This is not a question about Context object itself. I wonder what the best way to manage reference to it. When I create android objects like Activity, Service, etc - context already accessible everywhere.
But I have bunch of different classes for data access, various shared utilities, etc. I find myself writing every call and passing context alone. I wonder if there any good way to deal with context. Maybe static? Is that good idea to store reference in my own Application object like so?
public class MyApplication extends Application
{
public static Context Context;
This way I can access context from anywhere in application. Does it seem like a good idea?
Make public static Context mContext; as a global variable, and at the start of onCreate(), add mContext = this;. It makes access much easier. By making it public and static, other files can see the context of your main activity, and you won't have to pass it as a parameter to any function. In my opinion, this is probably the best/easiest way to manage reference to it. It also unclutters all sorts of references to MainActivity.this.
alextsc's comment shows a helpful link to problems that can occur from this as well.
Related
Android Studio now displays a warning when a class contains a static Context object. It says this causes a memory leak. However, I have noticed this is also done in android libraries. For instance, the LocalBroacastManager class has a static instance and it contains a context object.
So how bad is this as a memory leak?
I have a singleton geofencing class that runs in the background and saves a boolean to sharedPreferences indicating if the user is in the geofence or not. To save the preferences I need a context object, but since the method is an overridden method, I have no way of passing a context object in.
How can this be accomplished without a context instance variable?
Its about the worst you can have. Lets say that you have an Activity, and store that as a static Context. Unless you null it out when the activity finishes, you now have leaked a whole Activity. That means every variable that the activity holds is leaked, including the entire view hierarchy. Its basically preventing anything in that Context from freeing up.
The best thing to do is not to store a Context, but instead to pass it in as a parameter to functions that need it. If you must store a Context, do not make it static. A non-static variable won't leak it so long as nothing in the framework continues to hold a reference to the object after the activity is finished.
If you absolutely must use a static Context, make it the Application context. That one is valid over the length of your app, so it can't really be leaked.
Statics should be rarely used, especially in Android. There are ways and reasons of using statics, but in 90% of cases, it is just a misuse of them.
Keeping the context as a static variable is a big no-no. Imagine following scenario:
You go from activity A to activity B.
While in activity B you keep activity B reference as static Context somewhere.
You go back to activity A. Activity B should be destroyed, but instead it is kept, since you keep a static reference to it.
Now you go again to B from A, et voila - you have two B instances: the one you see and another one kept as static context.
You may NEVER have two instances of the same activity. It could cause a lot of issues.
Now, I'm aware that a lot of developers (even ones making libraries) every now and then make mistakes and use anti-patterns, so you should not blindly rely on practice/pattern seen in someone other's code. Hell, I've even seen a lot of garbage code in something written by Google devs.
If you really need a singleton (not Singleton pattern (uppercase S)), but single instance of some class, if you don't want to use libraries like Dagger and so, you can instantiate that class in your Application class, and then reference it from wherever you want.
Does it matter whether I access strings from strings.xml file from different contexts?
I accessed it in many ways such as: getApplicationContext().getString(R.string.name), this.getString(R.string.name),MainActivity.this.getApplicationContext().getString(R.string.name), or simply getString(R.string.name).
I understand that the application context lasts the whole application lifetime, but I honestly do not know when I should use it. Also, I cannot understand what is the difference between using this and MainActivity.this.
Please explain how I should access strings from both activities and Java classes.
The key argument for using the Application Context or the Activity Context is the dependency to the lifecycle of the whole App or a component (Activity in this case). If you wanna access the strings.xml it doesn't really matter which of both you use. Because you just use the Context get a reference to the ressource, which goes away when the Activity will be destroyed. A more importent case would be if you pass the Context to another class f.e. Because then there is a dependency to this specific Context and can lead to memory leaks if you don't clean it. There is a nice explanation about this with the usage of a BroadcastReceiver in the docs.
There is no difference between this and AnyClass.this because it points to the same class object. But you have to use AnyClass.this if you wanna access the outer class in an inner class. Or if you have an anonymous class and need to access fields of the outer class. You could see it as more strict because you refer to a specific class.
I have created a SettingsActivity for my app. In this Activity I am using the SharedPreferences class to do handle the user editable preferences.
While setting up the SharedPreferences, I have to load them in the onCreate of my main activity and then again in the SettingsActivity. The probably was that both calls to the getXXXX() methods require defaults and I figured that it would not be good to hard-code the default values into both places because I would imagine it would be problematic in the future if I ever changed them.
Which is the best/most popular (or accepted standard) of doing this?
Create a global variables class in which I import into each activity and define my default constants in there?
Use putExtra and getExtra to pass the data from the main activity to the settings activity?
Any other suggestions?
I think Squonk has a good answer, but if you're looking for an alternative, consider creating a Settings class with all of your settings as members. It could have a static method like loadFromPreferences(Context) that would return a Settings object constructed from SharedPreferences, using whatever defaults you need. It could also have a saveSettings(Context) method to save your edits. Hope that helps.
Personally, in this situation, I'd put the default values in a resource file. In that way there's no need to use a global variables class or a helper class. Android resources already do that for you.
See:
Providing resources
More resource types
Instead of using a class with static values why dont you extend the Application class which will always live when the application's process lives. you can keep shared methods and variables in it
I would highly recommend opening the SharedPreference in the onCreate of both activities. Every time I've tried to use global variables, the values disappear in a way that's difficult to detect and fix. Activities are destroyed when they are closed. Services can be removed from memory at any time. The application context will be destroyed if your services are sleeping and don't have activities in memory.
That being said, putting a variable in the application context is probably the best place. Create a class that extends Application and set AndroidManifest.xml to use this. Just don't expect the value to be there if you try to use it from services or broadcast receivers.
Also, unless you're having problems with the activities loading too slowly, you're better off spending time on features than optimization.
You can declare objects as public static and reference them from another class. ActivityA:
public static int testIntegerA = 42;
Intent intentInteger = new Intent(getActivityContext(), ActivityB.class);
intentInteger.putExtra("INTENT_EXTRA", testIntegerA);
startActivity(intentInteger);
ActivityB:
public static int intentInt, staticInt;
staticInt = ActivityA.testIntegerA;
intentInt = getIntent().getExtras().getInt("INTENT_EXTRA");
Now both intentInt and staticInt equal 42;
I am following some tutorials for learning android but the problem is they do not contain all the details.
Specifically, it would be great if some light could be shed upon the use cases for getApplicationContext().
You can think of a Context as a handle to your application's resources (i.e. everything in the res folder) and to the Android runtime. Classes like Activity, Service, Application inherit from Context, among others.
Typically, you'll pass in the current Activity whenever a Context is required, since Activity inherits from Context.
You'll also find information about using getApplicationContext() instead of your activities when a Context is required. This usually is to prevent whatever code that needs the Context from unnecessarily holding a reference to your Activity, which in the worst case might preventing it from being garbage collected (passing in the Application isn't really a huge problem in that scenario, because it's expected to be around for a long time).
However, there are certain scenarios where an argument of a method is of type Context, but an Activity is actually required. Using startActivity() is one example of this I believe (correct me if I'm wrong).
I have a simple Android application that uses an instance of a class, let's call it DataManager, to manage access to the domain classes in a Façade-like way. I initially designed it as a singleton which could be retrieved using static methods but I eventually got irritated with the messiness of my implementation and refactored to what I reckoned was a simpler and cleaner idea.
Now the idea is that for each file that is opened, one DataManager is created, which they handles both file I/O and modification of the domain classes (e.g. Book). When starting a new Activity, I pass this one instance as a Serializable extra (I haven't got on to using Parcelable yet, but expect I will when I have the basic concept working), and then I grab the DataManager from the Intent in the onCreate() method of the new Activity.
Yet comparison of the objects indicates that the object sent from one activity is not identical (different references) to the object retrieved from the Bundle in the second Activity. Reading up on Bundle (on StackOverflow, etc.) suggests that Bundles cannot do anything other than pass-by-value.
So what is likely to be the cleanest and safest strategy for passing an object between Activities? As I see it I could
Forget about passing by reference and live with each Activity having its own DataManager object. Pass back the new DataManager every time I close an activity so that the underlying activity can use it. (The simple solution, I think.)
Go back to using a singleton DataManager and use a static method to get it from each Activity. (Not keen on using singletons again.)
Extend Application to create a sort of global reference to DataManager. (Again, not keen on the idea of globals.)
Is that a fair summary? Is there some other healthy strategy I could use?
Another approach would be to create a service. The first activity would start the service and bind to it, when you launch a new intent, unbind the first activity and when second activity starts, bind to the service.
This way you don't have to ever stop the service or worry about passing data between activities.
Java does not have pass by reference so that option is out, I would suggest dependency injection for passing data between the activities. Otherwise definetely the singleton would be the way to go.
The prescribed one is Going by implementing Parcellable interface, thats the way to pass Objects between Activities.. and the 2nd and better choice is to make a Singleton to be sure its single Object.
Create your DataManager as a Singleton that implements Service. Bind the service to your application in the manifest xml (see the link), and you will have a persistent singleton your activities can access without issues.
Passing parcellable arguments can quickly get very messy if you need to get a lot of data. The singleton approach, although usually considered an anti-pattern, works like a charm in cases like these. Just remember to not create multiple singletons that interact with one another.
I would suggest using an Application Subclass. It allows you to hold a single reference to the DataManger class and is persistent as long as your app lives.
A singleton with a static field will also work, but there are some place in the documentation where it says that the content of static fields is not a safe place to store your data. As I understand static fields should persist as long as your ClassLoader stays in memory. Therefore a singleton should only vanish if the whole process leaves the memory and in that case the application class will also leave the memory, but it will call the onDestroy methods of the Application and that enables you to safely close your DataManager and persist important data to memory.
That said to your two variations.
The Android way to go would be to make your DataManager a ContentProvider. This will make it possible to access your Data from every Activity without holding a global reference. I'm not sure how you would build a caching Content Provider that stays in memory and is not reinstantiated too often.