android - getActionBar().setDisplayHomeAsUpEnabled(true); preferenceActivity 中的空指针

Question

我针对这个问题进行了很多搜索，但没有找到任何相关的内容。

我更新了我的应用程序项目以使用 api 21，现在当我运行 preferenceactivity 时，我在该行上得到了一个空指针。

这是我的代码(它与创建新设置 Activity 时从 eclipse 提供的代码相同)。

package com.example.prova;

import android.annotation.TargetApi;
import android.content.Context;
import android.content.res.Configuration;
import android.media.Ringtone;
import android.media.RingtoneManager;
import android.net.Uri;
import android.os.Build;
import android.os.Bundle;
import android.preference.ListPreference;
import android.preference.Preference;
import android.preference.PreferenceActivity;
import android.preference.PreferenceCategory;
import android.preference.PreferenceFragment;
import android.preference.PreferenceManager;
import android.preference.RingtonePreference;
import android.text.TextUtils;
import android.view.MenuItem;
import android.support.v4.app.NavUtils;

import java.util.List;

/**
 * A {@link PreferenceActivity} that presents a set of application settings. On
 * handset devices, settings are presented as a single list. On tablets,
 * settings are split by category, with category headers shown to the left of
 * the list of settings.
 * <p>
 * See <a href="http://developer.android.com/design/patterns/settings.html">
 * Android Design: Settings</a> for design guidelines and the <a
 * href="http://developer.android.com/guide/topics/ui/settings.html">Settings
 * API Guide</a> for more information on developing a Settings UI.
 */
public class SettingsActivity extends PreferenceActivity {
    /**
     * Determines whether to always show the simplified settings UI, where
     * settings are presented in a single list. When false, settings are shown
     * as a master/detail two-pane view on tablets. When true, a single pane is
     * shown on tablets.
     */
    private static final boolean ALWAYS_SIMPLE_PREFS = false;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setupActionBar();
    }

    /**
     * Set up the {@link android.app.ActionBar}, if the API is available.
     */
    @TargetApi(Build.VERSION_CODES.HONEYCOMB)
    private void setupActionBar() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
            // Show the Up button in the action bar.
            getActionBar().setDisplayHomeAsUpEnabled(true);
        }
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == android.R.id.home) {
            // This ID represents the Home or Up button. In the case of this
            // activity, the Up button is shown. Use NavUtils to allow users
            // to navigate up one level in the application structure. For
            // more details, see the Navigation pattern on Android Design:
            //
            // http://developer.android.com/design/patterns/navigation.html#up-vs-back
            //
            // TODO: If Settings has multiple levels, Up should navigate up
            // that hierarchy.
            NavUtils.navigateUpFromSameTask(this);
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);

        setupSimplePreferencesScreen();
    }

    /**
     * Shows the simplified settings UI if the device configuration if the
     * device configuration dictates that a simplified, single-pane UI should be
     * shown.
     */
    private void setupSimplePreferencesScreen() {
        if (!isSimplePreferences(this)) {
            return;
        }

        // In the simplified UI, fragments are not used at all and we instead
        // use the older PreferenceActivity APIs.

        // Add 'general' preferences.
        addPreferencesFromResource(R.xml.pref_general);

        // Add 'notifications' preferences, and a corresponding header.
        PreferenceCategory fakeHeader = new PreferenceCategory(this);
        fakeHeader.setTitle(R.string.pref_header_notifications);
        getPreferenceScreen().addPreference(fakeHeader);
        addPreferencesFromResource(R.xml.pref_notification);

        // Add 'data and sync' preferences, and a corresponding header.
        fakeHeader = new PreferenceCategory(this);
        fakeHeader.setTitle(R.string.pref_header_data_sync);
        getPreferenceScreen().addPreference(fakeHeader);
        addPreferencesFromResource(R.xml.pref_data_sync);

        // Bind the summaries of EditText/List/Dialog/Ringtone preferences to
        // their values. When their values change, their summaries are updated
        // to reflect the new value, per the Android Design guidelines.
        bindPreferenceSummaryToValue(findPreference("example_text"));
        bindPreferenceSummaryToValue(findPreference("example_list"));
        bindPreferenceSummaryToValue(findPreference("notifications_new_message_ringtone"));
        bindPreferenceSummaryToValue(findPreference("sync_frequency"));
    }

    /** {@inheritDoc} */
    @Override
    public boolean onIsMultiPane() {
        return isXLargeTablet(this) && !isSimplePreferences(this);
    }

    /**
     * Helper method to determine if the device has an extra-large screen. For
     * example, 10" tablets are extra-large.
     */
    private static boolean isXLargeTablet(Context context) {
        return (context.getResources().getConfiguration().screenLayout & Configuration.SCREENLAYOUT_SIZE_MASK) >= Configuration.SCREENLAYOUT_SIZE_XLARGE;
    }

    /**
     * Determines whether the simplified settings UI should be shown. This is
     * true if this is forced via {@link #ALWAYS_SIMPLE_PREFS}, or the device
     * doesn't have newer APIs like {@link PreferenceFragment}, or the device
     * doesn't have an extra-large screen. In these cases, a single-pane
     * "simplified" settings UI should be shown.
     */
    private static boolean isSimplePreferences(Context context) {
        return ALWAYS_SIMPLE_PREFS
                || Build.VERSION.SDK_INT < Build.VERSION_CODES.HONEYCOMB
                || !isXLargeTablet(context);
    }

    /** {@inheritDoc} */
    @Override
    @TargetApi(Build.VERSION_CODES.HONEYCOMB)
    public void onBuildHeaders(List<Header> target) {
        if (!isSimplePreferences(this)) {
            loadHeadersFromResource(R.xml.pref_headers, target);
        }
    }

    /**
     * A preference value change listener that updates the preference's summary
     * to reflect its new value.
     */
    private static Preference.OnPreferenceChangeListener sBindPreferenceSummaryToValueListener = new Preference.OnPreferenceChangeListener() {
        @Override
        public boolean onPreferenceChange(Preference preference, Object value) {
            String stringValue = value.toString();

            if (preference instanceof ListPreference) {
                // For list preferences, look up the correct display value in
                // the preference's 'entries' list.
                ListPreference listPreference = (ListPreference) preference;
                int index = listPreference.findIndexOfValue(stringValue);

                // Set the summary to reflect the new value.
                preference
                        .setSummary(index >= 0 ? listPreference.getEntries()[index]
                                : null);

            } else if (preference instanceof RingtonePreference) {
                // For ringtone preferences, look up the correct display value
                // using RingtoneManager.
                if (TextUtils.isEmpty(stringValue)) {
                    // Empty values correspond to 'silent' (no ringtone).
                    preference.setSummary(R.string.pref_ringtone_silent);

                } else {
                    Ringtone ringtone = RingtoneManager.getRingtone(
                            preference.getContext(), Uri.parse(stringValue));

                    if (ringtone == null) {
                        // Clear the summary if there was a lookup error.
                        preference.setSummary(null);
                    } else {
                        // Set the summary to reflect the new ringtone display
                        // name.
                        String name = ringtone
                                .getTitle(preference.getContext());
                        preference.setSummary(name);
                    }
                }

            } else {
                // For all other preferences, set the summary to the value's
                // simple string representation.
                preference.setSummary(stringValue);
            }
            return true;
        }
    };

    /**
     * Binds a preference's summary to its value. More specifically, when the
     * preference's value is changed, its summary (line of text below the
     * preference title) is updated to reflect the value. The summary is also
     * immediately updated upon calling this method. The exact display format is
     * dependent on the type of preference.
     * 
     * @see #sBindPreferenceSummaryToValueListener
     */
    private static void bindPreferenceSummaryToValue(Preference preference) {
        // Set the listener to watch for value changes.
        preference
                .setOnPreferenceChangeListener(sBindPreferenceSummaryToValueListener);

        // Trigger the listener immediately with the preference's
        // current value.
        sBindPreferenceSummaryToValueListener.onPreferenceChange(
                preference,
                PreferenceManager.getDefaultSharedPreferences(
                        preference.getContext()).getString(preference.getKey(),
                        ""));
    }

    /**
     * This fragment shows general preferences only. It is used when the
     * activity is showing a two-pane settings UI.
     */
    @TargetApi(Build.VERSION_CODES.HONEYCOMB)
    public static class GeneralPreferenceFragment extends PreferenceFragment {
        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            addPreferencesFromResource(R.xml.pref_general);

            // Bind the summaries of EditText/List/Dialog/Ringtone preferences
            // to their values. When their values change, their summaries are
            // updated to reflect the new value, per the Android Design
            // guidelines.
            bindPreferenceSummaryToValue(findPreference("example_text"));
            bindPreferenceSummaryToValue(findPreference("example_list"));
        }
    }

    /**
     * This fragment shows notification preferences only. It is used when the
     * activity is showing a two-pane settings UI.
     */
    @TargetApi(Build.VERSION_CODES.HONEYCOMB)
    public static class NotificationPreferenceFragment extends
            PreferenceFragment {
        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            addPreferencesFromResource(R.xml.pref_notification);

            // Bind the summaries of EditText/List/Dialog/Ringtone preferences
            // to their values. When their values change, their summaries are
            // updated to reflect the new value, per the Android Design
            // guidelines.
            bindPreferenceSummaryToValue(findPreference("notifications_new_message_ringtone"));
        }
    }

    /**
     * This fragment shows data and sync preferences only. It is used when the
     * activity is showing a two-pane settings UI.
     */
    @TargetApi(Build.VERSION_CODES.HONEYCOMB)
    public static class DataSyncPreferenceFragment extends PreferenceFragment {
        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            addPreferencesFromResource(R.xml.pref_data_sync);

            // Bind the summaries of EditText/List/Dialog/Ringtone preferences
            // to their values. When their values change, their summaries are
            // updated to reflect the new value, per the Android Design
            // guidelines.
            bindPreferenceSummaryToValue(findPreference("sync_frequency"));
        }
    }

}

我没有得到任何错误，但是当运行这个时我得到:

12-03 17:00:53.627: E/AndroidRuntime(10654): FATAL EXCEPTION: main
12-03 17:00:53.627: E/AndroidRuntime(10654): Process: com.example.prova, PID: 10654
12-03 17:00:53.627: E/AndroidRuntime(10654): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.prova/com.example.prova.SettingsActivity}: java.lang.NullPointerException
12-03 17:00:53.627: E/AndroidRuntime(10654):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2198)
12-03 17:00:53.627: E/AndroidRuntime(10654):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2257)
12-03 17:00:53.627: E/AndroidRuntime(10654):    at android.app.ActivityThread.access$800(ActivityThread.java:139)
12-03 17:00:53.627: E/AndroidRuntime(10654):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210)
12-03 17:00:53.627: E/AndroidRuntime(10654):    at android.os.Handler.dispatchMessage(Handler.java:102)
12-03 17:00:53.627: E/AndroidRuntime(10654):    at android.os.Looper.loop(Looper.java:136)
12-03 17:00:53.627: E/AndroidRuntime(10654):    at android.app.ActivityThread.main(ActivityThread.java:5097)
12-03 17:00:53.627: E/AndroidRuntime(10654):    at java.lang.reflect.Method.invokeNative(Native Method)
12-03 17:00:53.627: E/AndroidRuntime(10654):    at java.lang.reflect.Method.invoke(Method.java:515)
12-03 17:00:53.627: E/AndroidRuntime(10654):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
12-03 17:00:53.627: E/AndroidRuntime(10654):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
12-03 17:00:53.627: E/AndroidRuntime(10654):    at dalvik.system.NativeStart.main(Native Method)
12-03 17:00:53.627: E/AndroidRuntime(10654): Caused by: java.lang.NullPointerException
12-03 17:00:53.627: E/AndroidRuntime(10654):    at com.example.prova.SettingsActivity.setupActionBar(SettingsActivity.java:57)
12-03 17:00:53.627: E/AndroidRuntime(10654):    at com.example.prova.SettingsActivity.onCreate(SettingsActivity.java:47)
12-03 17:00:53.627: E/AndroidRuntime(10654):    at android.app.Activity.performCreate(Activity.java:5248)
12-03 17:00:53.627: E/AndroidRuntime(10654):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1110)
12-03 17:00:53.627: E/AndroidRuntime(10654):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2162)
12-03 17:00:53.627: E/AndroidRuntime(10654):    ... 11 more

那么，如何在创 build 置 Activity 的同时保持向后兼容性呢？此外，如果我删除该行，该应用程序可以运行，但我没有顶部栏。

---

更多信息:

我的AndroidManifest.xml:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.prova"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="21" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name=".SettingsActivity"
            android:label="@string/title_activity_settings"
            android:parentActivityName=".MainActivity" >
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="com.example.prova.MainActivity" />
        </activity>
    </application>

</manifest>

我的values/styles.xml:

    <resources>

    <!--
        Base application theme, dependent on API level. This theme is replaced
        by AppBaseTheme from res/values-vXX/styles.xml on newer devices.
    -->
    <style name="AppBaseTheme" parent="Theme.AppCompat.Light">
        <!--
            Theme customizations available in newer API levels can go in
            res/values-vXX/styles.xml, while customizations related to
            backward-compatibility can go here.
        -->
    </style>

    <!-- Application theme. -->
    <style name="AppTheme" parent="AppBaseTheme">
        <!-- All customizations that are NOT specific to a particular API-level can go here. -->
    </style>

</resources>

使用 android 支持库 v7 使用 android 5.0 进行编译。

Answer 1

使用 PreferenceActivcity 和 Theme.AppCompat.Light 方法 getActionBar() 返回 null。您应该使用不同的样式(仅适用于 API>=11)。

有一个替代方案(更好的替代方案)。

建议在官方谷歌doc :

如何使用带有首选项的 AppCompat？在 API v11+ 设备上运行时，您可以继续在 ActionBarActivity 中使用 PreferenceFragment。对于在此之前的设备，您将需要提供一个非 Material 样式的普通 PreferenceActivity。

然后您可以为 api>11 使用 PreferenceFragment 和 ActionBarActivity。通过这种方式，您可以使用 Theme.AppCompat.Light 主题。

注意使用getSupportActionBar()而不是getActionBar()。