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I want all my activities to have knowledge about the power state: is there a charger connected? When the registered receiver receives an ACTION_POWER_CONNECTED or an ACTION_POWER_DISCONNECTED a callback to the activity is made to inform the registered activity. Since I want all activities that are part of the application, and I work with this callback method I created an interface which forces the activity to implement powerDisconnected() and powerConnected();

public interface BrightnessActivityInterface {
    void powerDisconnected();
    void powerConnected();
}

Because my activities all use the same type of code (eg. screen on and bright lock when connected, and screen dim when power is disconnected) I extended the class Activity and implemented the BrightnessActivityInterface:

public class BrightnessActivity extends Activity implements BrightnessActivityInterface {

    private ChargingOffReceiver chargingOff;
    private ChargingOnReceiver chargingOn;

    public BrightnessActivity(){
        chargingOn = new ChargingOnReceiver(this);
        registerReceiver(chargingOn, new IntentFilter(Intent.ACTION_POWER_CONNECTED));


        chargingOff = new ChargingOffReceiver(this);
        registerReceiver(chargingOff, new IntentFilter(Intent.ACTION_POWER_DISCONNECTED));
    }


    @Override
    public void onResume() {
        super.onResume();
        getWindow().addFlags(WindowManager.LayoutParams.FLAG_DISMISS_KEYGUARD + WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON + WindowManager.LayoutParams.FLAG_TURN_SCREEN_ON);
    }

    @Override
    public void onPause() {
        super.onPause();
        getWindow().clearFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
    }

    public void powerDisconnected() {
        getWindow().clearFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
    }

    public void powerConnected() {
        getWindow().addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON + WindowManager.LayoutParams.FLAG_TURN_SCREEN_ON);
    }

All my activities extend now this BrightnessActivity instead of Activity. Sounds all good so far?

The problem is the first activity I start I get a nullpointer just after the line 

 registerReceiver(chargingOn, new IntentFilter(Intent.ACTION_POWER_CONNECTED));

'this' has a value (it is my launch activity), and chargingOn is a ChargingOnReceiver. I installed the source but I have no idea what I am looking at.

My class has the rights to register to the ACTION_POWER_CONNECTED and it worked fine until I derived all my classes from BrightnessActivity instead of Activity. Please advice.

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2 回答 2

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您不应覆盖默认的 Activity 构造函数。将您的代码移动到覆盖的 onCreate() 方法。

于 2012-12-06T09:38:27.690 回答
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您应该在其中注册您的接收器onCreate并在其中取消注册onPause

API：onRegisterReceiver API

“您可以使用 Context.registerReceiver() 动态注册此类的实例，也可以通过 AndroidManifest.xml 中的标记静态发布实现。注意：如果在 Activity.onResume() 实现中注册接收器，则应取消注册它在 Activity.onPause() 中。（暂停时您不会收到意图，这将减少不必要的系统开销）。不要在 Activity.onSaveInstanceState() 中取消注册。”

如果你不明白为什么你的 Activity 在构造函数之后没有处于“就绪”状态，你需要阅读Activity Lifecycle

于 2012-12-06T09:36:56.497 回答