Can anyone enlighten me about the safety of a class holding global values in Android?
Here's a short example of what I mean:
public class Globals {
public static int someVariable = 0;
public static User currentUser = null;
public static Handler onLogin = null;
}
Then somewhere in an Activity I do the following:
Globals.someVariable = 42;
Globals.currentUser = new User("John", "Doe");
I have to rely on Globals.currentUser at multiple places in my app as soon as the user is logged in, but I'm unsure if I should do it, and also if I could use a Handler like this.
I read everywhere that an Android app could be killed anytime, does this mean it is killed completely or maybe just a part of it, thus killing my Globals class only?
Or is there any other way to store globally available data in a safe way, without writing every member change to the database (in fact, my User class is a little more complex than in this example. ;-)
Thanks for your effort!
Edit: Ok, here's what I finally did:
public class MyApp extends Application {
private static MyApp _instance;
public MyApp() {
super();
_instance = this;
}
public static MyApp getContext() {
return _instance;
}
....
private User _user = null;
public User getUser() {
if (_user == null) _user = new User();
return _user;
}
}
Then modify the AndroidManifest.xml and add android:name=".MyApp" to your application node to tell the app to use your subclass.
So far everything works fine and I can easily access the current Context (f.ex. in SQLiteOpenHelper) by calling MyApp.getContext().
It would be better to use the Android Application class. It's meant to store global application state
http://developer.android.com/reference/android/app/Application.html
Just create a subclass and make sure to update your manifest file to use your version. Then you can store whatever you need to in it. Activities have a method getApplication() which you can cast to your class to access your implementation
The pattern is discouraged--you will run into problems when unit testing.
Can you explain how you unit-test a class that must supply different custom "Users" here? You are either forcing a mock/fake class into "User" which will probably have a cross-effect on other tests or you are putting an if(test) into your code which gets ugly quick.
Over time populating this class artificially for testing gets more complex and starts to have relationships and dependencies.
More simply it makes it difficult to unit test a class in isolation.
It's one of those patterns that a given programmer either doesn't see a problem with or never uses because he's been burnt--you'll see little middle ground.
Related
I'm in the process of completely redesigning my Android app. Before, EVERYTHING was in the same class.
So I tried to redraw everything so that the code is clearer apart Admob than the doc advice to put in the Main thread, I separate the different part of my code in class. So I used two technique: I created a songleton that contains variables that I want to have access to constantly,and I call my classes via weak reference.
Here is what it looks like:
For example, the UIManager class that needs to update the game's IU have a weak reference looks like this:
private static SoftReference<UIManager> ManageUI;
static{ManageUI= new SoftReference<>(null);}
static UIManager get()
{
if(ManageUI.get()==null)
{
ManageUI= new SoftReference<>(new UIManager());
}
return ManageUI.get();
}
GameManager Manager=GameManager.getInstance();
to be able to use the findviewbyid for example I place in method argument the main class that is the mainthread
the singleton that contains all my variables that I want to have permanent access to looks like this:
private GameManager()
{}
/** Holder */
private static class Manager
{
/** Instance unique non préinitialisée */
private final static GameManager instance = new GameManager();
}
/** Point d'accès pour l'instance unique du singleton */
public static GameManager getInstance()
{
return Manager.instance;
}
To separate all in different class, I pass argument to my method so I can call au stuff belong to Activity like that:
(My main class is called GamePlay)
void OpenGlobalScene(GamePlay activity)
{
Manager.OnTitle=false;
if (!checkLayout(activity,R.id.globalscene)) {
LayoutInflater(activity,9, true);
LinearLayout GamePlan = (LinearLayout) activity.findViewById(R.id.globalscene);
GamePlan.setAlpha(Manager.AlphaBord);
}
}
For now, I have not noticed any problems except a few slownesses on old android phone 4.4.2.
Also compared to my old code were EVERYTHING was in the same class, it's much easier to change pieces of code (going to the inapp billing V3 was simpler since everything was in one class that I call like the others with weak referencre)
My questions are:
-What are the problems that such a structure might pose?
I had also chosen that structure to not load or leave in memory things that are not useful
-How are chance that Android will erase from memory an action in progress called with weak reference?
-As you can see I pass the activity has argument to the method, sometimes I pass it from a method to another. Is that fact can cause some trouble?
Thank you for your help.
Check Dagger2 is better than the clasic singleton https://developer.android.com/training/dependency-injection/dagger-android?hl=es-419
thanks for your answer and your tips. I'am gonna check this out.
Anyone else know something about consequences on memory when using weak references ?
Is creating a Singleton to keep track of the current logged in user of a mobile app a bad idea? The class below is something along the lines of what I've used before and it works fine, but I always feel it's not the best way of doing things.
public class LoggedInUser {
private static LoggedInUser ourInstance = null;
User user;
public static LoggedInUser getInstance() {
return ourInstance != null ? ourInstance : new LoggedInUser();
}
private LoggedInUser () {
user = new User();
}
public void setUser(User user) {
this.user = user;
}
public User getUser() {
return this.user;
}
}
I use the info of the user pretty often throughout the app, and sometimes the User object isn't trivially small so passing the object between each view doesn't sound like the best option either. What is normal practice for this?
The user has to log in every time they use the app, so I also don't want to write the information to the phone necessarily.
I have found a similar question (Is Singleton the best way to keep data in memory on an Android application?) and the answer posted by charlag is actually pretty mature:
I would use the following: Interface CurrentUser. CurrentUser has methods to retrieve and update user. User may be stored in DB, preferences or mixed. I would use Dagger to inject CurrentUser when needed. CurrentUser implementation should itself be a singleton, because Dagger doesn't guarantee qualities of singleton. Bonus points if your CurrentUser has something like RxJava stream or LiveData to keep observers up-to-date.
As an alternative to Singleton you may want to implement Fragment
Holder pattern with retained fragments but in your situation Singleton
seems better. Just make sure that you use interfaces and injection to
not compromise testability and to keep your components separated.
Do not fall into keeping your objects within the App class, unless it is crucial to instantiate your app's dependencies correctly - it sounds nice and easy, but can also get out of control pretty quick.
Singleton is normal.
But you can always use Application class and its variables (both static and object) to store the data.
public class MyApp extends Application {
public static LoggedInUser user;
#Override
public void onCreate() {
super.onCreate();
user = LoggedInUser.getInstance();
//...
}
//or even
public LoggedInUser user_obj;
}
AndroidManifest.xml:
<application
android:allowBackup="true"
android:largeHeap="true"
android:name=".MyApp"
android:icon="#drawable/iconlite"
android:label="#string/app_alias"
android:theme="#style/AppTheme" >
somewhere in the code use MyApp.user or ((MyApp)getApplication()).user_obj
The work flow of my program is:
Launch app
Splash screen, check the server api, from the api get a list of file name
Download some of the file in file list , remove the downloaded file name from the list
App opened
when the download is finished , jump to main page that will start download another file in the list
The problem is , the list I was keep in the download manager , when I select don't leave activities in android setting , it will be killed. If I need a class that is some Data Class , that means I put a share data (A several hash map , array list) in it, and it keep updating (delete after async download finish) , and it never get killed. How can it be done? Thanks
The more general problem you are encountering is how to save state across several Activities and all parts of your application. A static variable (for instance, a singleton) is a common Java way of achieving this. I have found however, that a more elegant way in Android is to associate your state with the Application context. As you know, each Activity is also a Context, which is information about its execution environment in the broadest sense. Your application also has a context, and Android guarantees that it will exist as a single instance across your application. The way to do this is to create your own subclass of android.app.Application, and then specify that class in the application tag in your manifest. Now Android will automatically create an instance of that class and make it available for your entire application. You can access it from any context using the Context.getApplicationContext() method (Activity also provides a method getApplication() which has the exact same effect):
class MyApp extends Application {
private String myState;
public String getState() {
return myState;
}
public void setState(String s) {
myState = s;
}
}
class Blah extends Activity {
#Override public void onCreate(Bundle b) {
...
MyApp appState = ((MyApp) getApplicationContext());
String state = appState.getState();
...
}
}
This has essentially the same effect as using a static variable or singleton, but integrates quite well into the existing Android framework. Note that this will not work across processes (should your app be one of the rare ones that has multiple processes).
I am looking for how to share functions and data across multiple activities within a single application. I researched the daylights out of it and find some ideology war between overriding the extend for the application and doing a singleton, neither of which I can find examples sufficient to make me understand. Basically I want to share data and share functions. All activities need the same functions and data so this is not one activity sharing data with another activity. It is all activities needing to have access to the same functions and data.
What I want to know is what is the way to go and how do I do it. I need to see what I need to do in my 34 activities, what the class that is going to be common looks like, and what the Manifest entry needs to be. I also need to be sure the common data area will not be closed by the OS.
This is my first Android - Java program and now find my 15,000 line, 34 activity application needs some structure. I know, should have done things differently but the app works really well with two exceptions. One is that it is structurally a mess. Two is that the fact it is a mess is making it hard to fix one behavior I would like to fix.
This is a GPS based application for racing sailboats. It is timing critical and every activity basically runs a once a second loop inside the location manager onLocationChanged function. That part is fine and I do not want to put the GPS code in one place. The problem is that most activities need to filter the data so a lot of code is copied and pasted to the activities. The filter needs history so it needs to remember a state. There are other functions that are used by several activities so these have been copied as well. Think of a function that averages the last three GPS speed readings. It needs to save some history, do its thing, and give a result. All activities need to do the exact same thing. All this works but the problem is that the averaging starts over every time I switch activities because every activity has its own filter. That gives a glitch in the data that I need to get rid of. I need common place to save the data and hopefully a common place to run the filtering and other functions that are common. If every activity can call the filter function that is using common state data, there will be no glitch across activity changes.
I would appreciate some guidance.
Why you don't just make a Class with only static functions, passing needed Parameters? An example if you want to show an ErrorDialog
public class SharedHelper{
public static Dialog showErrorDialog(Context ctx, String message, String title, DialogInterface.OnClickListener okListener, DialogInterface.OnClickListener cancelListener){
AlertDialog.Builder builder = new AlertDialog.Builder(ctx);
builder.setMessage(message).setTitle(tilte);
if (okListener != null){
builder.setPositiveButton(R.string.button_positive, okListener);
}
if (cancelListener != null){
builder.setNegativeButton(R.string.button_negative, cancelListener);
}
return builder.show();
}
}
Singletons are (from my point of view) one of the uglyest design pattern and will bite you sooner or later. Putting anything in Application requires you to cast it everytime to the Special Application class you designed. A class with only statics however is very flexible in its usage and doesn't need an instance to work.
For the storage-issue:
lookup "SharedPreferences" & "SQLite" and decide afterwards which storage-type suits your needs more.
For the methods-issue:
This question is a bit more complex and there are different ways to do it. For example you could write a parent-class that implements all your globally needed questions and you let all your activity-classes inherit from it.
public class MyParentActivity extends Activity {
public void myMethod() {
}
}
and:
public class Activity1of34 extends MyParentActivity {
myMethod();
}
I think what this comes down to is not an Android problem but an Object-Oriented Programming problem. If I understand the situation correctly, I'm betting the best solution would be to take your shared filter and create a new Filter class that is instantiated within each Activity (this is likely more manageable than a singleton, but not having seen your use case, it's hard to say for sure). If you need to centrally track the averaging, you can simply create a static variable within the Filter class that maintains the same value during the life of the application. If you really want to maintain that average (even past the application's current lifecycle), you can persist it in a database or other local data options. However, I don't see any reason to put everything in a singleton just to maintain that average. Singletons (and all static data structures) can be potentially troublesome if used incorrectly.
I, for one, do not mind the singleton pattern. Of course as everything else it should not be abused.
This is the construction I use for my shared objects. My app is divided into modules this way but can just as well be used in your case.
public class SharedDataObject {
private Context context;
private static SharedDataObject instance;
public static SharedDataObject getInstance() {
if (instance == null) throw new RuntimeException("Reference to SharedDataObject was null");
return instance;
}
public static SharedDataObject createInstance(Context context) {
if (instance != null) {
return instance;
}
return instance = new SharedDataObject(context.getApplicationContext());
}
// notice the constructor is private
private SharedDataObject(Context context) {
this.context = context;
}
...
public void myMethod() {
// do stuff
}
}
Notice that it uses the application context, that means among other things, means that the context owned by SharedDataObject cannot be used for GUI operations. But, the context will live for the entire lifetime of the application, which is nice.
Furthermore I hate having to pass a context everytime I wish to call methods on my SharedDataObject, thus I have a splashscreen calling SharedDataObject.createInstance() on all my modules.
Once an instance is create, I can call:
SharedDataObject.getInstance().myMethod();
Anywhere in my code, regardless of a context being present or not (from the place calling this code that is).
I was wondering what is the best way to handle global variables for android apps. For example, I'm just trying to create a basic login/register system. I've created a user class (that has various attributes like username, password, etc..), so that when we go to the register activity, the User class constructor is called to create a unique user object once all the fields are filled out. I was then thinking of simply having a global arrayList of type User so that I could just loop through all the users on a login attempt.
So far (due to a combined lack of experience in java, and being very new to this android stuff), I haven't been able to implement this successfully. I have a class which I call "globalStuff" which has a bunch of public static variables (i.e. the list of users and current user), which I thought could be accessed from any activity the user navigates to.
There must be a better way to go about this. I've been reading through a few tutorials and a few posts on here, but none address this very basic idea. So what would be the best way to approach something like this?
Thanks for any help!
It's called a static singleton and it looks like this:
public class Global {
private static Global instance = null;
public static Global getInstance() {
if( instance == null )
instance = new Global();
return instance;
}
// additional methods, members, etc...
}
Then you just refer to it in your code as:
Global.getInstance().getUserList();
etc.