java - 如何将上下文传递给android库中的Intent？

Question

我在 android 中为循环屏幕创建了一个库，当我尝试在我的 Activity 中实现它时，我收到了此错误消息。

java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.app.Activity.getApplicationContext()' on a null object reference
        at com.expert.recur.ScreenReco.<init>(ScreenReco.java:15)
        at com.expert.recurringscreen.MainActivity.onCreate(MainActivity.java:18)

我的代码。 MainActivity.java:

public class MainActivity extends AppCompatActivity {

    ScreenReco screenReco;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        screenReco=new ScreenReco(MainActivity.this);//line 18
        screenReco.value = 1000;
        screenReco.runnable.run();

    }
}

我的图书馆:

public class ScreenReco {
    Activity activity;

    public ScreenReco(Activity activity) {
        this.activity = activity;
    }

    public Context context = activity.getApplicationContext();//line 15
    public int value;
    public Handler handler = new Handler();
    public Runnable runnable = new Runnable() {
        @Override
        public void run() {
            Intent i = new Intent(context, context.getClass());
            handler.postDelayed((Runnable) context,value);
            context.startActivity(i);
        }
    };
}

Answer 1

您应该在为其变量赋值之前创建一个 ScreenReco 类的对象...

public class MainActivity extends AppCompatActivity {

ScreenReco screenReco;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      screenReco=new ScreenReco(); // you are missing this
      screenReco.context = this;
      screenReco.value = 1000;
      screenReco.runnable.run();
  }

}

但我强烈建议您为此使用构造函数......这是一个很好的做法