试图动态禁用preference当空指针 [英] Null Pointer when trying to dynamically disable a preference

问题描述

我收到的米preference =（preference）一个空指针异常... 在以下code（即调试在米preference 显示空; NB关键在于非空，我可以用文字代替，它仍然给了我同样的除外）：

I am getting a null pointer exception on mPreference = (Preference)... in the following code (i.e. with debugger on, mPreference shows null; NB key is non-null, I can substitute with a literal and it still gives me the same exception):

public class SettingsManager extends PreferenceActivity {

    private Preference mPreference;
    private static boolean toggle;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // Load the XML preferences file
        addPreferencesFromResource(R.xml.preferences);
    }

    public void setPrefEnabled(Context context, String key, String enabled) {
        if (enabled.contentEquals("true")) {
            toggle = true;
        } else {
            toggle = false;
        }
        mPreference = (Preference) getPreferenceScreen().findPreference(key);
        mPreference.setEnabled(toggle);
    }
}

类是由以下code调用，在一个类中的 applySettings 方法，其中设定管理在构造函数中实例化：

The class is being called by the following code, in the applySettings method of a class where settingsManager is instantiated in the constructor:

public class ConfigurationSetter {

    private static Context mContext;
    private static SettingsManager settingsManager;

    public ConfigurationSetter(Context context) {
        mContext = context;
        settingsManager = new SettingsManager();
    }

    private static void applySettings(Context context, String key,
            String value, String enabled) {
        settingsManager.setPrefEnabled(mContext, key, enabled);
    }
}

我一直在抓我在这头，但可能有点太累了，弄明白我自己。任何帮助是AP preciated。

I've been scratching my head on this but may be a little too tired to figure it out on my own. Any help is appreciated.

与此相关的问题，我asked这里有关意图是否为这项活动所需的instantiater的问题。

Related to this problem, I asked a question here about whether intents are the required instantiater for this activity.

推荐答案

您的onCreate 方法永远不会在这个code称为（因此 preferenceScreen 在通话期间，不填充添加preferencesFromResource 像预想的那样）。 的onCreate 是活动生命周期，如果你启动你的活动使用意图。人们通常不会使用创建活动新，在这种情况下，它们不会链接到正确的上下文和显示其他形式的未定义行为，像你这样的。

Your onCreate method is never called in this code (and thus the PreferenceScreen is not filled during the call to addPreferencesFromResource as supposed). onCreate is a part of the Activity lifecycle which gets called if you launch your Activity using an Intent. One usually does not create Activities using new, as in that case they don't get linked to the proper Context and display other forms of undefined behaviour, like yours.

这篇关于试图动态禁用preference当空指针的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！