无法实例化class.Tabs； Android 中没有空的构造函数，actionBar.Tab

Question

我已经为一个新的标签系统实现了 FragmentTabHost。但很快意识到我也需要 actionBars 和 tabListener 支持。我的问题如下：我在没有 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）我无法使代码正常工作。我错过了什么……？

我正在尝试的代码：

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

Answer 1

由于您有一个参数化的Tabs 构造函数，因此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，您需要显式定义一个默认构造函数。

但是当前的实现还有另一个问题：

ActionBar.Tab tab = actionBar.newTab().setText(R.string.LabelSearchTabTitle).setTabListener((ActionBar.TabListener)new Tabs(this, LandingSearch.class, TAG ));

当前的方法会为添加的每个选项卡创建一个 Activity 实例。而是需要将当前实例（即this）传递给setTabListener，这是不可能的。

解决方案： 更简洁的方法是代替 Tabs 实现 ActionBar.TabListener，在 Tabs 中定义一个内部类，它将实现 ActionBar.TabListener 并在每个将要创建的选项卡上显式设置它的实例。

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。

Answer 2

我在您的代码中看不到这个特定问题，但如果您使用 Eclipse 向导创建了操作栏，您的问题可能会通过以下方式解决：How can we retain a simple State Array across Configuration changes using Headless Fragments?