What is the significance of:
super.onCreate(null);
instead of
super.onCreate(savedInstanceState);
With this change, I am able to avoid many problems that otherwise plague my Activitys each time a configuration change occurs (rotation, locale shift, permission toggle). It seems that with this change, the Activity is started afresh whenever a configuration change triggers it to restart. And I don't seem to lose any data or process state by doing this: all my Activitys are restored exactly to their former state.
My question is, can I do this with impunity henceforth, or am losing something in the bargain? I don't really understand why this works, whether it is safe or not, and what unintended effects it may have on my app.
I chanced upon this trick here.
Related Questions:
Calling super.onCreate() with null parameter?
Will 'Bundle savedInstanceState' be alive after Application is being killed?
Activity state instance - insights?
Activity's instance state: what is automatically stored and restored
onCreate() call first when activity is about to create, Also Android System manage activity lifecycle and can kill the activity with saving its instanceState, in case if acitvity out of focus for user for long time and system is on low memory situation.
An activity has essentially four states
super.onCreate(null) : Would always create activity as it is creating fisrt time , even Android system provide its savedInstanceState, and does't matter what orientation configurations are.
super.onCreate(savedInstanceState) : Activity can use 'savedInstanceState' to reset its state or component where it was last.
To achive this, acitivty's instance state need to be persist before activity lost user attention( that could be onStop or onDestroy)
savedInstaceState can also be important to handle if activity configuration got changed, Please check acitvity life cycle behavior on Configuration change
am losing something in the bargain?
Only if you are working with Fragments. see Calling super.onCreate() with null parameter?
Yes, onCreate(...) is necessary to start an Activity, but passing Bundle as an argument is required when you are working with fragments.
What did you infer from that?
The argument savedInstanceState is anyway null by default. So you aren't really losing anything in a bargain.
But wait, we usually use Bundles to maintain orientation change, right?
the following manifest code declares an activity that handles both the screen orientation change and keyboard availability change:
<activity android:name=".MyActivity"
android:configChanges="orientation|keyboardHidden"
android:label="#string/app_name">
Now, when one of these configurations change, MyActivity does not restart. Instead, the MyActivity receives a call to onConfigurationChanged(). This method is passed a Configuration object that specifies the new device configuration. By reading fields in the Configuration, you can determine the new configuration and make appropriate changes by updating the resources used in your interface. At the time this method is called, your activity's Resources object is updated to return resources based on the new configuration, so you can easily reset elements of your UI without the system restarting your activity.
the following onConfigurationChanged() implementation checks the current device orientation:
#Override
public void onConfigurationChanged(Configuration newConfig) {
super.onConfigurationChanged(newConfig);
// Checks the orientation of the screen
if (newConfig.orientation == Configuration.ORIENTATION_LANDSCAPE) {
Toast.makeText(this, "landscape", Toast.LENGTH_SHORT).show();
} else if (newConfig.orientation == Configuration.ORIENTATION_PORTRAIT){
Toast.makeText(this, "portrait", Toast.LENGTH_SHORT).show();
}
}
But Remember: When you declare your activity to handle a configuration change, you are responsible for resetting any elements for which you provide alternatives. If you declare your activity to handle the orientation change and have images that should change between landscape and portrait, you must re-assign each resource to each element during onConfigurationChanged().
As far as I know a lot of data is saved in the bundle savedInstanceState.
E.g. all the views' states in your current layout, such as the current content of any EditText or CheckBox.
You could also look up some official sources to check whether you need to keep some data.
Here's a nice article about it
Basically it says that all View class implements the methods onRestoreInstanceState and onSaveInstanceState, which are saving and restoring any temporary states they were in before the state change.
The savedInstanceState is a reference to a Bundle object that is passed into the onCreate method of every Android Activity. Activities have the ability, under special circumstances, to restore themselves to a previous state using the data stored in this bundle.
It is very important to use savedInstantState to get values from Intent which is saved in the bundle.
All your data stored in class variables or local variables is lost whenever you change rotation of device, but in your activity it looks like you have not stored any data as long as user enters any data, but instead you are perhaps reading data on click of a button or something like that, your activity will behave and work normally, and all user inputs like text inside EditText will be restored by Android itself, because it identifies "IDs" (android:id="#+id/anyID") allotted to each view and can restore by itself all the values inserted by user.
I hope this this helps you...
Happy coding :)
Related
I have an activity class that locks itself with a custom PIN whenever it is resumed/recreated (so that I can lend my phone to someone secure in the knowledge they can't see that app's data).
The problem this causes is that rotating the device recreates the Activity and redisplays the PIN lock, which is a bit unsmooth.
So, is there some way the Activity to know either
I am being destroyed because of an orientation change.
I am being created as a result of an orientation change.
I would like to avoid solutions based on android:configChanges="orientation|screenSize" if possible.
EDIT: For posterity, I ended up doing this by having all Activities in the app inherit from this LockableActivity class.
A very simple solution is to check the time passed between 'onPause' and 'onResume'. If it is less than 0.2 seconds then you haven't handed your phone over ...
In fact, you could make this a user controllable security feature: how long away from the activity before pin entry is needed again could be set by the user.
For the truly obsessive, you could ask the user to reorient their phone during the set up phase to determine the associated time lapse and set that as the minimum.
As noted elsewhere, you could also use onSaveInstanceState. In this approach that is when you would store the time for comparison later.
Put the below code in your activity, it will get triggered when the device is rotated
#Override
public void onConfigurationChanged(Configuration newConfig) {
super.onConfigurationChanged(newConfig);
//perform your operations here
}
If you also want to check whether it is in landscape or portrait mode , you can apply below conditions
if(newConfig.orientation == Configuration.ORIENTATION_LANDSCAPE){
}
else {
}
If your activity is normally destroyed, onSaveInstanceState() will never be called. During orientation change (or any other change that needs to load different resources), onSaveInstanceState() will be called. In this method, you can save your data that you want your activity to have during it's re-creation.
If your activity is normally created, savedInstanceState will be null. But if it was created because of re-creation (like change in orientation), then savedInstanceState will not be null and you can use this to fetch the data that you saved during onSaveInstanceState().
It's a bad idea to override onConfigurationChanged(), instead you could just use these methods that Android provide to persist your data or application logic . In your case, it could be something like this in your onCreate().
if(savedInstanceState != null){
boolean loggedIn= savedInstanceState.getBoolean("LoggedIn",false);
if(!loggedIn){
// Not logged in, hence show pinlock
}
}
You could do something like this.
I am an Android Beginner. According to the lifecycle of an android application, when screen orientation is changed following methods are called:
onPause()
onStop()
onDestroy()
OnCreate()
onStart()
onResume()
But I have now added onConfigChanged in my XML file. When activity starts, onCreate, onStart and onResume are called. But when my screen orientation changes, only onConfigurationChanged method is called, the above mentioned methods are not called as shown below:
Any reason for this?
Thanks in advance.
Normally when an orientation , language , ... change happen the activity get destroyed and restarted and start the lifecycle from the begining unless you override the onConfigurationChanged function.
More details from Android documentation:
Configuration Changes
If the configuration of the device (as defined by the
Resources.Configuration class) changes, then anything displaying a
user interface will need to update to match that configuration.
Because Activity is the primary mechanism for interacting with the
user, it includes special support for handling configuration changes.
Unless you specify otherwise, a configuration change (such as a change
in screen orientation, language, input devices, etc) will cause your
current activity to be destroyed, going through the normal activity
lifecycle process of onPause(), onStop(), and onDestroy() as
appropriate. If the activity had been in the foreground or visible to
the user, once onDestroy() is called in that instance then a new
instance of the activity will be created, with whatever
savedInstanceState the previous instance had generated from
onSaveInstanceState(Bundle).
This is done because any application resource, including layout files,
can change based on any configuration value. Thus the only safe way to
handle a configuration change is to re-retrieve all resources,
including layouts, drawables, and strings. Because activities must
already know how to save their state and re-create themselves from
that state, this is a convenient way to have an activity restart
itself with a new configuration.
In some special cases, you may want to bypass restarting of your
activity based on one or more types of configuration changes. This is
done with the android:configChanges attribute in its manifest. For any
types of configuration changes you say that you handle there, you will
receive a call to your current activity's
onConfigurationChanged(Configuration) method instead of being
restarted. If a configuration change involves any that you do not
handle, however, the activity will still be restarted and
onConfigurationChanged(Configuration) will not be called.
It's a very good thing that only onConfigurationChanged() is called with orientation changes. It's actually preferred because it means that your initializations in onCreate() DON'T have to happen again -- you can reuse them with the new views that you rebind in onConfigurationChanged(). This results in a faster application that uses less memory and is less clunky.
Case in point:
You have a ListView. That ListView uses an ArrayAdapter. In onCreate(), you find the listView using findViewById(R.id.xxx) and you CREATE a NEW ArrayAdapter and set it for that listView. If every time onCreate() is called when the orientation changes, you would be creatting a new ArrayAdapter each and every time the orientation changes. Imagine switching the orientation 3 times -- what a waste!
Like this, you can create the ArrayAdapter in onCreate(), once, ever (when your application is created). And when the orientation changes, in onConfigurationChanged(), simply reset the ArrayAdapter for the new ListView you pick up again using findViewById(R.id.xxxx). Done. Only 1 ArrayAdapter has ever been created and it's data remains in tact for the new orientation.
Beautiful! :)
In the onCreate() of my MainActivity, my app does an intensive operation to generate some data set (runs in a separate thread, but takes some 2 - 3 seconds to complete normally). Now my problem is that, when orientation changes, the app again does this complex computation again.
Since I haven't done anything like this before, I was wondering if there is a way around this. My first thought was to store the computed data in a static variable, so that the data is persisted between different instances of MainActivity. I am guessing this is not the best approach.
My data set consists of a Map and an ArrayList and not a simple data type, if it helps.
I have looked at the onSaveInstanceState(), but it only provides to store values like int, String, etc.
In addition to what #Raghunandan suggested you can read the official docs on Recreating an Activity .
It says:
Your activity will be destroyed and recreated each time the user rotates the screen. When the screen changes orientation, the system destroys and recreates the foreground activity because the screen configuration has changed and your activity might need to load alternative resources (such as the layout.
It also introduces the concept of
onSaveInstanceState()(To save additional data about the activity state, you must override this method) and
onRestoreInstanceState() method (The default implementation of this method performs a restore of any view state that had previously been frozen by onSaveInstanceState(Bundle)).
You can also see the sample implementation of these methods here Saving Android Activity state using Save Instance State thread, which will help you to save and restore the state.
Update
You may also try using:
<activity name= ".YourActivity" android:configChanges="orientation|screenSize"/>
The docs for android:configChanges say;
It lists configuration changes that the activity will handle itself. When a configuration change occurs at runtime, the activity is shut down and restarted by default, but declaring a configuration with this attribute will prevent the activity from being restarted. Source: How to save state during orientation change in Android if the state is made of my classes?
Hope this helps.
I have a small activity with an EditText and an imageview and a button. When you press the button it launches camera for result, and when it returns it changes the imageview to the picture you've just taken.
But when the orientation changes, the imageview resets back to the default one on the layout.
What I tried doing was setting a boolean called custom, and when you take a picture it sets it to true. I overrid onConfigurationChanged() and if custom is set to true I restore the image.
My problem now is the EditText becomes erased -- How can I restore the EditText after configuration change? My first attempt was storing it's content into a String onPause() and then restoring it, but it always comes up blank.
Usually when UI view does not keep its state, first thing to check is that this UI view has id assigned. Without this id views cannot restore their state.
<EditText android:id="#+id/text" ... />
If this doesn't help, you need to save and restore state yourself. Take a look at Handling Runtime Changes. It pretty much explains what you should do:
To properly handle a restart, it is important that your Activity restores its previous state through the normal Activity lifecycle, in which Android calls onSaveInstanceState() before it destroys your Activity so that you can save data about the application state. You can then restore the state during onCreate() or onRestoreInstanceState(). To test that your application restarts itself with the application state intact, you should invoke configuration changes (such as changing the screen orientation) while performing various tasks in your application.
You should override onSaveInstanceState() and save your Acitivity state when its called:
#Override
protected void onSaveInstanceState(Bundle outState)
{
super.onSaveInstanceState(outState);
outState.putString("textKey", mEditText.getText().toString());
}
And then restore state in onCreate() or onRestoreInstanceState():
public void onCreate(Bundle savedInstanceState)
{
if(savedInstanceState != null)
{
mEditText.setText(savedInstanceState.getString("textKey"));
}
}
If this is still not sufficient, you can override onRetainNonConfigurationInstance() and return any custom Object that will be passed to new activity object, when its recreated. More details about how to use it can be found in Handling Runtime Changes. But this function is deprecated in Android 3.0+ (specifically for FragmentActivity where its final). So this cannot be used together with Fragments (which is fine, they have their mechanism to retain objects accross configuration changes).
And final one - never use android:configChanges. You must have very good reasons to use it, and usually these are performance reasons. It wasn't meant to be abused the way it is now: just to prevent UI state reset. If this attribute is used, then yes, Activity UI will not be re-set on config change, but Activity state still will be reset when destroyed and re-created later.
The documentation explains this option pretty well:
Note: Handling the configuration change yourself can make it much more
difficult to use alternative resources, because the system does not
automatically apply them for you. This technique should be considered
a last resort and is not recommended for most applications
You can force your activity not to reload after orientation changed by adding android:configChanges="orientation" to your activity line in manifest.
I have a dialog popup to indicate how long I want an activity to run.
Seems simple enough, but its difficult to determine when the activity is really starting.
If the user opens the keyboard on a G1 and thereby goes into landscape, that reruns the activities onCreate method.
I can't have a static for the class or even create an application class, because I can't determine when the 'application' goes away. Even after destroying the only activity, the application is apparently still alive.
Is there a way to determine that onCreate has been called because of the keyboard being extended or landscape mode invoked?
Thanks
In onSaveInstanceState you could store a flag indicating if it had run. If the app was being restored then in onCreate(Bundle savedInstanceState), the savedInstanceState will have the variable so you could check if savedInstanceState != null and saveInstanceState.get("restoring") != null then don't show the dialog.
I tried creating an application subclass, but still I could not determine when it would go away.
I tried another approach.
I added in the manifest within the activity,
android:configChanges="orientation|keyboardHidden"
Then within the activity I added,
public void onConfigurationChanged(Configuration newConfig) {
super.onConfigurationChanged(newConfig);
}
This results in my onCreate method NOT being called when the orientation changes or the keyboard is hidden. I'm not sure why my view still looks correct in either case, but it works great. In fact it even handle text typed into the displayed dialog. The text is maintained when the orientation is changed.
Is there a way to determine that
onCreate has been called because of
the keyboard being extended or
landscape mode invoked?
There is a way to:
1. Check if the keyboard is being extended
2. a way to check the current orientation (landscape or portrait)
Determining whether onCreate() has been called or not will require some work on your part. For instance, you can have a variable to flag completion of onCreate() and save it in Activity's state Bundle.
You can put a check in onCreate() to determine if this is a first run or a restart due to Configuration changes, by making some sense out of values from 1,2 and the flag.
This is just a suggestion; better solutions might exist