An extended Application class can declare global variables. Are there other reasons?
Introduction:
If we consider an apk file in our mobile, it is comprised of
multiple useful blocks such as, Activitys, Services and
others.
These components do not communicate with each other regularly and
not forget they have their own life cycle. which indicate that
they may be active at one time and inactive the other moment.
Requirements:
Sometimes we may require a scenario where we need to access a
variable and its states across the entire Application regardless of
the Activity the user is using,
An example is that a user might need to access a variable that holds his
personnel information (e.g. name) that has to be accessed across the
Application,
We can use SQLite but creating a Cursor and closing it again and
again is not good on performance,
We could use Intents to pass the data but it's clumsy and activity
itself may not exist at a certain scenario depending on the memory-availability.
Uses of Application Class:
Access to variables across the Application,
You can use the Application to start certain things like analytics
etc. since the application class is started before Activitys or
Servicess are being run,
There is an overridden method called onConfigurationChanged() that is
triggered when the application configuration is changed (horizontal
to vertical & vice-versa),
There is also an event called onLowMemory() that is triggered when
the Android device is low on memory.
Application class is the object that has the full lifecycle of your application. It is your highest layer as an application. example possible usages:
You can add what you need when the application is started by overriding onCreate in the Application class.
store global variables that jump from Activity to Activity. Like Asynctask.
etc
Sometimes you want to store data, like global variables which need to be accessed from multiple Activities - sometimes everywhere within the application. In this case, the Application object will help you.
For example, if you want to get the basic authentication data for each http request, you can implement the methods for authentication data in the application object.
After this,you can get the username and password in any of the activities like this:
MyApplication mApplication = (MyApplication)getApplicationContext();
String username = mApplication.getUsername();
String password = mApplication.getPassword();
And finally, do remember to use the Application object as a singleton object:
public class MyApplication extends Application {
private static MyApplication singleton;
public MyApplication getInstance(){
return singleton;
}
#Override
public void onCreate() {
super.onCreate();
singleton = this;
}
}
For more information, please Click Application Class
Offhand, I can't think of a real scenario in which extending Application is either preferable to another approach or necessary to accomplish something. If you have an expensive, frequently used object you can initialize it in an IntentService when you detect that the object isn't currently present. Application itself runs on the UI thread, while IntentService runs on its own thread.
I prefer to pass data from Activity to Activity with explicit Intents, or use SharedPreferences. There are also ways to pass data from a Fragment to its parent Activity using interfaces.
The Application class is a singleton that you can access from any activity or anywhere else you have a Context object.
You also get a little bit of lifecycle.
You could use the Application's onCreate method to instantiate expensive, but frequently used objects like an analytics helper. Then you can access and use those objects everywhere.
Best use of application class.
Example: Suppose you need to restart your alarm manager on boot completed.
public class BaseJuiceApplication extends Application implements BootListener {
public static BaseJuiceApplication instance = null;
public static Context getInstance() {
if (null == instance) {
instance = new BaseJuiceApplication();
}
return instance;
}
#Override
public void onCreate() {
super.onCreate();
}
#Override
public void onBootCompleted(Context context, Intent intent) {
new PushService().scheduleService(getInstance());
//startToNotify(context);
}
Not an answer but an observation: keep in mind that the data in the extended application object should not be tied to an instance of an activity, as it is possible that you have two instances of the same activity running at the same time (one in the foreground and one not being visible).
For example, you start your activity normally through the launcher, then "minimize" it. You then start another app (ie Tasker) which starts another instance of your activitiy, for example in order to create a shortcut, because your app supports android.intent.action.CREATE_SHORTCUT. If the shortcut is then created and this shortcut-creating invocation of the activity modified the data the application object, then the activity running in the background will start to use this modified application object once it is brought back to the foreground.
I see that this question is missing an answer. I extend Application because I use Bill Pugh Singleton implementation (see reference) and some of my singletons need context. The Application class looks like this:
public class MyApplication extends Application {
private static final String TAG = MyApplication.class.getSimpleName();
private static MyApplication sInstance;
#Contract(pure = true)
#Nullable
public static Context getAppContext() {
return sInstance;
}
#Override
public void onCreate() {
super.onCreate();
Log.d(TAG, "onCreate() called");
sInstance = this;
}
}
And the singletons look like this:
public class DataManager {
private static final String TAG = DataManager.class.getSimpleName();
#Contract(pure = true)
public static DataManager getInstance() {
return InstanceHolder.INSTANCE;
}
private DataManager() {
doStuffRequiringContext(MyApplication.getAppContext());
}
private static final class InstanceHolder {
#SuppressLint("StaticFieldLeak")
private static final DataManager INSTANCE = new DataManager();
}
}
This way I don't need to have a context every time I'm using a singleton and get lazy synchronized initialization with minimal amount of code.
Tip: updating Android Studio singleton template saves a lot of time.
I think you can use the Application class for many things, but they are all tied to your need to do some stuff BEFORE any of your Activities or Services are started.
For instance, in my application I use custom fonts. Instead of calling
Typeface.createFromAsset()
from every Activity to get references for my fonts from the Assets folder (this is bad because it will result in memory leak as you are keeping a reference to assets every time you call that method), I do this from the onCreate() method in my Application class:
private App appInstance;
Typeface quickSandRegular;
...
public void onCreate() {
super.onCreate();
appInstance = this;
quicksandRegular = Typeface.createFromAsset(getApplicationContext().getAssets(),
"fonts/Quicksand-Regular.otf");
...
}
Now, I also have a method defined like this:
public static App getAppInstance() {
return appInstance;
}
and this:
public Typeface getQuickSandRegular() {
return quicksandRegular;
}
So, from anywhere in my application, all I have to do is:
App.getAppInstance().getQuickSandRegular()
Another use for the Application class for me is to check if the device is connected to the Internet BEFORE activities and services that require a connection actually start and take necessary action.
Source: https://github.com/codepath/android_guides/wiki/Understanding-the-Android-Application-Class
In many apps, there's no need to work with an application class directly. However, there are a few acceptable uses of a custom application class:
Specialized tasks that need to run before the creation of your first activity
Global initialization that needs to be shared across all components (crash reporting, persistence)
Static methods for easy access to static immutable data such as a shared network client object
You should never store mutable instance data inside the Application object because if you assume that your data will stay there, your application will inevitably crash at some point with a NullPointerException. The application object is not guaranteed to stay in memory forever, it will get killed. Contrary to popular belief, the app won’t be restarted from scratch. Android will create a new Application object and start the activity where the user was before to give the illusion that the application was never killed in the first place.
To add onto the other answers that state that you might wish store variables in the application scope, for any long-running threads or other objects that need binding to your application where you are NOT using an activity (application is not an activity).. such as not being able to request a binded service.. then binding to the application instance is preferred. The only obvious warning with this approach is that the objects live for as long as the application is alive, so more implicit control over memory is required else you'll encounter memory-related problems like leaks.
Something else you may find useful is that in the order of operations, the application starts first before any activities. In this timeframe, you can prepare any necessary housekeeping that would occur before your first activity if you so desired.
2018-10-19 11:31:55.246 8643-8643/: application created
2018-10-19 11:31:55.630 8643-8643/: activity created
You can access variables to any class without creating objects, if its extended by Application. They can be called globally and their state is maintained till application is not killed.
The use of extending application just make your application sure for any kind of operation that you want throughout your application running period. Now it may be any kind of variables and suppose if you want to fetch some data from server then you can put your asynctask in application so it will fetch each time and continuously, so that you will get a updated data automatically.. Use this link for more knowledge....
http://www.intridea.com/blog/2011/5/24/how-to-use-application-object-of-android
Related
I created a global variable called GlobalVariables in my app:
public class GlobalVariables extends Notifications {
private boolean purchased = false;
public boolean getPurchaseState() {
return this.purchased;
}
public void setPurchaseState(boolean result) {
purchased = result;
}
}
As you may have noticed it extends a class called Notifications instead of extending Application. This is because I already had a class called Notifications which itself extends Application, and I read that it's not possible for two classes to extend Application since only one class can be added in the <application tag in Manifest, and in my case that class is Notifications. I read that the solution was simply to have one class subclass other, so I had GlobalVariables subclassing Notifications.
So I then wrote this in my activity
GlobalVariables globalVariables = (GlobalVariables) getApplication();
which causes a crash that says getApplication() cannot be cast to GlobalVariables.
How do I get globalVariables?
You can't, if Notifications is your application class you can't cast it to a subclass as Android still considers it a Notifications class. You could set GlobalVariables as the Application class though (inheriting Notifications).
I don't know what exactly you want to achieve but SharedPreferences might help to access variables across the app and also keep their state when the app gets closed.
Otherwise ViewModels (MVVM) help you access variables across Fragments or different parts of the App, same as Repositories.
I made a static class (NetworkUtils.java) with a few static AsyncTasks, each sending some data to server and retrieving a response.
It is working great (in a test environment) and it made the code very clean in the Activities:
NetworkUtils.SomeNetworkingTaskCallbackInterface callbacks =
new NetworkUtils.SomeNetworkingTaskCallbackInterface() {
#Override
public void onFinished(String result) {
}
#Override
public void onFailed() {
}
};
NetworkUtils.SomeNetworkingTask task = new NetworkUtils.SomeNetworkingTask(callbacks);
task.execute();
Now I am doing a little extra research if there is anything wrong with this approach. I've seen a lot of use-cases of nested static AsyncTasks, but I need them completely decoupled and modular, that's why I put them into a separate static class. I can't think of any downside. Can someone more experienced weigh in?
The only disadvantages I can think about is that you won't have any access to non static members of the parent class. If for example, your static Async Task object had an Activity parent class you wouldn't be able to call methods that require a context. That means you won't be able to start services, activities or broadcast events from the Async Task.
However, you could simply pass a context object to the static class to solve that. But then what happens when your Async Task takes long and your activity gets destroyed before it's onFinish is called? You're get an error as your context object is from a destroyed activity.
I'm utilizing Firebase in my application and I figure it'd be better practice if I constantly observed the AuthState once the user has signed up or logged in and updated one class that can be accessed globally throughout my application (but that can only be instantiated once). Do I use a Singleton design pattern for this? How do I accomplish something like this in Android?
A common pattern is to create your own subclass of Android's Application class (specified in AndroidManifest.xml as shown below for example) and store variable there....effectively a singleton.
<application
android:name=".MyApplication"
In my app I use a static way for doing something similar.. maybe it helps to find solution for your situation:
I created a class which can not be instantiated because the constructor is set private and the class is used as some "variable-holder" accessible from all app statically. Here is the class:
VarHolder.java
package your.package.name;
public class VarHolder {
private VarHolder() {}
static int myInt;
static String myString;
}
than in app I use it this way:
VarHolder.myInt = 1;
VarHolder.myString = "Hello";
on another place in app :
textView.setText(VarHolder.myString);
there is Class Named "Configs.java" contains :
public Contect context;
public Activity currentActivity;
On Global Class Named "App.java" :
public static Configs configs = new Configs();
On Main Activity Java Class Oncreate :
App.configs.context = getApplicationContext();
App.configs.currentActivity = this;
On Products Activity Java Class Oncreate (Main Activity Not Finished And Must Work In Background):
App.configs.currentActivity = this;
After Long Time Minimized , it shows An Error On Bellow Line From Products Activity Java Class :
App.configs.currentActivity = this;
NullPointerException
Unable To Use "Bundle savedInstanceState" Because It's Activity Valiable
Try moving this code to onResume() instead of onCreate() and see if the error still exists.
Using static variables on Android is no good. This post explains that Android is very likely to kill your app (kills your Application instance, hence the static variable becomes null when your app is recreated and the class loader reinstantiates your vars) to save memory. The post also refers to Saving Activity state in Android which offers an alternative to save stuff with Bundles.
Anyway, if you need a quick solution, make your Configs class a singleton, and use getters and setters instead of making it public. Make the getter check if your singleton is null, and in this case, instantiate a new and return it. You won't get NPEs but it is very risky since it won't restore the previous state.
I'm going through some memory issues with my Android development.
I was wondering if my actual model would work, and also would like some input on how to do this in a better way:
I need public static final globals
I need public global variables that never gets garbage collected
I need to have an Engine running and never destroy without me calling stop()
MainApplication : Application
public static final Boolean DEBUG = false;
onCreate()
Engine.getEngine().prepare()
MainActivity : Activity
onResume()
Engine.getEngine().start()
onPause()
Engine.getEngine().stop()
Engine
prepare()
MainApplication.DEBUG = true;
start()
LocationManager.requestLocationUpdates()
stop()
LocationManager.removeUpdates()
Engine is a Singleton class, receiving location updates and such.
It is imperative that my Engine class does not get released not the DEBUG variable.
For your "Type 1" constants use a dedicated public class containing public static final T NAME = Val; - declarations.
For the Engine, I'd recommend using a Service.
For your "Type 2" vars, the Service could offer getters or you could make use of SQLite.
I see there's some "Location"-Stuff going on there ... the developer website has some good "best practices" on using the Location Services. Maybe there you'll get some more inspiration.