我已经为新的标签系统实现了 FragmentTabHost。但很快意识到我还需要 actionBars 和 tabListener 支持。我的问题如下:我已经在没有 actionBar 和 tabListener 的情况下管理了 FragmentTabHost 的实现(请参阅下面的代码)。但是使用 FragmentActivity(使用 actionBars/实现 tabListener)我无法让代码工作。我错过了什么……？
我尝试使用的代码:
import android.app.ActionBar.Tab;
import android.app.ActionBar;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentTransaction;
import android.widget.Toast;
public class Tabs extends FragmentActivity {
protected static final String TAG = Tabs.class.toString();
private boolean haveShownStartDialog = false;
protected ISettingsDataProvider settings;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
final ActionBar bar = getActionBar();
bar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
bar.setDisplayOptions(0, ActionBar.DISPLAY_SHOW_TITLE);
bar.addTab(bar.newTab()
.setText("Search")
.setTabListener(new TabListener<LandingSearch>(
this, "search", LandingSearch.class)));
bar.addTab(bar.newTab()
.setText("Bookmark")
.setTabListener(new TabListener<LandingBookmark>(
this, "bookmark", LandingBookmark.class)));
bar.addTab(bar.newTab()
.setText("History")
.setTabListener(new TabListener<LandingHistory>(
this, "search", LandingHistory.class)));
bar.addTab(bar.newTab()
.setText("Forum")
.setTabListener(new TabListener<LandingForum>(
this, "search", LandingForum.class)));
}
public static class TabListener<T extends Fragment> implements ActionBar.TabListener {
private final FragmentActivity mActivity;
private final String mTag;
private final Class<T> mClass;
private final Bundle mArgs;
private Fragment mFragment;
public TabListener(FragmentActivity activity, String tag, Class<T> clz) {
this(activity, tag, clz, null);
}
public TabListener(FragmentActivity activity, String tag, Class<T> clz, Bundle args) {
mActivity = activity;
mTag = tag;
mClass = clz;
mArgs = args;
// Check to see if we already have a fragment for this tab, probably
// from a previously saved state. If so, deactivate it, because our
// initial state is that a tab isn't shown.
mFragment = mActivity.getSupportFragmentManager().findFragmentByTag(mTag);
if (mFragment != null && !mFragment.isDetached()) {
FragmentTransaction ft = mActivity.getSupportFragmentManager().beginTransaction();
ft.detach(mFragment);
ft.commit();
}
}
public void onTabSelected(Tab tab, android.app.FragmentTransaction ft) {
// Since ActionBar.TabListener needs android.app.FragmentTransaction, define the method signature with it, but don't use it
// Instead use the support fragment manager
FragmentTransaction fft = mActivity.getSupportFragmentManager().beginTransaction();
if (mFragment == null) {
mFragment = Fragment.instantiate(mActivity, mClass.getName(), mArgs);
fft.add(android.R.id.content, mFragment, mTag);
} else {
fft.attach(mFragment);
}
// Don't forget to call commit, as we are not using FragmentTransaction passed by ActionBar.
fft.commit();
}
public void onTabUnselected(Tab tab, android.app.FragmentTransaction ft) {
FragmentTransaction fft = mActivity.getSupportFragmentManager().beginTransaction();
if (mFragment != null) {
fft.detach(mFragment);
}
fft.commit();
}
public void onTabReselected(Tab tab, android.app.FragmentTransaction ft) {
Toast.makeText(mActivity, "Reselected!", Toast.LENGTH_SHORT).show();
}
}
}
一个选项卡类的示例..
public class LandingSearch extends Fragment
{
protected static final String TAG = LandingSearch.class.toString();
…….
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.landing_search, container, false);
super.onCreateView(inflater, container, savedInstanceState, v);
setHasOptionsMenu(true);
initList(v);
code continues….
新的空指针错误:
05-13 11:57:41.322: E/AndroidRuntime(18118): FATAL EXCEPTION: main
05-13 11:57:41.322: E/AndroidRuntime(18118): java.lang.RuntimeException: Unable to start activity ComponentInfo{com./com.Tabs}: java.lang.NullPointerException
05-13 11:57:41.322: E/AndroidRuntime(18118): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
05-13 11:57:41.322: E/AndroidRuntime(18118): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
05-13 11:57:41.322: E/AndroidRuntime(18118): at android.app.ActivityThread.access$600(ActivityThread.java:130)
05-13 11:57:41.322: E/AndroidRuntime(18118): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
05-13 11:57:41.322: E/AndroidRuntime(18118): at android.os.Handler.dispatchMessage(Handler.java:99)
05-13 11:57:41.322: E/AndroidRuntime(18118): at android.os.Looper.loop(Looper.java:137)
05-13 11:57:41.322: E/AndroidRuntime(18118): at android.app.ActivityThread.main(ActivityThread.java:4745)
05-13 11:57:41.322: E/AndroidRuntime(18118): at java.lang.reflect.Method.invokeNative(Native Method)
05-13 11:57:41.322: E/AndroidRuntime(18118): at java.lang.reflect.Method.invoke(Method.java:511)
05-13 11:57:41.322: E/AndroidRuntime(18118): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
05-13 11:57:41.322: E/AndroidRuntime(18118): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
05-13 11:57:41.322: E/AndroidRuntime(18118): at dalvik.system.NativeStart.main(Native Method)
05-13 11:57:41.322: E/AndroidRuntime(18118): Caused by: java.lang.NullPointerException
05-13 11:57:41.322: E/AndroidRuntime(18118): at com.Tabs.onCreate(Tabs.java:42)
05-13 11:57:41.322: E/AndroidRuntime(18118): at android.app.Activity.performCreate(Activity.java:5008)
05-13 11:57:41.322: E/AndroidRuntime(18118): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
05-13 11:57:41.322: E/AndroidRuntime(18118): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
05-13 11:57:41.322: E/AndroidRuntime(18118): ... 11 more
最佳答案
由于您有一个参数化的Tabs 构造函数，系统无法使用默认构造函数来创建它的实例。要克服这个问题，您需要显式定义默认构造函数。
但是当前的实现还有另外一个问题:
ActionBar.Tab tab = actionBar.newTab().setText(R.string.LabelSearchTabTitle).setTabListener((ActionBar.TabListener)new Tabs(this, LandingSearch.class, TAG ));
当前的方法导致为每个添加的选项卡创建一个 Activity 实例。相反，当前实例，即 this 需要传递给 setTabListener，这是不可能的。
解决方案:
更清晰的方法是代替 Tabs 实现 ActionBar.TabListener，在 Tabs 中定义一个内部类，它将实现 ActionBar.TabListener 并在将要创建的每个选项卡上显式设置它的一个实例。
代码如下:
import android.app.ActionBar.Tab;
import android.app.ActionBar;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentTransaction;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Toast;
public class MainActivity extends FragmentActivity {
protected static final String TAG = MainActivity.class.toString();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
final ActionBar bar = getActionBar();
bar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
bar.setDisplayOptions(0, ActionBar.DISPLAY_SHOW_TITLE);
bar.addTab(bar.newTab()
.setText("Search")
.setTabListener(new TabListener<LandingSearch>(
this, "search", LandingSearch.class)));
bar.addTab(bar.newTab()
.setText("Book")
.setTabListener(new TabListener<LandingBook>(
this, "book", LandingBook.class)));
}
public static class TabListener<T extends Fragment> implements ActionBar.TabListener {
private final FragmentActivity mActivity;
private final String mTag;
private final Class<T> mClass;
private final Bundle mArgs;
private Fragment mFragment;
public TabListener(FragmentActivity activity, String tag, Class<T> clz) {
this(activity, tag, clz, null);
}
public TabListener(FragmentActivity activity, String tag, Class<T> clz, Bundle args) {
mActivity = activity;
mTag = tag;
mClass = clz;
mArgs = args;
// Check to see if we already have a fragment for this tab, probably
// from a previously saved state. If so, deactivate it, because our
// initial state is that a tab isn't shown.
mFragment = mActivity.getSupportFragmentManager().findFragmentByTag(mTag);
if (mFragment != null && !mFragment.isDetached()) {
FragmentTransaction ft = mActivity.getSupportFragmentManager().beginTransaction();
ft.detach(mFragment);
ft.commit();
}
}
public void onTabSelected(Tab tab, android.app.FragmentTransaction ft) {
// Since ActionBar.TabListener needs android.app.FragmentTransaction, define the method signature with it, but don't use it
// Instead use the support fragment manager
FragmentTransaction fft = mActivity.getSupportFragmentManager().beginTransaction();
if (mFragment == null) {
mFragment = Fragment.instantiate(mActivity, mClass.getName(), mArgs);
fft.add(android.R.id.content, mFragment, mTag);
} else {
fft.attach(mFragment);
}
// Don't forget to call commit, as we are not using FragmentTransaction passed by ActionBar.
fft.commit();
}
public void onTabUnselected(Tab tab, android.app.FragmentTransaction ft) {
FragmentTransaction fft = mActivity.getSupportFragmentManager().beginTransaction();
if (mFragment != null) {
fft.detach(mFragment);
}
fft.commit();
}
public void onTabReselected(Tab tab, android.app.FragmentTransaction ft) {
Toast.makeText(mActivity, "Reselected!", Toast.LENGTH_SHORT).show();
}
}
public static class LandingSearch extends Fragment
{
protected static final String TAG = LandingSearch.class.toString();
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.landing_search, container, false);
setHasOptionsMenu(true);
// Other code.
return v;
}
}
public static class LandingBook extends Fragment
{
protected static final String TAG = LandingBook.class.toString();
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.landing_book, container, false);
setHasOptionsMenu(true);
// Other code
return v;
}
}
}
看看这个Android example .
关于android - 无法实例化 class.Tabs；没有空的构造函数，Android 中的 actionBar.Tab，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/23461876/