First of all: I am rather new to Android App programming and I have a rather basic question:
Already with the sandbox app I am currently working on, the code in the Activity class get quite huge because all the callback methods / listeners (click listener, callbacks from GoogleApiClient) are in there (either by implementing the respective interface or by creating a private class). But I would rather put those into separate classes.
But the question that I ask myself is this: how would I then be able to access the class attributes of the activity class? Sure, I would then probably create setter/getter, but still I first need a reference to the Activity object. How would I get this?
Thanks and regards!
It's a really wide question, since the answer depends by your project and by your programming style. The first suggestion is: move what you can move in one or more fragment. All stuffs related to google play services can be nicely handled in a fragment for example. Listener and callback are UI related components, so they need a Context of an Activity to work, but you can split your UI (again) with Fragment and keep a piece of logic in a Fragment and another piece somewhere else. If you have some logic that runs in background, then you should consider using Service. I tend to have empty Activities, but this is not a rule.
A common situation I face is that I wish to test a custom View visibly on screen while it is being developed as part of a large application. I realise that unit testing exists, particularly using ActivityUnitTestCase, but as far as I understand these frameworks don't actually attach the components visibly on screen.
At present, the way I tend to do this is to just place the component wherever it will actually be used within the application. This often means needing to wait some time for the application to start or to navigate through parts of the application before it is visible, which can become cumbersome.
The better way I've found is to create an Activity within the application that exists purely for test purposes (e.g. to simply display a custom View I'm developing), which is launched to display the custom View (or whatever) that I am working on. To do this, the only intent filter I've assigned to that Activity in the manifest is android.intent.action.MAIN, and not android.intent.category.LAUNCHER. Then, I can simply create a Run/Debug Configuration in Android Studio to launch that Activity directly. This, as far as I believe, effectively allows me to have Activity classes which may only be launched by me from the IDE.
My questions are:
Does omitting android.intent.category.LAUNCHER guarantee that users of the application won't be able to launch that Activity by any means nor be aware of its existence? Is this a safe to have Activity classes for development only?
Is there a workflow or test framework of any kind I could use to improve on how I am doing this?
I realise that unit testing exists, particularly using ActivityUnitTestCase, but as far as I understand these frameworks don't actually attach the components visibly on screen.
Well, ActivityInstrumentationTestCase2 does, as does the new ActivityTestRule. I don't recall playing with ActivityUnitTestCase.
Does omitting android.intent.category.LAUNCHER guarantee that users of the application won't be able to launch that Activity by any means nor be aware of its existence?
No. It won't be easy for them to start it, but it is exported and has an <intent-filter>, so somebody could find a way.
Is this a safe to have Activity classes for development only?
I would put them in your debug sourceset, assuming that you are using Android Studio. Here is a sample project that has a production launcher activity in main and a diagnostic activity in debug. In a release build, the diagnostic activity does not even ship.
Why don't declared some value like
public static boolean IS_DEBUG = true;
then in any part you want to show it, just put in condition like
if (IS_DEBUG) {
your_develop_view.setVisibility(View.VISIBLE);
//...
}
else {
your_develop_view.setVisibility(View.GONE);
}
after you finish develop. before you publish, just change
IS_DEBUG = false;
Instead of defining your own IS_DEBUG, just use BuildConfig.DEBUG
There are many examples of taking a screenshot using a reference to the current activity. However, in my case, I need to take a screenshot of an activity which comes from an external SDK (but still within my app). For obvious reasons I don't have a reference to that activity object within my code. I saw solutions using Instrumentation and UiDevice, but they seem to work only when implementing a testing application using the TestCase framework, while I need it to work in a normal application.
Is there another way?
Well, if anyone finds this question useful, here's the answer I found:
In your activity or service, call getApplication().registerActivityLifecycleCallbacks(), passing it an object that implements the Application.ActivityLifecycleCallbacks interface. That object will get a callback on every activity's onCreate/onStart etc. with a reference to the Activity object, from which you can get the view and take a screenshot. You can identify that it's an activity from an external SDK by its package name.
NOTE: Application.ActivityLifecycleCallbacks is only available since API level 14 (a.k.a. ICS).
I've seen this in a few tutorials now... but how in the world can Android source code not have a main method and still run.
For example (from http://developer.android.com/guide/tutorials/hello-world.html):
public class HelloAndroid extends Activity {
/** Called when the activity is first created. */
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}
}
That runs but there is no main!!!
I've also thought that using things like onCreate (or formLoad, etc.) was bad becuase a constructor should do that work and such built-in methods can be smelly sometimes. But onCreate is an entry point? Even without a main?
What if there is more than one activity... is there a hierarchy to these built in event handlers? OnCreate trumps everything else? Otherwise, how would the app know what to run or where to enter the program?
Thanks!
Each application will be having it's own Virtual Machine. To run an app, within it's space (VM), must have a main method.
Activities are not the actual classes to be invoked for start of application. There is a class called Application, which will be the root class for an application to be launched.
If there is no main method, how can a VM recognize how to start an app?
Framework has classes called Process, VMRuntime which are responsible for starting an application. Which indeed deal with main method.
For better understanding, study the Zygote service of Android. deals with Applicationmanager Service, ActivityStack Activity Threadds etc.
That runs but there is no main!!!
Of course. Many things that you might think of as a Java "application" do not have their own main() method. For example, IIRC, servlets, WARs, and the like do not have main() methods -- the main() method, if there is one, is in the container.
But onCreate is an entry point?
onCreate() is a method.
What if there is more than one activity... is there a hierarchy to these built in event handlers?
Not really.
OnCreate trumps everything else?
Not really.
Otherwise, how would the app know what to run or where to enter the program?
An app does not "know what to run or where to enter the program".
An Android application is a basket of components. Some components may be tied to icons in a home screen launcher. Some components may be tied to scheduled timers, like cron jobs or Windows scheduled tasks. Some components may be tied to system events, such as when the device is placed into or removed from a car dock. Those components will be automatically created and used when appropriate (e.g., when a user taps the icon in the home screen launcher). Yet other components are only created and used when your code specifically asks for them.
Thinking of an Android application as if it were a monolithic console-mode Java program will cause you no end of trouble.
You tell it which one to run on startup in the manifest file. There isn't a main() because there doesn't have to be, main may be a convention used for "regular" java apps, but it isn't for things like browser applets. The system creates the activity object and calls methods within it, which may or may not be called main. In this case, it's not.
onCreate is different from a main, and from a constructor, in that it can be called twice on a single activity, such as if the process is killed and the user navigates back to the activity. See http://developer.android.com/reference/android/app/Activity.html#ActivityLifecycle
Actually, this type of pattern is not peculiar of Android, but happens whenever you have some framework in the middle. Some basic examples are java Applets and Servlets. Some of the answers already provide give the correct response, but I will try to elaborate a bit.
When you launch a Java app, you start a JVM and then you need to load something into it: so you need a static method (the main) because there are no objects (yet) living in the JVM that you can refer to.
If you have some sort of framework in the middle, it is the framework that will start the JVM and will start populating it with its own service objects: writing your code then means writing your own objects (which will be subclasses of given "template"). Your objects can then be injected (loaded) by the framework. The framework service objects manage the lifecycle of the injected objects by calling the lifecycle methods defined in the "template" superclass.
So for instance when you provide an applet to a browser, you do not launch a static main method: you rather only provide a subclass of java.applet.Applet that implements some instance methods which act as callback to manage the lifecycle (init, paint, stop...). It is the browser that will launch the JVM, instantiate what's needed for the launching an applet, load your applet and call it.
Similarly, with servlets you subclass the javax.servlet.http.HttpServlet class and implement some instance (non static) methods (doGet, doPost...). The Web container (e.g. Tomcat) will be in charge to launch the JVM, instantiate what's needed for launching a servlet, load your servlet and call it.
The pattern in Android is pretty much the same: what do you do is to create a subclass of android.app.Activity. When you launch an app, the system looks in the manifest to find out which activity should be started, then the "framework" loads it and calls its instance methods (onCreate, onPause, onResume...).
In Java programs we need a main() method, because while executing the byte code the JVM will search for the main() method in the class and start executing there.
In Android, the Dalvik Virtual Machine is designed to find a class which is a subclass of Activity and which is set to start the execution of the application from its onCreate() method, so there is no need of a main() method.
The order in which Dalvik Virtual Machine calls methods is based on order of priorities called android life cycle for more information on android life cycle check the link below
Android Life Cycle: https://developer.android.com/guide/components/activities/activity-lifecycle.html
While there is no specific main entry point, intent filters describe which activity is started when the application is launched.
They are controlled in AndroidManifest.xml as described here:
http://developer.android.com/guide/topics/intents/intents-filters.html
where a note pad application example is described:
This filter declares the main entry point into the Note Pad application. The standard MAIN action is an entry point that does not require any other information in the Intent (no data specification, for example), and the LAUNCHER category says that this entry point should be listed in the application launcher.
An android programmer should learn this like the back of their hands it simply explains everything and would help in the future when creating activities.
http://developer.android.com/reference/android/app/Activity.html
There is a main of sorts, it just happens to be out of your hands. After all, there's nothing special about a main function in any language. It's just the entry point where your code starts executing. The Android operating system expects applications to have a certain structure and it calls your code based on the conventions you follow.
I found this particularly useful...
http://developer.android.com/guide/topics/fundamentals.html#appcomp
Applets don't have main() methods either. It just depends on how your code is packaged.
The Android UI frame encapsulate some Java common details, you can study the source code of the android UI framework
I think that Jonathon's answer is going in the right direction. He says the OS expects a certain structure. There's a name for that structure which is a "state machine". In this case Android calls it the "activity lifecycle". Rob gives a link to the documentation which contains an important diagram of that state machine though the text is a bit dry. A quick search also found me the following link that explains it fairly clearly: http://www.android-app-market.com/android-activity-lifecycle.html
In Java, there is a main even if it isn't listed as main(). The page you get after the icon click, whatever its name, is the main().
I have an application that is driven by a configuration XML: various
app properties are loaded at the app start-time by parsing the XML and
initializing static variables of some class. The data read from this
XML drives different Activities of the application. Presently, I have
called the "parsing and the properties-initialization" from the
onCreate() of my Main Activity.
I have a few questions as regards this case/approach:
Should I invoke the app initialization method from the Application
Object or is the current approach correct? What advantages/
disadvantages do/would we get/have if I choose to invoke it from the
Application object?
Do we really need a static class to store app properties? Or can we have all the properties as a static Collection variable in the application object?
Parsing a XML(~200 nodes) at app load time might take some time(not
sure how long tho); How can I avoid the dreaded ANRs? I am using a
Pull Parser.
Please help me find answers to these questions.
Thank you.
It depends on what you're initializing. Application's onCreate() should be used when you're doing things that need to be done before any part of your app works correctly and only needs to be done once, whereas Activity/Service/etc's onCreate() should be used for things that are needed for that component alone and needs to be done multiple times.
The main concern I have for putting all your initialization into a component is that it will make extending your application more difficult later on. Suppose you want to make some Activity in your application accessible by outside intents - now you've got to either move the initialization code to Application or you have to duplicate initialization code in the non-launcher Activity.
It sounds like you should check out SharedPreferences, especially PreferenceManager.getDefaultSharedPreferences(). The preferences will be stored between sessions and it gives you easy access to simple properties from any Context.
Threading. I find AsyncTask to be the easiest way to accomplish this task; there's a good write-up on it at Google. Alternatively, you could fire up a Service to do this in the background while having a foreground Activity inform the user that you're booting up the app.
The Application object is used for sharing non-persistent state across the application. I don't think you'll need to use an Application class at all. You can do your initialisation in the onCreate() method of the Activity that is called first. To quote the documentation:
The subclass is optional; most applications won't need one. In the absence of a subclass, Android uses an instance of the base Application class.
You don't need to create your own class to store application properties. This is done for you by SharedPreferences.
You should also have a look at the setDefaultValues() method in the PreferenceManager
class as this will set preferences from the data in an XML file. What's nice about this method is that use the readAgain parameter so that the XML is only parsed once - the first time you start up your application - rather than every time.