I noticed that one of my colleagues is declaring all the member variables of an activity as static. His point is that there will never be two instances of the activity, and static member variables are more efficient than non-static. Of course, when the activity is created, he will initialize all the members variables as one would do.
I feel this is wrong. I don't think the gain is even justifiable. But I wanted to run this by you guys. Whether or not it is a good programming practice is probably beyond the scope of stackoverflow's mandate. So my specific question is if using static member variables for an activity ever break anything (besides coder's error of not initializing them properly in onCreate).
Thank you in advance for your help.
Yes, android can create 2 instances of the same activity, so your friend is wrong.
To have only one instance, you must declare the following in manifest
<activity android:name=".YourActivity"
android:launchMode="singleTask"
android:label="#string/app_name" />
Yes, you can have multiple instances on the same activity and you often want to.
For example, if I have 3 views that simply compose of a ListView. You can create 3 activities that had the same code duplicated and small changes, or one activity that handles the loading of the layout and getting a reference to the ListView and extend it to add functionality.
You can limit an activity to one instance by declaring it that way in the manifest file.
Related
If I have two class of extended with Application Class. How do I handle the App? I have two Class. How do I manage the two Global Class in manifest. I have created two Module now I am integrating so it creating a problem for me.
if i have two class of extended with Application Class. How do i Handle the App?
Have one subclass the other. Or merge them together. Or delete one. Or delete both.
In first class i have done Network parsing Work. in Second class i have done Cart Controller. Both should be Global so extended with Application class
It is unclear if either of those things should be global in scope. Even if they should be global in scope, then neither of those things need to extend Application. Have them be standard Java singletons.
Only the <manifest> and <application> elements are required.
They each must be present and can occur only once. Most of the other elements can occur many times or not at all. However, at least some of them must be present before the manifest file becomes useful.
If an element contains anything at all, it contains other elements. All of the values are set through attributes, not as character data within an element.
Elements at the same level are generally not ordered. For example, the <activity>, <provider>, and <service> elements can be intermixed in any sequence. There are two key exceptions to this rule:
An <activity-alias> element must follow the <activity> for which it is an alias.
The <application> element must be the last element inside the element. In other words, the </application> closing tag must appear immediately before the </manifest> closing tag.
I have seen a JS talk about Redux recently. The speaker programmed a working application without any classes, just with const variables. He even excluded classes in his lint file (https://github.com/MartinSeeler/redux-presentation/tree/master/demo).
So I tried to do the same in my Android apps, which are written in Kotlin. Unfortunately, I get the following error when I try to make my MainActivity class an object:
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{blog.app.niklas.flyingbytes/blog.app.niklas.flyingbytes.MainActivity}: java.lang.IllegalAccessException: void blog.app.niklas.flyingbytes.MainActivity.<init>() is not accessible from java.lang.Class<android.app.Instrumentation>
Caused by: java.lang.IllegalAccessException: void blog.app.niklas.flyingbytes.MainActivity.<init>() is not accessible from java.lang.Class<android.app.Instrumentation>
Is there a way to work around this? I really like the idea of having as many singleton objects as possible in my app.
Thank you in advance,
Niklas
You can't replace Activity class by kotlin object on Android, for the same reason you don't instantiate Activities by calling it's constructor:
activities are managed by the platform.
When you ask (via an Intent) the platform for an Activity, the platform will try to instantiate it by calling it's constructor with zero parameter. Which doesn't exist for an object, giving the mentioned error.
That's said, some developer dodge around the frame given by Android, like mortar, by having only one activity and using custom logic for navigation between views.
As Geoffrey said, the Android platform manages your activities, but that also means that the platform is responsible for keeping track of your Activity's instances - which is the purpose of Singleton.
So, while you can't make your Activity an object, you can still make it a Singleton by using the android:launchMode attribute in your AndroidManifest:
<activity
android:name=".MainActivity"
android:label="#string/app_name"
android:launchMode="singleInstance">
For more information on the launchMode attribute (including other possible values), check out the Official Documentation
May be its simple question, may be its repeated question, this question s not for upvote and all.
I just want to pass my object from one activity to second and second to third activity. I know there are a lot of ways using shared preferences, Intent bundle from one to another activity.
The reason I want to know that why I can't use an object globally for all my activities and if I can how it is possible?
Thanks
There is a handful of ways in which you can achieve this.
Using a bus/listener:
https://github.com/greenrobot/EventBus
https://square.github.io/otto/
Using the application class (ready the first answer carefully, there are caveats):
How to declare global variables in Android?
And using utility classes with static methods.
Also, if you are have more than one fragment running at the same time, you can create your own interfaces and implement them where you need to receive the information.
I have an app with multiple activities and multiple layouts. However, one piece of layout is included on several activities. I also have a thread which updates this layout. However, when i switch activity it doesn't work. Since the layout is included the elements have the same ID's, shouldn't it just work? Or do I really need to fetch an object for each element in the layout and feed it into my thread in order to make it update the elements in a new activity?
You should run the update code for each Activity/View, although the XML included is the same, each is a different instance.
My suggestion is on Restart verify is there is any modification to do in each activity, a simple way is to each Activity extend a BaseActivity that has this code.
I include a layout for adverts in my app, but on each activity that uses it, the adverts need to be reloaded.
If I call an activity from one that is using the same included layout when I go back to the previous activity it's still there.
I guess this is what you are seeing....
So you can also save that data inside sharedPreferences (if it is little data and primitive objets or parceable objects).
Also you can extend the Application class and store the data there and update every activity inside the onResume() method. that i believe is the best way to handle this. and this is quite simple to do.
Ask google about extending the application class and he will provide tons of results on how to do it. its an easy way to pass data between activities and/or keep a reference to a single object which you will use throughout the app. Just be carefull to clear it when you wont need it anymore because it will stay in existance untill the application is finished() (which comes with the application extension living thru the whole application lifetime).
I'm new to this and I'm sorry if this is a really dumb question. I'm just trying to clarify things. My book says I can retrieve application context for process by using the getApplicationContext() method. I just really don't know where to type this or what to do with any of it. I can go to the hierarchy but what do I do with all the script there. Also where would I write Activity Callbacks, in the main.xml? An exercise wants me to add a logging tag to my project but I'm not sure how to do this. The exact text says:
"Within the onCreate() callback method, add an informational logging message, using the Log.i() method."
and another exercise says to:
"Implement some of the Activity callback methods in addition to onCreate(), such as onStart(). Add a log message to each callback method and then run the application normally".
As these seem like basic questions, can someone please help me.
I am using the Android SDK, and Eclipse. I have made the Hello World application, but I have no idea what to do with Context or Retrieving resources. Please help!
The first rule I would give you: if you don't know why you need it, you probably don't need it. Use your activity object as the Context when you need a context.
The callbacks you talk about are on the Activity class. The Application Fundamentals describes what an Activity is: http://developer.android.com/guide/topics/fundamentals.html#Components
The only time you want to use getApplicationContext() is when you need a Context that exists outside of the lifecycle of an Activity class (or other component). You'll want to find documentation on specific cases where this is desired, there is a lot floating around. For example this one is part of the Android documentation: http://android-developers.blogspot.de/2009/01/avoiding-memory-leaks.html
For the tasks you're working with here, you'll be using the Java code that defines the behavior of the application, not the XML files that define resources and layouts or the AndroidManifest.xml file that declares basic application properties.
If you're working with Hour 3 of the Sam's Teach Yourself... book, then you need to open the src\com.androidbook.droid1\DroidActivity.java file. In general, you would need src\<package-name>\<class-name>.java. When you open that file, you'll see a class (in this case, DroidActivity) that extends Activity and already has the onCreate() callback method. Anything that you want to happen during onCreate() goes inside that method. Other callback methods can be added inside the activity class. To see an example that has all the lifecycle callbacks (but doesn't do anything in them), look here.
A logging tag is just a string. You can declare it, for example, as a private static final String inside the activity class.
If there's confusion about where methods belong, where and how to define variables or constants, how to call methods, how to use classes, and so forth, then it might be best to go through an introductory Java text before starting with Android. There are plenty of free resources available for that.