如何添加带有片段的新选项卡
How do I add a new tab with fragments
所以我想创建 3 个选项卡，每个选项卡都各不相同。 xml 我已经涵盖了。但不是如何在不使我的应用程序崩溃的情况下创建选项卡。
我已经实现了带有viewpage的滑动标签。
http://developer.android.com/training/implementing-navigation/lateral.html
http://www.androidhive.info/2013/10/android-tab-layout-with-swipeable-views-1/
和一部分
http://www.java2s.com/Code/Android/UI/Demonstrateshowfragmentscanparticipateintheoptionsmenu.htm
当我尝试转到下一个选项卡时（滑动确实没问题）我收到错误消息：
07-08 22:07:57.414: E/AndroidRuntime(6865): FATAL EXCEPTION: main
07-08 22:07:57.414: E/AndroidRuntime(6865): Process: com.cyberdog.magiceasydraft, PID: 6865
07-08 22:07:57.414: E/AndroidRuntime(6865): java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.view.ViewPager.setCurrentItem(int)' on a null object reference
07-08 22:07:57.414: E/AndroidRuntime(6865): at com.cyberdog.magiceasydraft.addPlayersfragmentTab.doPositiveClick(addPlayersfragmentTab.java:113)
07-08 22:07:57.414: E/AndroidRuntime(6865): at com.cyberdog.magiceasydraft.addPlayersfragmentTab$MyAlertDialogFragment.onClick(addPlayersfragmentTab.java:165)
07-08 22:07:57.414: E/AndroidRuntime(6865): at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:160)
07-08 22:07:57.414: E/AndroidRuntime(6865): at android.os.Handler.dispatchMessage(Handler.java:102)
07-08 22:07:57.414: E/AndroidRuntime(6865): at android.os.Looper.loop(Looper.java:135)
07-08 22:07:57.414: E/AndroidRuntime(6865): at android.app.ActivityThread.main(ActivityThread.java:5274)
07-08 22:07:57.414: E/AndroidRuntime(6865): at java.lang.reflect.Method.invoke(Native Method)
07-08 22:07:57.414: E/AndroidRuntime(6865): at java.lang.reflect.Method.invoke(Method.java:372)
07-08 22:07:57.414: E/AndroidRuntime(6865): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:909)
07-08 22:07:57.414: E/AndroidRuntime(6865): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:704)
所以..我做错了什么？我可以看到对我创建的圆形片段有一个空引用......但我不知道如何解决这个问题。这是我第一次使用片段和选项卡。
代码：
主要 activity:
public class CreateDraft extends FragmentActivity implements
ActionBar.TabListener {
private ViewPager viewPager;
private fragmentPageAdapter mAdapter;
private ActionBar actionBar;
private Fragment addPlayer, createRound,score;
private String[] tabs = { "Add players", "Create round", "Score" };
public static final String CuSToM_FRAGMENT_KEY ="ADD_PLAYER_TAB";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_create_draft);
viewPager = (ViewPager) findViewById(R.id.activity_viewpager_main);
actionBar = getActionBar();
mAdapter = new fragmentPageAdapter(getSupportFragmentManager());
viewPager.setAdapter(mAdapter);
actionBar.setHomeButtonEnabled(false);
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
if(savedInstanceState == null){
FragmentManager fm = getSupportFragmentManager();
android.support.v4.app.FragmentTransaction ft = fm.beginTransaction();
addPlayer = fm.findFragmentByTag("Add Players");
if(addPlayer == null){
addPlayer = new addPlayersfragmentTab();
ft.add(addPlayer, "Add players");
}
createRound = fm.findFragmentByTag("Create round");
if(createRound == null){
createRound = new createRoundfragmentTab();
ft.add(createRound, "Create round");
}
score = fm.findFragmentByTag("Score");
if(score == null){
score = new scoreResultfragmentTab();
ft.add(score, "Score");
}
ft.commit();
}
//
for (String tab_name : tabs) {
actionBar.addTab(actionBar.newTab().setText(tab_name)
.setTabListener(this));
}
viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
@Override
public void onPageSelected(int position) {
// on changing the page
// make respected tab selected
actionBar.setSelectedNavigationItem(position);
}
@Override
public void onPageScrolled(int arg0, float arg1, int arg2) {
}
@Override
public void onPageScrollStateChanged(int arg0) {
}
});
}
public void passBundleToFragment(int position){
if(true){
}
}
@Override
public void onTabReselected(Tab tab, FragmentTransaction ft) {
View focus = getCurrentFocus();
if (focus != null) {
hiddenKeyboard(focus);
}
}
@Override
public void onTabSelected(Tab tab, FragmentTransaction ft) {
// on tab selected
// show respected fragment view
viewPager.setCurrentItem(tab.getPosition());
View focus = getCurrentFocus();
if (focus != null) {
hiddenKeyboard(focus);
}
}
@Override
public void onTabUnselected(Tab tab, FragmentTransaction ft) {
View focus = getCurrentFocus();
if (focus != null) {
hiddenKeyboard(focus);
}
}
private void hiddenKeyboard(View v) {
InputMethodManager keyboard = (InputMethodManager) getSystemService(Context.INPUT_METHOD_SERVICE);
keyboard.hideSoftInputFromWindow(v.getWindowToken(), 0);
}
}
其中一个片段（其他的还没有包含太多代码）：
public class addPlayersfragmentTab extends Fragment {
private Button addPlayer, createDraft;
private EditText evAddPlayer;
private ViewPager viewPager;
private List<Player> players;
private ListView lvAddedPlayers;
ListAdapter adapter;
private boolean buttonCreateDraftPressed= false;
public boolean isButtonCreateDraftPressed() {
return buttonCreateDraftPressed;
}
public void setButtonCreateDraftPressed(boolean buttonCreateDraftPressed) {
this.buttonCreateDraftPressed = buttonCreateDraftPressed;
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.draft_adding_players,
container, false);
initialiseEditAndText(rootView);
players = new ArrayList<Player>();
lvAddedPlayers = (ListView) rootView.findViewById(
R.id.lv_players_adding);
adapter = new ListAdapter(rootView.getContext(), players);
lvAddedPlayers.setAdapter(adapter);
addPlayer = (Button) rootView.findViewById(R.id.button_total_player);
createDraft = (Button) rootView.findViewById(R.id.button_create_draft);
addPlayer.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
Player player = new Player(evAddPlayer.getText().toString());
players.add(player);
adapter.notifyDataSetChanged();
InputMethodManager imm = (InputMethodManager) getActivity()
.getSystemService(Context.INPUT_METHOD_SERVICE);
imm.hideSoftInputFromWindow(getView().getWindowToken(), 0);
}
});
createDraft.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
if (adapter != null && players != null) {
createdraft(players.size());
} else {
Toast.makeText(getActivity(),
"Add minimal 3 players to start the draft.",
Toast.LENGTH_LONG).show();
}
}
});
return rootView;
}
public void showDialog(){
DialogFragment newFragment = new MyAlertDialogFragment().newInstance(R.string.recreate_draft);
newFragment.show(getFragmentManager(), "dialog");
}
public void doPositiveClick(){
viewPager.setCurrentItem(1);
}
public void doNegativeClick(){
}
public int getTotalPlayers() {
return players.size();
}
public List<Player> getPlayersNames() {
return players;
}
public void initialiseEditAndText(View rootView) {
evAddPlayer = (EditText) rootView.findViewById(R.id.et_player_name);
}
public void createdraft(int totalplayers) {
if (totalplayers >= 3 && totalplayers <= 16) {
showDialog();
} else {
Toast.makeText(getActivity(),
"Minimum of 3 and maximum of 16 players.",
Toast.LENGTH_LONG).show();
}
}
public class MyAlertDialogFragment extends DialogFragment{
public MyAlertDialogFragment newInstance(int title){
MyAlertDialogFragment frag = new MyAlertDialogFragment();
Bundle args = new Bundle();
args.putInt("title", title);
frag.setArguments(args);
return frag;
}
@Override
public Dialog onCreateDialog(Bundle savedInstanceState) {
int title = getArguments().getInt("title");
return new AlertDialog.Builder(getActivity())
.setIcon(R.drawable.abc_ic_search)
.setTitle(title)
.setPositiveButton("R.string.alert_dialog_ok",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int whichButton) {
addPlayersfragmentTab.this.doPositiveClick();
}
}
)
.setNegativeButton("R.string.alert_dialog_cancel",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int whichButton) {
addPlayersfragmentTab.this.doNegativeClick();
}
}
)
.create();
}
}
}
编辑：
页面适配器：
public fragmentPageAdapter(FragmentManager fm) {
super(fm);
// TODO Auto-generated constructor stub
}
@Override
public Fragment getItem(int index) {
switch (index) {
case 0:
// Top Rated fragment activity
return new addPlayersfragmentTab();
case 1:
// Games fragment activity
return new createRoundfragmentTab();
case 2:
// Movies fragment activity
return new scoreResultfragmentTab();
}
return null;
}
@Override
public int getCount() {
// TODO Auto-generated method stub
return 3;
}
}
编辑 2：
我跟随数据传递到 activity，这确实有效，但是如果我想从 activity 传递数据（从 addPlayerTab 获取数据）我得到又是一个空引用，这次我认为是因为 id 是错误的。但我的布局文件不使用 .那我怎么找到片段呢？
代码：
addPlayerTab的接口实现：
@Override
public void onButtonClickCreateRound(int position, List<Player> players) {
viewPager.setCurrentItem(position);
this.players = players;
GetPlayer();
}
addPlayerTab 中的代码：
public interface GoToCreateRoundListener{
public void onButtonClickCreateRound(int position, List<Player> players);
}
public void onAttach(Activity activity){
super.onAttach(activity);
try{
goCreateRound = (GoToCreateRoundListener)activity;
}catch(ClassCastException e){
throw new ClassCastException(activity.toString()+" must implement GoToRoundListener");
}
}
public void createdraft(int totalplayers) {
if (totalplayers >= 3 && totalplayers <= 16) {
goCreateRound.onButtonClickCreateRound(1, players);
} else {
Toast.makeText(getActivity(),
"Minimum of 3 and maximum of 16 players.",
Toast.LENGTH_LONG).show();
}
}
这是将数据传递给createroundtab的接口实现：
@Override
public void GetPlayer() {
createRoundfragmentTab createRoundfragmentTab = (createRoundfragmentTab)getSupportFragmentManager().findFragmentById(R.layout.create_round);
createRoundfragmentTab.makeRandomFirstRound(players);
}
createroundTab 中的代码：
public interface GetPlayersFromAddPlayersListener{
public void GetPlayer();
}
public void onAttach(Activity activity){
super.onAttach(activity);
try{
getPlayers = (GetPlayersFromAddPlayersListener)activity;
}catch(ClassCastException e){
throw new ClassCastException(activity.toString()+" must implement GetPlayersFromAddPlayersListener");
}
}
您没有初始化片段的 viewPager 字段 (addPlayersfragmentTab.viewPager)。这就是你得到 NullPointerException 的原因。
您可以在 activity 中执行 getViewPager() 方法，然后从片段中调用它，即 ((CreateDraft) getActivity()).getViewPager().setCurrentItem(1)
我已经在上面分享了关于 Fragment 实例化的观点，所以我将再次引用我自己的话。
That whole if (savedInstance == null) block is not required because Fragment instantiation is the job of your PagerAdapter. The ViewPager will call PagerAdapter#getItem() when it sees fit to.
虽然，很明显 NullPointerException 是由 addPlayersfragmentTab 中未初始化的 ViewPager 引用引起的，在创建时将视图寻呼机传递给片段的整个方法或者通过 getter 访问它的片段是错误的。
原因是片段是您的 UI 一部分的独立可重用实现，因此它不应与托管它的 activity 耦合。一旦你将它与你的视图寻呼机结合起来，你就再也不能在任何其他正常的 activity 中托管这个片段了。简而言之，该片段不再保持可重用状态。
那么，片段应该如何与其宿主通信 activity？好吧，使用 回调接口 。这是来自 Android 开发者官方网站的简短教程：Communicating with Other Fragments 但它同样适用于请求 activity 代表片段执行任何操作。
在您的情况下，您会通知主要 activity 用户在警报对话框中按下了“确定”，activity 将通过使用查看寻呼机移至下一个选项卡来对此做出响应。
通常当您想再次抓住 Fragment 时，您可以使用 findFragmentById() 或它的带有 findFragmentByTag() 的自定义标签。但是，整个 ViewPager 及其 PagerAdapter 不会向我们公开这些属性。
因此，要启用片段查找，您需要在 PagerAdapter 中保存对它们的引用。大致如下。
Map<String, Fragment> mFragments = new HashMap<>(3);
...
@Override
public Fragment getItem(int index) {
...
}
@Override
public Object instantiateItem(ViewGroup container, int position) {
// super calls getItem() if fragment is not already instantiated
Fragment fragment = (Fragment) super.instantiateItem(container, position);
mFragments.put(fragment.getClass().getSimpleName(), fragment);
return fragment;
}
public Fragment getFragment(String name) {
return mFragments.get(name);
}
@Override
public int getCount() {
return 3;
}
通过上述更改，您的 GetPlayer() 实施现在将变为
@Override
public void GetPlayer() {
createRoundfragmentTab createRoundfragmentTab = (createRoundfragmentTab)
((fragmentPageAdapter) mAdapter).getFragment("createRoundfragmentTab");
createRoundfragmentTab.makeRandomFirstRound(players);
}
要更详细地了解此方法，请阅读 ViewPager and fragments — what's the right way to store fragment's state?。
顺便说一句，也请看看Oracle推荐的Java naming conventions。长话短说，您的 class 名称需要以大写字母开头，方法名称则相反。遵循这些约定将使您的代码更易于阅读，从而更易于理解。目前，您的所有 class 名称对经验丰富的 Java 开发人员都显示为变量名称。
所以我想创建 3 个选项卡，每个选项卡都各不相同。 xml 我已经涵盖了。但不是如何在不使我的应用程序崩溃的情况下创建选项卡。
我已经实现了带有viewpage的滑动标签。
http://developer.android.com/training/implementing-navigation/lateral.html http://www.androidhive.info/2013/10/android-tab-layout-with-swipeable-views-1/
和一部分 http://www.java2s.com/Code/Android/UI/Demonstrateshowfragmentscanparticipateintheoptionsmenu.htm
当我尝试转到下一个选项卡时（滑动确实没问题）我收到错误消息：
07-08 22:07:57.414: E/AndroidRuntime(6865): FATAL EXCEPTION: main
07-08 22:07:57.414: E/AndroidRuntime(6865): Process: com.cyberdog.magiceasydraft, PID: 6865
07-08 22:07:57.414: E/AndroidRuntime(6865): java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.view.ViewPager.setCurrentItem(int)' on a null object reference
07-08 22:07:57.414: E/AndroidRuntime(6865): at com.cyberdog.magiceasydraft.addPlayersfragmentTab.doPositiveClick(addPlayersfragmentTab.java:113)
07-08 22:07:57.414: E/AndroidRuntime(6865): at com.cyberdog.magiceasydraft.addPlayersfragmentTab$MyAlertDialogFragment.onClick(addPlayersfragmentTab.java:165)
07-08 22:07:57.414: E/AndroidRuntime(6865): at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:160)
07-08 22:07:57.414: E/AndroidRuntime(6865): at android.os.Handler.dispatchMessage(Handler.java:102)
07-08 22:07:57.414: E/AndroidRuntime(6865): at android.os.Looper.loop(Looper.java:135)
07-08 22:07:57.414: E/AndroidRuntime(6865): at android.app.ActivityThread.main(ActivityThread.java:5274)
07-08 22:07:57.414: E/AndroidRuntime(6865): at java.lang.reflect.Method.invoke(Native Method)
07-08 22:07:57.414: E/AndroidRuntime(6865): at java.lang.reflect.Method.invoke(Method.java:372)
07-08 22:07:57.414: E/AndroidRuntime(6865): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:909)
07-08 22:07:57.414: E/AndroidRuntime(6865): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:704)
所以..我做错了什么？我可以看到对我创建的圆形片段有一个空引用......但我不知道如何解决这个问题。这是我第一次使用片段和选项卡。
代码： 主要 activity:
public class CreateDraft extends FragmentActivity implements
ActionBar.TabListener {
private ViewPager viewPager;
private fragmentPageAdapter mAdapter;
private ActionBar actionBar;
private Fragment addPlayer, createRound,score;
private String[] tabs = { "Add players", "Create round", "Score" };
public static final String CuSToM_FRAGMENT_KEY ="ADD_PLAYER_TAB";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_create_draft);
viewPager = (ViewPager) findViewById(R.id.activity_viewpager_main);
actionBar = getActionBar();
mAdapter = new fragmentPageAdapter(getSupportFragmentManager());
viewPager.setAdapter(mAdapter);
actionBar.setHomeButtonEnabled(false);
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
if(savedInstanceState == null){
FragmentManager fm = getSupportFragmentManager();
android.support.v4.app.FragmentTransaction ft = fm.beginTransaction();
addPlayer = fm.findFragmentByTag("Add Players");
if(addPlayer == null){
addPlayer = new addPlayersfragmentTab();
ft.add(addPlayer, "Add players");
}
createRound = fm.findFragmentByTag("Create round");
if(createRound == null){
createRound = new createRoundfragmentTab();
ft.add(createRound, "Create round");
}
score = fm.findFragmentByTag("Score");
if(score == null){
score = new scoreResultfragmentTab();
ft.add(score, "Score");
}
ft.commit();
}
//
for (String tab_name : tabs) {
actionBar.addTab(actionBar.newTab().setText(tab_name)
.setTabListener(this));
}
viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
@Override
public void onPageSelected(int position) {
// on changing the page
// make respected tab selected
actionBar.setSelectedNavigationItem(position);
}
@Override
public void onPageScrolled(int arg0, float arg1, int arg2) {
}
@Override
public void onPageScrollStateChanged(int arg0) {
}
});
}
public void passBundleToFragment(int position){
if(true){
}
}
@Override
public void onTabReselected(Tab tab, FragmentTransaction ft) {
View focus = getCurrentFocus();
if (focus != null) {
hiddenKeyboard(focus);
}
}
@Override
public void onTabSelected(Tab tab, FragmentTransaction ft) {
// on tab selected
// show respected fragment view
viewPager.setCurrentItem(tab.getPosition());
View focus = getCurrentFocus();
if (focus != null) {
hiddenKeyboard(focus);
}
}
@Override
public void onTabUnselected(Tab tab, FragmentTransaction ft) {
View focus = getCurrentFocus();
if (focus != null) {
hiddenKeyboard(focus);
}
}
private void hiddenKeyboard(View v) {
InputMethodManager keyboard = (InputMethodManager) getSystemService(Context.INPUT_METHOD_SERVICE);
keyboard.hideSoftInputFromWindow(v.getWindowToken(), 0);
}
}
其中一个片段（其他的还没有包含太多代码）：
public class addPlayersfragmentTab extends Fragment {
private Button addPlayer, createDraft;
private EditText evAddPlayer;
private ViewPager viewPager;
private List<Player> players;
private ListView lvAddedPlayers;
ListAdapter adapter;
private boolean buttonCreateDraftPressed= false;
public boolean isButtonCreateDraftPressed() {
return buttonCreateDraftPressed;
}
public void setButtonCreateDraftPressed(boolean buttonCreateDraftPressed) {
this.buttonCreateDraftPressed = buttonCreateDraftPressed;
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.draft_adding_players,
container, false);
initialiseEditAndText(rootView);
players = new ArrayList<Player>();
lvAddedPlayers = (ListView) rootView.findViewById(
R.id.lv_players_adding);
adapter = new ListAdapter(rootView.getContext(), players);
lvAddedPlayers.setAdapter(adapter);
addPlayer = (Button) rootView.findViewById(R.id.button_total_player);
createDraft = (Button) rootView.findViewById(R.id.button_create_draft);
addPlayer.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
Player player = new Player(evAddPlayer.getText().toString());
players.add(player);
adapter.notifyDataSetChanged();
InputMethodManager imm = (InputMethodManager) getActivity()
.getSystemService(Context.INPUT_METHOD_SERVICE);
imm.hideSoftInputFromWindow(getView().getWindowToken(), 0);
}
});
createDraft.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
if (adapter != null && players != null) {
createdraft(players.size());
} else {
Toast.makeText(getActivity(),
"Add minimal 3 players to start the draft.",
Toast.LENGTH_LONG).show();
}
}
});
return rootView;
}
public void showDialog(){
DialogFragment newFragment = new MyAlertDialogFragment().newInstance(R.string.recreate_draft);
newFragment.show(getFragmentManager(), "dialog");
}
public void doPositiveClick(){
viewPager.setCurrentItem(1);
}
public void doNegativeClick(){
}
public int getTotalPlayers() {
return players.size();
}
public List<Player> getPlayersNames() {
return players;
}
public void initialiseEditAndText(View rootView) {
evAddPlayer = (EditText) rootView.findViewById(R.id.et_player_name);
}
public void createdraft(int totalplayers) {
if (totalplayers >= 3 && totalplayers <= 16) {
showDialog();
} else {
Toast.makeText(getActivity(),
"Minimum of 3 and maximum of 16 players.",
Toast.LENGTH_LONG).show();
}
}
public class MyAlertDialogFragment extends DialogFragment{
public MyAlertDialogFragment newInstance(int title){
MyAlertDialogFragment frag = new MyAlertDialogFragment();
Bundle args = new Bundle();
args.putInt("title", title);
frag.setArguments(args);
return frag;
}
@Override
public Dialog onCreateDialog(Bundle savedInstanceState) {
int title = getArguments().getInt("title");
return new AlertDialog.Builder(getActivity())
.setIcon(R.drawable.abc_ic_search)
.setTitle(title)
.setPositiveButton("R.string.alert_dialog_ok",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int whichButton) {
addPlayersfragmentTab.this.doPositiveClick();
}
}
)
.setNegativeButton("R.string.alert_dialog_cancel",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int whichButton) {
addPlayersfragmentTab.this.doNegativeClick();
}
}
)
.create();
}
}
}
编辑：
页面适配器：
public fragmentPageAdapter(FragmentManager fm) {
super(fm);
// TODO Auto-generated constructor stub
}
@Override
public Fragment getItem(int index) {
switch (index) {
case 0:
// Top Rated fragment activity
return new addPlayersfragmentTab();
case 1:
// Games fragment activity
return new createRoundfragmentTab();
case 2:
// Movies fragment activity
return new scoreResultfragmentTab();
}
return null;
}
@Override
public int getCount() {
// TODO Auto-generated method stub
return 3;
}
}
编辑 2：
我跟随数据传递到 activity，这确实有效，但是如果我想从 activity 传递数据（从 addPlayerTab 获取数据）我得到又是一个空引用，这次我认为是因为 id 是错误的。但我的布局文件不使用 .那我怎么找到片段呢？
代码： addPlayerTab的接口实现：
@Override
public void onButtonClickCreateRound(int position, List<Player> players) {
viewPager.setCurrentItem(position);
this.players = players;
GetPlayer();
}
addPlayerTab 中的代码：
public interface GoToCreateRoundListener{
public void onButtonClickCreateRound(int position, List<Player> players);
}
public void onAttach(Activity activity){
super.onAttach(activity);
try{
goCreateRound = (GoToCreateRoundListener)activity;
}catch(ClassCastException e){
throw new ClassCastException(activity.toString()+" must implement GoToRoundListener");
}
}
public void createdraft(int totalplayers) {
if (totalplayers >= 3 && totalplayers <= 16) {
goCreateRound.onButtonClickCreateRound(1, players);
} else {
Toast.makeText(getActivity(),
"Minimum of 3 and maximum of 16 players.",
Toast.LENGTH_LONG).show();
}
}
这是将数据传递给createroundtab的接口实现：
@Override
public void GetPlayer() {
createRoundfragmentTab createRoundfragmentTab = (createRoundfragmentTab)getSupportFragmentManager().findFragmentById(R.layout.create_round);
createRoundfragmentTab.makeRandomFirstRound(players);
}
createroundTab 中的代码：
public interface GetPlayersFromAddPlayersListener{
public void GetPlayer();
}
public void onAttach(Activity activity){
super.onAttach(activity);
try{
getPlayers = (GetPlayersFromAddPlayersListener)activity;
}catch(ClassCastException e){
throw new ClassCastException(activity.toString()+" must implement GetPlayersFromAddPlayersListener");
}
}
您没有初始化片段的 viewPager 字段 (addPlayersfragmentTab.viewPager)。这就是你得到 NullPointerException 的原因。
您可以在 activity 中执行 getViewPager() 方法，然后从片段中调用它，即 ((CreateDraft) getActivity()).getViewPager().setCurrentItem(1)
我已经在上面分享了关于 Fragment 实例化的观点，所以我将再次引用我自己的话。
That whole
if (savedInstance == null)block is not required becauseFragmentinstantiation is the job of yourPagerAdapter. TheViewPagerwill callPagerAdapter#getItem()when it sees fit to.
虽然，很明显 NullPointerException 是由 addPlayersfragmentTab 中未初始化的 ViewPager 引用引起的，在创建时将视图寻呼机传递给片段的整个方法或者通过 getter 访问它的片段是错误的。
原因是片段是您的 UI 一部分的独立可重用实现，因此它不应与托管它的 activity 耦合。一旦你将它与你的视图寻呼机结合起来，你就再也不能在任何其他正常的 activity 中托管这个片段了。简而言之，该片段不再保持可重用状态。
那么，片段应该如何与其宿主通信 activity？好吧，使用 回调接口 。这是来自 Android 开发者官方网站的简短教程：Communicating with Other Fragments 但它同样适用于请求 activity 代表片段执行任何操作。
在您的情况下，您会通知主要 activity 用户在警报对话框中按下了“确定”，activity 将通过使用查看寻呼机移至下一个选项卡来对此做出响应。
通常当您想再次抓住 Fragment 时，您可以使用 findFragmentById() 或它的带有 findFragmentByTag() 的自定义标签。但是，整个 ViewPager 及其 PagerAdapter 不会向我们公开这些属性。
因此，要启用片段查找，您需要在 PagerAdapter 中保存对它们的引用。大致如下。
Map<String, Fragment> mFragments = new HashMap<>(3);
...
@Override
public Fragment getItem(int index) {
...
}
@Override
public Object instantiateItem(ViewGroup container, int position) {
// super calls getItem() if fragment is not already instantiated
Fragment fragment = (Fragment) super.instantiateItem(container, position);
mFragments.put(fragment.getClass().getSimpleName(), fragment);
return fragment;
}
public Fragment getFragment(String name) {
return mFragments.get(name);
}
@Override
public int getCount() {
return 3;
}
通过上述更改，您的 GetPlayer() 实施现在将变为
@Override
public void GetPlayer() {
createRoundfragmentTab createRoundfragmentTab = (createRoundfragmentTab)
((fragmentPageAdapter) mAdapter).getFragment("createRoundfragmentTab");
createRoundfragmentTab.makeRandomFirstRound(players);
}
要更详细地了解此方法，请阅读 ViewPager and fragments — what's the right way to store fragment's state?。
顺便说一句，也请看看Oracle推荐的Java naming conventions。长话短说，您的 class 名称需要以大写字母开头，方法名称则相反。遵循这些约定将使您的代码更易于阅读，从而更易于理解。目前，您的所有 class 名称对经验丰富的 Java 开发人员都显示为变量名称。