So I am developing this small game, it currently uses 2 activities:
The Menu Activity and the actual Game Activity;
The Game Activity is started by the Menu Activity (no problems at that)
Then I can switch from the Game Activity to the Menu Activity by using the back Key:
public void onBackPressed() {
super.onBackPressed();
}
Now I am back in the Menu Activity. The Game Activity I've created has been stopped(I guess);
How do I go back to the Game Activity, as I left it(how do I restart it)?
You need to manually save the state of your game using any of the methods described in Data Storage.
However if you start a new copy of the Menu Activity:
Menu Activity ->
Game Activity ->
Menu Activity
You can return to the Game Activity rather than completely rebuilding it. But of course, there is no guarantee that game Activity won't be destroyed by the garbage collector, so you still need to save the Game's state in onPause().
It is absolutely pointless to override a method just to call super. Please re-check what overriding means : if you don't override, then your subclass method A will be the same as super class method A. Thus, you can override to create a different version of A in your subclass, but not to call the super class method A, it is useless.
About the Activity, in android, you can't hope than activity that is not on the stack will be preserved. You can add this to your manifest :
<activity name=".GameActivity"
...
android:launchMode="singleTop" />
but even though, it will not guarantee that android will just clean up your activity and recreate it when it is relaunched. That's the way android preserves resources.
But a better way would be to call (startActivity) MenuActivity again from inside GameActivity. That way, your old GameActivity will still be in the activity stack. But really, there will always be some limit cases where android will recreate GameActivity. For instance, if system is under memory pressure.
The best is to persist the state of GameActivity so that even a new instance will be re-put in the same state as the old instance was.
Assuming you are using the menu as a sort of "pause" menu where the back button pauses the game, you could have the back button create a new intent for the menu activity and then let the back button from the menu activity function as normal.
Of course, you should still preserve game data when you launch the menu activity - since the user could close the screen from the menu - but the system will inherently keep your game activity (and data) in memory when you launch the menu.
#Override
public void onBackPressed() {
Intent intent = new Intent(GameActivity.this, MenuActivity.class);
startActivity(intent);
}
Related
I launch main activity, change some settings in UI, press back button and then reopen the activity. onCreate() is called again and activity is back to default state. Why is this?
I would expect only onResume() to be called since I have this in Manifest:
android:launchMode="singleInstance"
Try using moveTaskToBack(true); in onBackPressed() to make your current activity hide instead of destroying, so you can reopen it again
#Override
public void onBackPresses(){
moveTaskToBack(true);
}
I launch main activity, change some settings in UI, press back button and then reopen the activity. onCreate() is called again and activity is back to default state. Why is this?
This is expected behavior of activity life cycle. When you press back button then activity get destroyed, it will start from onCreate method.
This is because when you press back, it destroyes the activity.
I think that what you want is to keep the state when you come back to this activity, not really to have one instance of it. So you should save the information you need and then restore them in onCreate.
Single instance only means (if I remember it well) that if you launch multiple activities without finishing them, the ones that have singleInstance will go back to front when you will call startActivity() and will not call onCreate. But this means you can't have this behavior by pressing back.
Maybe you can override onBackPressed and start another activity (the one you should go back to) instead of calling super.onBackPressed(). this should do what you want but if think it will be kind of difficult to manage.
By the way remember that Activities might be killed by the system itself, so you should not rely on the fact that your activity as not been killed
I have a simple launcher app, that doesn't launch any apps, just shows a WebView. It is set as the default launcher. I don't use any other apps on this tablet. When I restart the tablet, this launcher starts as expected. My problem is if I press the back button of the device, the activity is destroyed, and it is created again as a new activity. I don't get what's the point of this behaviour, as the activity is already visible... The view's launch mode is singleTask, so I expect it to just simply stay where it is, rather than being recreated. How can this be done? As far as I understand the documentation, this should work as I expect it, instead of the way it does now.
The system creates the activity at the root of a new task and routes
the intent to it. However, if an instance of the activity already
exists, the system routes the intent to existing instance through a
call to its onNewIntent() method, rather than creating a new one.
http://developer.android.com/guide/topics/manifest/activity-element.html#lmode
You can override onBackPressed() and do nothing like
#Override
public void onBackPressed() { }
I have two activities; let's say A and B. In activity A there is a broadcast receiver registered that listens for a particular event which will finish activity A. I am registering the broadcast receiver in onCreate(), and destroying it in onDestroy() of activity A.
For simplicity, there is one button in activity B named "Destroy Activity A". When a user clicks on button, activity A should be destroyed.
Normally all of this is running smoothly without any issues,but the problem occurs in following scenarios:
1) Suppose I am in activity B and i press the Home key to move the application to the background then if i use other resource-heavy applications, Android system will kill my application to free memory. Then If I open my application from recent tasks, activity B will be resumed, and it's onCreate(), onResume() etc method will be called. Now I press button to destroy activity A, but activity A has already been destroyed, so activity A's onCreate(), onResume() etc methods will not be called until and unless i go to activity A by pressing the back button. Thus broadcast receiver is not registered to listen for the event.
2) The same problem will arise when user has selected "Don't keep activities" from Developer options in the device's settings.
I have been looking to solve this issue for a long time, but i am unable to find a proper answer. What is the best way to handle this scenario? Is this an Android bug? There should be some solution for this issue.
Please help me.
If your Activity A has destroyed by Android OS itself then there are
no way to track.
Some people has suggested to track that Activity A by listning event in onDestroy method BUT if your Activity killed by system OS then note here it wont call those method .
This cannot be fixed while keeping your current broadcast logic.
Killing activities from the backstack, imo, is not a correct approach. You should strongly consider changing the logic of your navigation.
But if your project is big and time is a problem, and refactoring is out of the question, A.J. 's approach works, but you mentioned that you have lots of activities that needs to be killed, his solution becomes very tricky to implement.
What I suggest is the following. This might not be the best idea, but I cannot think of another. So maybe that could help.
You should have the following:
A Base Activity for all your activities.
A ArrayList<String> activitiesToKill object at the application level. (If you did not extend Application you can have it as static variable
First we have to make sure that the activitiesToKill is not lost when the OS kills the app in low memory. In the BaseActivity we save the list during onSaveInstanceState and restore it in the onRestoreInstanceState
#Override
protected void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
outState.putSerializable("activitiesToKill", activitiesToKill);
}
private void onRestoreInstanceState(Bundle state) {
if (state != null) {
activitiesToKill = (ArrayList<String>) state.getSerializable("activitiesToKill");
super.onRestoreInstanceState(state);
}
}
The idea here is to save which activities should be killed in the list, by using their name.
The logic is as follow:
Let's say you have Activities A, B, C, D and E
From Activity E, you press the button and you want to kill B and D
When you press the Button in E, you add the names of B and D to the activitiesToKill object.
activitiesToKill.add(B.class.getSimpleName()
activitiesToKill.add(D.class.getSimpleName()
In the onCreate method of the BaseActivity, we have to check if the
if(savedInstanceState != null)
{
//The activity is being restored. We check if the it is in the lest to Kill and we finish it
if(activitiesToKill.contains(this.getClass().getSimpleName()))
{
activitiesToKill.remove(this.getClass().getSimpleName())
finish();
}
}
Make sure to remove the name of the activity if it is killed through the broadcast.
So basically this is what happens in every scenario.
If the app is running normally, and you click the button, the broadcast gets sent and B and D will get killed. Make sure to remove B and D from the activitiesToKill
If the app was killed and restored, you press the button, the broadcast will have no effect, but you have added B and D to the activitiesToKill object. So when you click back, the activity is created and the savedInstanceState is not null, the activity is finished.
This approach consider that activity E knows which activities it has to kill.
In case you DON'T know which activities to kill from E, you have to modify this logic slightly:
Instead of using an ArrayList use a HashMap<String, bool>
When Activity B is created, it will register it self to the hashmap:
activitiesToKill.put(this.class.getSimpleName(), false)
Then from Activity E, all you have to do is set all the entries to true
Then in the on create of the base activity you have to check if this activity is registered in the activitiesToKill (the hashmap contains the key) AND the boolean is true you kill it (don't forget to return it to false, or remove the key)
This ensure that each activity register itself to the HashMap and Activity E doesn't have top know all the activities to kill. And don't forget to remove them in case the broadcast kills them.
This approach also ensure that the activity is not killed when opened normally from an intent because in that case onSaveInstanceState would be null in the onCreate, so nothing will happen.
More advanced checks can be accomplished in case you have groups of activities that needs to be terminated through different conditions (not only a button click) so you can have a HashMap of a HashMap to divide them in categories.
Also note, that you can use getName instead of getSimpleName if you have multiple activities with same name but different bundles.
I hope my explanation is clear enough as I wrote it from my head, let me know if any area is not clear.
Best of luck
One of the main rules with Activities is you can't rely on any activity being alive except the foreground activity. The thing you're trying to do with broadcasts has nothing to do with back stack -- back stack doesn't guarantee all activities are alive at all times, but it will make sure they're recreated when it's time to go foreground.
In your example (if my understanding of what you're aiming to do) you need to navigate to something underneath A -- say, Activity Z, and the stack looks like this: Z-A-[B]. There's normal course of events where you hit back and it takes you to A, then after another hit -- to Z but in a certain case (say pressing a button) you want to move back to Z bypassing A -- this is a classic case to use FLAG_ACTIVITY_CLEAR_TOP and launch Z explicitly:
Intent intent = new Intent(this, ActivityZ.class);
intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(intent);
This will finish both B and A, and deliver the intent to Z. You will, probably also need the FLAG_ACTIVITY_SINGLE_TOP flag, pay close attention to the description of FLAG_ACTIVITY_CLEAR_TOP, there's some trickery you should consider.
I don't know if it's possible to handle this on a "proper" way.
What it comes to my mind, is to flag the A activity in some way. You can't use startActivityForResult() because you will receive the result before onResume() is called i.e. UI was already inflated.
If you using an Otto, you could try with a sticky event. Otherwise you will need a singleton to handle the flag or save it to shared preferences.
You will have to check that flag on your onCreate() method before calling setContentView(), if the flag is true just finish the activity.
With the information you have given, how about you register the broadcast in onCreate of Activity B after checking if its registered already or not. If onDestroy of Activity A has been called in either of the scenarios you have mentioned, then the deregister of the Broadcast would have been called. So in that case, you can register your Broadcast in onCreate of Activity B, so that you can listen to it, even if you have only the Activity B in your backstack.
Have you considered using Sticky Broadcast?
Also you can register your receiver on application level (in manifest) and listen to this event regardless of Activity A state.
But, like already said Youssef, killing activities from the backstack is not a correct approach. You should strongly consider changing the logic of your navigation.
Many solutions came to my mind but as you have not provided much information about your app, so I think this should work in general.
Instead of firing a broadcast to kill Activity A, just execute the following code when the "Kill Activity A" button is pressed in Activity B.
Intent intent = new Intent(getApplicationContext(),
ActivityA.class);
intent.setFlags(Intent.FLAG_ACTIVITY_REORDER_TO_FRONT);
intent.putExtra("EXIT", true);
startActivity(intent);
Add the following code in activity A
#Override
protected void onNewIntent(Intent intent) {
super.onNewIntent(intent);
if (intent.getBooleanExtra("EXIT", false)) {
finish();
}
}
protected void onCreate(Bundle savedInstanceState) {
//Ideally, there should not be anything before this
super.onCreate(savedInstanceState);
if(getIntent().getBooleanExtra("EXIT", false)){
finish();
return;
}
In the manifest set "singleTop" launch mode for activity A.
<activity
android:name=".ActivityA"
...
android:launchMode="singleTop" />
This will have the following consequences:
If Activity A is already running it will brought to the front of the activity stack and finished, thus removing it from the stack.
If Activity A has been destroyed but still present in the activity stack (to be launched when back button is pressed), it will be started, brought to front and finished, thus removing it from the activity stack.
If Activity A has already has already been destroyed and not present in the activity stack, and you still press the "Remove Activity A" button, it will be started, brought to front and finished.
Generally, you should not see any flicker.
Based on this idea, you may build a better performing solution for your particular app. For example, you may use the FLAG_ACTIVITY_CLEAR_TOP and finish the Activity A in onBackPressed() of Activity B.
The books I have are abysmal at explaining how to work with the lifecycle, there's a lot I'm missing that I'm hoping somebody can fill in.
My app structure is that when it's first started, it starts an activity full of legalbabble that the user has to accept. When he says 'ok', I start my main activity and then I call finish like this:
public void onClick(View view) { //as a result of "I accept"
Intent mainIntent = new Intent(mParent, EtMain.class);
startActivity(mainIntent); // Start the main program
finish();
}
Then in EtMain in the onCreate method, I've got some tabs and I instantiate some classes:
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
mTabHost = (TabHost)findViewById(android.R.id.tabhost);
SetupTabs.setMyTabs(mTabHost, this);
mComData = new ComFields(this); // Create the objects
mDepWx = new WxFields(this, DepArr.Departure);
mArrWx = new WxFields(this, DepArr.Arrival);
mDepVs = new DepFields(this);
mArrVs = new ArrFields(this);
mTabHost.setOnTabChangedListener(new OnTabChangeListener(){
}
Questions:
The 'finish' in the first fragment should terminate the legalbabble activity so it'll never be restarted, right? And the EtMain one will remain forever (until killed externally), even if my app gets pushed to the background, right?
The way it is now, when EtMain gets pushed and later brought to the foreground (by tapping on the icon), it goes through the legalbabble screen as though it's a complete start - that's what I'd like to prevent - going thru the legalbabble screen again.
It would seem that I'd want to override onRestart in the second code fragment and put something in there to restart the app, right? That's the part I'm unclear about.
My question then is what needs to be done in onRestart. Do I have to recreate all the tabs and data in the tabs and all my object instantiations? Or is the memory state of the app saved someplace and then is restored back to the state that it was in before something else was brought to the foreground in which case not much needs to be done because all the objects and listeners will still be there?
Yes after the first activity has ended you shouldn't have to view that activity again. You could also write to the shared preferences that the user has previously seen legal info.
If you're UI object creation is in the onCreate method, this should only be called once. Pausing or resuming will not call the onCreate method again.
Unless you explicitly remove your objects and tabChangedListeners in the onPause method, you should not have to touch them in the onRestart method.
Correct, the state of the app is saved automatically. You shouldn't have to touch the onRestart method.
Hope this helps!
I think the problem is that the launch activity in your manifest is the legalbabble activity, so when you click on the icon, the system launches another one. A better architecture would be to launch the legalbabble activity it from your EtMain activity in the onCreate method of the latter, using startActivityForResult. From the docs:
As a special case, if you call startActivityForResult() with a requestCode >= 0 during the initial onCreate(Bundle savedInstanceState)/onResume() of your activity, then your window will not be displayed until a result is returned back from the started activity.
When you get the result in onActivityResult, you can call finish() if the legal stuff was declined; otherwise everything will proceed normally.
This avoids the problem that the launch activity defined in your manifest finishes when the legal stuff is accepted.
EtMain will not remain forever, if the user backs out (by pressing the BACK key) the Activity will be finished (onPause, then onStop, then onDestroy will be called).
In general you can ignore onRestore until you are doing something complicated.
Once the user has exited your application and re-enters (or presses the icon on the Homescreen), onCreate (followed by onStart and onResume) will be called for your first activity, so you do not need any logic in onRestart, your code in onCreate will do the setting up for you as it did the first time. Because of this your legal babble will appear again when the user starts the app after exiting unless you store a preference (in SharedPreferences or a database or file) to indicate you have already displayed it - in which case finish it straight away and start the main activity.
onRestart is only called when the application goes from the stopped state (onStop has been called but not onDestroy) to the started state (onStart is called but onResume has not yet).
For saving data - some components save their state automatically (e.g. EditTexts remember the text in them, TabHosts remember the currently selected tab etc). Some components will not. If you wish to save extra data then make use of onSaveInstanceState and onRestoreInstanceState. You should only use these methods to restore the state of your application or temporary data, not important things, e.g. the id of the resource what the user was looking at, what zoom level they were at etc. For things like contacts or actual data you should commit these changes to a database, SharedPreferences or other permanent storage (e.g. file) when onPause is called.
I recommend taking a look at the Android Activity lifecycle if you are confused. Or ask more questions!
Right now an activity gets destroyed when the BACK key is pressed. How can I make it just stop ( i.e. keep all the variables, etc. alive ), rather then be destroyed?
Thanks!
Why is it that you need to keep the variables alive? Given the established lifecycle of an Android application, I'm not sure that preventing the activity from being destroyed "just to keep the variables" makes sense.
Even if you stop the application without destroying it, there is always the chance that Android will kill it to free up memory. You will have to account for this in your code anyway, and so preventing the application from destroying doesn't save you from writing code.
Variables can be saved and restored relatively easily and quickly using SharedPreferences in your onPause() and onResume() methods. Unless you are storing a ton of data, preventing the application from destroying might not make much of a difference.
It sounds like you want to keep the variables in memory because you intend to return to this activity. Typically, you don't use the back button to navigate away from activities that you intend to come back to. Instead you would create an Intent and start a new activity. When you do this, Android places the current activity on the Back Stack calling onPause() and onStop(), which seems like exactly the sort of behavior you are looking for.
So if you still really want to prevent your activity from being destroyed (at least until Android decides it's using too much memory and kills it on it's own) you could always use Sagar's code and start a new activity in onBackPressed().
#Override
public void onBackPressed()
{
Intent intent = new Intent(this, Other.class);
startActivity(intent);
}
Just be certain that that is what you really want to do.
Simple one line
#Override
public void onBackPressed() {
mActivity.moveTaskToBack(true);
}
Pressing the BACK key triggers the onBackPressed callback method of Activity class. The default implementation of this callback calls the finish() method.
http://developer.android.com/reference/android/app/Activity.html#onBackPressed()
You can override this method to move the activity to background (mimick the action of pressing the HOME key.
eg:
#Override
public void onBackPressed() {
onKeyDown(KeyEvent.KEYCODE_HOME);
}
You could also instead consider moveTaskToBackground() mentioned here:
Override back button to act like home button
I have managed to work out exactly what you want: switch between 2 activities using Back button and keep them all not to be destroyed!
For example: you have 2 activities A & B. A will be started first, then A calls B. When B is loaded, user press Back button and switches back to activity A from B. From now B should not be destroyed and just goes to background, and when user starts activity B from A again, activity B will be brought to foreground, instead of being re-created again or created new instance! How to implement this:
1. Override onBackPressed() of activity B:
#Override
public void onBackPressed() {
Intent backIntent = new Intent(this, ActivityA.class);
backIntent.addFlags(Intent.FLAG_ACTIVITY_REORDER_TO_FRONT);
startActivity(backIntent);
}
2. On activity A, call activity B:
public void callActivityB() {
Intent toBintent = new Intent(this, ActivityB.class);
toBIntent.addFlags(Intent.FLAG_ACTIVITY_REORDER_TO_FRONT);
startActivity(toBintent);
}
remember to add flag: Intent.FLAG_ACTIVITY_REORDER_TO_FRONT when you call A&B.
This is similar to this question that was asked earlier.
Hope this helps!
N.S.
First of all, sorry for not answering the question, cause, I still have no optimal answer for it.
But, I really like when people start asking "what do you need this for". And, very rarely, the person who asked the question, really deserves this kind of question. I think not this time, but ok, this is not the issue...
Anyway, I will try to point out why some of us are convinced that
going from Activity A to Activity B(creating UI based on some data fetching) AND
going back from B to A(destroying all the created UI and/or fetched data in B) is sometimes a bad concept. Better solution would be to keep the stack as it is, so using something like finish() in Activity B, but keeping the Activity B in Pause state, so later when calling it again from Activity A - it just goes in onResume = nothing recreated in UI, no additional data fetching. The bonus of course is a fast and responsive UI, and the difference is really if you have a more complicated UI layout.
Just specify in the manifest for the activity as
android:persistent="true"
That should prevent your activity getting destroyed. To know more about this please refer to these below links which were answered by me
How to prevent call of onDestroy() after onPause()?
Prevent activity from being destroyed as long as possible
In the above posts I have explained in detail with a use case