Can I create custom global methods in my Android Application class?

Question

I currently have an app that has many activities and needs to have a way of maintaining state between these activities.

I use the Application class to do this, declaring my global variables and using getters and setters to interact with my activities.

I was hoping to place a few custom methods in there, so that when I want to do a common task like, for instance, display an error message, I can declare the method in my application class and call it from any activity that uses it

EscarApplication application = (EscarApplication) this.getApplication();

EscarApplication being the name of my application class above.

I have tried to include this method in my application class:

public void showError(String title, String message) {
        Log.i("Application level",message);
        this.alertDialog.setTitle(title);
        alertDialog.setMessage(message);
        alertDialog.setButton("OK", new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog,int which) {
                return;
            }
        });
        alertDialog.show();
    }

In the hope that I can call this method from activity without having to redeclare it, but when I call it using something like below I get an null pointer exception:

Visit.this.application.showError("Update error", "An error has occurred while trying to communicate with the server");

Visit being the name of my current activity above.

Should this work, or can I only use getters and setters to change global vars in an Application Class.

Stack Trace:

java.lang.RuntimeException: Unable to start activity ComponentInfo{escar.beedge/escar.beedge.HomeScreen}: android.view.WindowManager$BadTokenException: Unable to add window -- token null is not for an application
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2401)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2417)
at android.app.ActivityThread.access$2100(ActivityThread.java:116)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1794)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:123)
at android.app.ActivityThread.main(ActivityThread.java:4203)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:521)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:549)
at dalvik.system.NativeStart.main(Native Method)
ERROR/AndroidRuntime(375): Caused by: android.view.WindowManager$BadTokenException: Unable to add window -- token null is not for an application
at android.view.ViewRoot.setView(ViewRoot.java:460)
at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:177)
at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:91)
at android.app.Dialog.show(Dialog.java:238)
at escar.beedge.EscarApplication.showError(EscarApplication.java:98)
at escar.beedge.HomeScreen.onCreate(HomeScreen.java:30)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1123)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2364)

The dialog is declared as such in the application class:

AlertDialog alertDialog;

Created in that same class:

alertDialog = new AlertDialog.Builder(this).create();

and the method to call it in that class is as follows:

public void showError(String title, String message) {
        alertDialog.setTitle(title);
        alertDialog.setMessage(message);
        alertDialog.setButton("OK", new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog,int which) {
                return;
            }
        });
        alertDialog.show();
    }

And finally, it is called from an activity like so:

EscarApplication application;
application = (EscarApplication) this.getApplication();
application.showError("test", "display this message");

Answer 1

If you need to maintain state between activities, then use a service. Anything else is a hack

Answer 2

Someone correct me if Im wrong, but an Application class wont be able to execute view related objects as they need to be bound to a view which needs to be bound to an activity.

In that sense, you could use your Application class to implement a static method that customises the dialog

public static void setDialog(String title, String message,AlertDialog alertDialog){
        alertDialog.setTitle(title);
        alertDialog.setMessage(message);
        alertDialog.setButton("OK", new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog,int which) {
                return;
            }
        });
}

but you would have to create the dialog and call the show method on the activities themselves (actually maybe even the button to be set in the dialog would need to be created on the activity)

Another option could be to implement a class that extends the AlertDialog class and whose button is pre-set to the behavior you want.

Answer 3

You could use the Singleton pattern.

Answer 4

I'm looking to achieve something similar to you.

I haven't found an official answer, but it looks like you shouldn't be using the application context for Toast and Dialogs. Instead, try using the context of an Activity like so :

// From inside your activity
Dialog dialog = new Dialog(this);

instead of this:

// From inside your Application instance
Dialog dialog = new Dialog(getApplicationContext());

Read this : Android: ProgressDialog.show() crashes with getApplicationContext