method stub
}
@Override
public void onPageScrollStateChanged(int arg0) {
// TODO Auto-generated method stub
}
});
}
@Override
public void onTabSelected(Tab arg0, FragmentTransaction arg1) {
viewpager.setCurrentItem(arg0.getPosition()); //here throws error
Log.d("tab", String.valueOf(arg0.getPosition())); //gives the correct position if comments the above line
}
@Override
public void onTabUnselected(Tab arg0, FragmentTransaction arg1) {
}
@Override
public void onTabReselected(Tab arg0, FragmentTransaction arg1) {
viewpager.setCurrentItem(arg0.getPosition()); //works fine here
}
}
这是 class 用于 tabpager 适配器
package co.howlabs.cafe.lahore;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentStatePagerAdapter;
public class TabPagerAdapter extends FragmentStatePagerAdapter {
public TabPagerAdapter(FragmentManager fm) {
super(fm);
}
@Override
public Fragment getItem(int i) {
switch (i) {
case 0:
return new Promotions();
case 1:
return new AllCafes();
case 2:
return new Categories();
case 3:
return new Recommended();
}
return null;
}
@Override
public int getCount() {
return 4; //No of Tabs
}
}
这是我的LogCat错误信息
04-11 15:16:40.624: E/AndroidRuntime(1841): FATAL EXCEPTION: main
04-11 15:16:40.624: E/AndroidRuntime(1841): java.lang.RuntimeException: Unable to start activity ComponentInfo{co.howlabs.cafe.lahore/co.howlabs.cafe.lahore.MainActivity}: java.lang.NullPointerException
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.app.ActivityThread.access0(ActivityThread.java:130)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.os.Handler.dispatchMessage(Handler.java:99)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.os.Looper.loop(Looper.java:137)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.app.ActivityThread.main(ActivityThread.java:4745)
04-11 15:16:40.624: E/AndroidRuntime(1841): at java.lang.reflect.Method.invokeNative(Native Method)
04-11 15:16:40.624: E/AndroidRuntime(1841): at java.lang.reflect.Method.invoke(Method.java:511)
04-11 15:16:40.624: E/AndroidRuntime(1841): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
04-11 15:16:40.624: E/AndroidRuntime(1841): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
04-11 15:16:40.624: E/AndroidRuntime(1841): at dalvik.system.NativeStart.main(Native Method)
04-11 15:16:40.624: E/AndroidRuntime(1841): Caused by: java.lang.NullPointerException
04-11 15:16:40.624: E/AndroidRuntime(1841): at co.howlabs.cafe.lahore.MainActivity.onTabSelected(MainActivity.java:75)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.support.v7.app.ActionBarImplICS$TabWrapper.onTabSelected(ActionBarImplICS.java:558)
04-11 15:16:40.624: E/AndroidRuntime(1841): at com.android.internal.app.ActionBarImpl.selectTab(ActionBarImpl.java:570)
04-11 15:16:40.624: E/AndroidRuntime(1841): at com.android.internal.app.ActionBarImpl.addTab(ActionBarImpl.java:499)
04-11 15:16:40.624: E/AndroidRuntime(1841): at com.android.internal.app.ActionBarImpl.addTab(ActionBarImpl.java:485)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.support.v7.app.ActionBarImplICS.addTab(ActionBarImplICS.java:247)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.support.v7.app.ActionBarImplJB.addTab(ActionBarImplJB.java:20)
04-11 15:16:40.624: E/AndroidRuntime(1841): at co.howlabs.cafe.lahore.MainActivity.onCreate(MainActivity.java:43)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.app.Activity.performCreate(Activity.java:5008)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
04-11 15:16:40.624: E/AndroidRuntime(1841): ... 11 more
如堆栈跟踪所示，调用 actionbar.addTab(...).
后触发了 onTabSelected()
知道了这一点，就应该清楚为什么 viewpager.setCurrentItem() 会抛出 NPE。添加第一个选项卡时，该字段尚未初始化，只是因为当前您在 viewpager = (ViewPager)findViewById(R.id.pager) 设置选项卡后
解决方案是将 viewpager = (ViewPager)findViewById(R.id.pager) 移动到某个地方 在 第一次 addTab() 调用之前（当然仍然在 setContentView(R.layout.activity_main) 之后）。
一个小问题找了好久也没搞清楚。如果能帮我解决问题，那就太好了。
我有一个 ActionbarActivity，我实际上想要四个选项卡，所以我只使用了 viewpager 和 actionbar 选项卡。一切正常但是当我使用
viewPager.setCurrentItem() 在我的 OnTabSelected() 方法中，它给出空点异常，即使它在 OnTabReselect() 或 onTabUnselect() 中工作正常
这是我的主要代码 activity
包裹 co.howlabs.cafe.lahore;
import android.os.Bundle;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.support.v4.view.ViewPager;
import android.support.v7.app.ActionBar;
import android.support.v7.app.ActionBar.Tab;
import android.support.v7.app.ActionBar.TabListener;
import android.support.v7.app.ActionBarActivity;
import android.util.Log;
public class MainActivity extends ActionBarActivity implements TabListener {
ViewPager viewpager;
TabPagerAdapter TabAdapter;
ActionBar actionbar;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// actionBar.setDisplayShowTitleEnabled(false);
// actionBar.setDisplayShowHomeEnabled(false);
setContentView(R.layout.activity_main);
actionbar = getSupportActionBar();
actionbar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
ActionBar.Tab tab1 = actionbar.newTab();
tab1.setText("Promotions");
tab1.setTabListener(this);
ActionBar.Tab tab2 = actionbar.newTab();
tab2.setText("All Cafes");
tab2.setTabListener(this);
ActionBar.Tab tab3 = actionbar.newTab();
tab3.setText("Categories");
tab3.setTabListener(this);
ActionBar.Tab tab4 = actionbar.newTab();
tab4.setText("Recommended");
tab4.setTabListener(this);
actionbar.addTab(tab1);
actionbar.addTab(tab2);
actionbar.addTab(tab3);
actionbar.addTab(tab4);
viewpager = (ViewPager)findViewById(R.id.pager);
FragmentManager fragmentmanager = getSupportFragmentManager();
viewpager.setAdapter(new TabPagerAdapter(fragmentmanager));
viewpager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
@Override
public void onPageSelected(int arg0) {
actionbar.setSelectedNavigationItem(arg0);
}
@Override
public void onPageScrolled(int arg0, float arg1, int arg2) {
// TODO Auto-generated method stub
}
@Override
public void onPageScrollStateChanged(int arg0) {
// TODO Auto-generated method stub
}
});
}
@Override
public void onTabSelected(Tab arg0, FragmentTransaction arg1) {
viewpager.setCurrentItem(arg0.getPosition()); //here throws error
Log.d("tab", String.valueOf(arg0.getPosition())); //gives the correct position if comments the above line
}
@Override
public void onTabUnselected(Tab arg0, FragmentTransaction arg1) {
}
@Override
public void onTabReselected(Tab arg0, FragmentTransaction arg1) {
viewpager.setCurrentItem(arg0.getPosition()); //works fine here
}
}
这是 class 用于 tabpager 适配器
package co.howlabs.cafe.lahore;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentStatePagerAdapter;
public class TabPagerAdapter extends FragmentStatePagerAdapter {
public TabPagerAdapter(FragmentManager fm) {
super(fm);
}
@Override
public Fragment getItem(int i) {
switch (i) {
case 0:
return new Promotions();
case 1:
return new AllCafes();
case 2:
return new Categories();
case 3:
return new Recommended();
}
return null;
}
@Override
public int getCount() {
return 4; //No of Tabs
}
}
这是我的LogCat错误信息
04-11 15:16:40.624: E/AndroidRuntime(1841): FATAL EXCEPTION: main
04-11 15:16:40.624: E/AndroidRuntime(1841): java.lang.RuntimeException: Unable to start activity ComponentInfo{co.howlabs.cafe.lahore/co.howlabs.cafe.lahore.MainActivity}: java.lang.NullPointerException
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.app.ActivityThread.access0(ActivityThread.java:130)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.os.Handler.dispatchMessage(Handler.java:99)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.os.Looper.loop(Looper.java:137)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.app.ActivityThread.main(ActivityThread.java:4745)
04-11 15:16:40.624: E/AndroidRuntime(1841): at java.lang.reflect.Method.invokeNative(Native Method)
04-11 15:16:40.624: E/AndroidRuntime(1841): at java.lang.reflect.Method.invoke(Method.java:511)
04-11 15:16:40.624: E/AndroidRuntime(1841): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
04-11 15:16:40.624: E/AndroidRuntime(1841): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
04-11 15:16:40.624: E/AndroidRuntime(1841): at dalvik.system.NativeStart.main(Native Method)
04-11 15:16:40.624: E/AndroidRuntime(1841): Caused by: java.lang.NullPointerException
04-11 15:16:40.624: E/AndroidRuntime(1841): at co.howlabs.cafe.lahore.MainActivity.onTabSelected(MainActivity.java:75)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.support.v7.app.ActionBarImplICS$TabWrapper.onTabSelected(ActionBarImplICS.java:558)
04-11 15:16:40.624: E/AndroidRuntime(1841): at com.android.internal.app.ActionBarImpl.selectTab(ActionBarImpl.java:570)
04-11 15:16:40.624: E/AndroidRuntime(1841): at com.android.internal.app.ActionBarImpl.addTab(ActionBarImpl.java:499)
04-11 15:16:40.624: E/AndroidRuntime(1841): at com.android.internal.app.ActionBarImpl.addTab(ActionBarImpl.java:485)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.support.v7.app.ActionBarImplICS.addTab(ActionBarImplICS.java:247)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.support.v7.app.ActionBarImplJB.addTab(ActionBarImplJB.java:20)
04-11 15:16:40.624: E/AndroidRuntime(1841): at co.howlabs.cafe.lahore.MainActivity.onCreate(MainActivity.java:43)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.app.Activity.performCreate(Activity.java:5008)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
04-11 15:16:40.624: E/AndroidRuntime(1841): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
04-11 15:16:40.624: E/AndroidRuntime(1841): ... 11 more
如堆栈跟踪所示，调用 actionbar.addTab(...).
onTabSelected()
知道了这一点，就应该清楚为什么 viewpager.setCurrentItem() 会抛出 NPE。添加第一个选项卡时，该字段尚未初始化，只是因为当前您在 viewpager = (ViewPager)findViewById(R.id.pager) 设置选项卡后
解决方案是将 viewpager = (ViewPager)findViewById(R.id.pager) 移动到某个地方 在 第一次 addTab() 调用之前（当然仍然在 setContentView(R.layout.activity_main) 之后）。