如何使用服务的上下文创建ImageView

Question

Working on Android I am creating an SDK that creates a service to display an overlay on the client app. I want to be able to create ImageViews and other widgets using the context of my class which extends the Service class. This is cut down version of what I am doing.

public class MyService extends Service {

private WindowManager windowManager;
private ImageView myImage;


@Override
public void onCreate(){
    super.onCreate();
}

@Override
public int onStartCommand(Intent intent, int flags, int startId){
    return START_NOT_STICKY;
}

public  void startService (Context ctx) {
    context = ctx;
    Intent intent = new Intent(ctx, MyService.class);
    ctx.startService(intent);
   // have also tried new Intent(this, MyService.class);
   // startService(intent);

}

public void displayMyImage () {
    if (chatHead == null) {
        chatHead = new ImageView(this);// explodes here due to null 
    }
    chatHead.setImageResource(R.drawable.myimage);

    windowManager = (WindowManager) getSystemService(WINDOW_SERVICE);
    WindowManager.LayoutParams params = new WindowManager.LayoutParams(
            WindowManager.LayoutParams.WRAP_CONTENT,
            WindowManager.LayoutParams.WRAP_CONTENT,
            WindowManager.LayoutParams.TYPE_PHONE,
            WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE | WindowManager.LayoutParams.FLAG_LAYOUT_NO_LIMITS,
            PixelFormat.TRANSLUCENT);

    params.gravity = Gravity.BOTTOM | Gravity.LEFT;
    params.x = 10;
    params.y = 100;


    windowManager.addView(chatHead, params);


}

@Override
public IBinder onBind(Intent intent) {
    return null;
}

@Override
public void onDestroy() {
    super.onDestroy();
    if (chatHead != null) windowManager.removeView(chatHead);
}

I get a java.lang.NullPointerException on the line chatHead = new ImageView(this)

The reason I am trying this is from this article http://www.piwai.info/chatheads-basics/ where they are using this to create an image view within the service. How can I get it to use the context of the service and not need me to pass in the context of the client/host app?

Edit: just putting in full error from log

java.lang.NullPointerException
at android.content.ContextWrapper.getResources(ContextWrapper.java:89)
at android.view.View.<init>(View.java:3438)
at android.widget.ImageView.<init>(ImageView.java:114)

Answer 1

The most easiest solution is create a Global Application context under the Application class of your project.

in that application class write the below code to intitialize your context and access it from anywhere.It will be initialized when your application starts.and it will solve your NPE

For Application class code:

public class MyApplication extends Application{

private static Context context;

public void onCreate(){
    super.onCreate();
    MyApplication.context = getApplicationContext();
}

public static Context getAppContext() {
    return MyApplication.context;
}

}

Now Modify your displayImage like this:

public void displayMyImage () {
if (chatHead == null) {
    chatHead = new ImageView(MyApplication.getAppContext());// NPE won't occur
......
}
}