Difference between back press and home button in android - android

I am starting a service in my app. On click of service I am launching an activity. The service click event works fine when we go to any app and press back button and exit the app.
But if we are in any app and then press home button and click on the service the activity is not launched. If I click it more then 2 times,it opens the activity and sometimes it opens the app also.
So i am unable to understand the difference between pressing back button and home button.

After you start an activity, if HOME key is pressed, then the current activity is stopped and its task goes into the background. The system retains the state of the activity - i.e. onSaveInstanceState will be called. If the user later resumes the task by selecting the launcher icon that began the task again, the task comes to the foreground and resumes the activity at the top of the stack.
However, if BACK key is pressed, the current activity is popped from the stack and destroyed. The assmuption is the activity is done and will not be used again. So the system does not retain the activity's state - i.e. onSaveInstanceState will not be called.
Home Task :
Pressing the Home switches you from the app to the home screen, whilst leaving your app running in the background. This is a bit like switching between windows on a Windows PC.
Except that when your phone is running low on resources like memory it will start to close apps that are running in the background, so that your phone has enough resources for what you're trying to do now. Games are often amongst the first apps the phone will "kill" to save resources as they often use a lot more memory and CPU than other apps. This is why sometimes your game is still running paused, and sometimes Android has closed it for you.
The Back button is the way to close apps so that they are actually closed.

onPause() is called in Activity A when it launches Activity B. After the back button is called in Activity B, onResume() is called in Activity A.

In case of activities their default implementation is LIFO based in stack and works like:
On Back button Pressed: finish the current activity by calling stop method.
On Home button pressed: activity is being paused and then it may either resume if come back to it, otherwise system will call stop() method of activity to save unused resources and utilize memory.
but these functions can be edited by overriding if required.

Related

Intent launches Activity, but Activity disappears when pressing the home button

I have made an app.
When I launch that app in the normal way, then it works fine. When I press the home button, the app is paused and is still there as it should be.
Now I want to start my app by an intent...so e.g. when a NFC tag is detected the app should launch. This also works, but when I am pressing the home button then the app disappears, but I would like that it stays open (in pause mode) like when I launch it in the "normal wayy". Does anyone know why this can happen? My observation is that the onStop() function is called but not the onDestroy() function. Therefore it is very strange that the app just "disappears".
Thanks a lot in advance.
Please see Android Activity Lifecycle. onDestroy() is not always called when application goes to background and generally here are no guarantee that onDestory() will be called before app/activity is killed.
Could you please explain what you mean 'pause mode'? You mean that app saves the state?

home button pressed

What exactly happens when I press home button in Android?
Because when I open it again after home button pressed, it has series of bugs.
I need to know it to track down the point that causes that bugs.
UPDATE:
when pressing home button, application goes to background and onPause() is called and saves the state of UI, however it does not save state of application, like variables, custom views. And you have to save them manually, as Oren explained.
When you press Home Button your Application/Activity goes in background and when you open it again it resume from the same position as it was until it was being killed/closed by the OS.
Activity Life Cycle will give you a clear idea about it.
Technichally? Anything can happen, from just onPause being called, to the device killing the app to free up memory, to the user shutting down and restarting the device. Your app should handle all of these possibilities.
Further reading : android activity lifecycle

Android - keeping the activity alive

I have a puzzling problem. If I deploy my application from Eclipse on any device/emulator, I can use the Back and Home buttons to minimize it and then I can click the launcher icon to resume it.
If I take the apk, put it on the phone and install it (using the Astro file manager), then press Back or Home, when I click the launcher icon again, my application will be recreated instead of resumed: the activity is not destroyed.
This goes against everything written about on the android developer website regarding activities and their lifecycle.
For instance, this is what I do when the user uses the Back button:
#Override
public void onBackPressed() {
moveTaskToBack(true);
}
Yet my application is recreated when I click its launcher icon. Any ideas?
This goes exactly with everything written about the activity life cycle.
If an activity is paused or stopped, the system can drop the activity
from memory by either asking it to finish, or simply killing its
process. When it is displayed again to the user, it must be completely
restarted and restored to its previous state.
http://developer.android.com/reference/android/app/Activity.html
You application is not "minimised". There is not such thing as minimised in Android. It's not Linux or Windows or OSX.
It's true, pressing Back will kill your Activity. However, why is that a problem? Android provides a mechanism to save your Activity's state and be able to resume.
View the Activity lifecycle: http://developer.android.com/reference/android/app/Activity.html#ActivityLifecycle
This is a must read on saving Activity state: http://developer.android.com/reference/android/app/Activity.html#SavingPersistentState

what exactly android's home button doing?

What I do know is that the current activity get into the onPause() mode, and the home screen activity brought to front.
My confusion starts with situation you can re-open the application from the recent tasks menu.
so what exactly happening when I'm opening the application from the recent tasks manager?
Is the activity that was foreground when the home button pressed is still somewhere in the stack?
Is there more then one activity stack on the same time?
As far as I understand it, there is no real stack (of applications). Is just that your activity has states, so whenever you are pressing the HOME button your activity in your current application just "pauses" like if it was a stand-by state so multiple applications can be in this state as long as the system doesnt require memory and kill the tasks.
So whenever you open the activity from the recent tasks its just telling the application to wake up.
First of all, here is everything you need to know about the concept of the "Up Button":
Navigation with Back and Up and some of the implementation details: Providing Ancestral and Temporal Navigation.
Generally speaking, the Up button lets you navigate up in the application hierarchy, instead of just navigating back in the application(s) back-stack.
For example, if you work with some kind of app and you get the email notification, you can open the mail client by pressing the notification. Then you can go back to you application you were working with by pressing the Back button ( back-stack ) or you can press the Up button in order to go to the mail client's 'parent' activity ( for example from some EmailMessageActivity to EmailHomeActivity ) to work with the mail client application instead of the initial application ( the back-stack usually is cleared then, so you can only go back with the Back button as far as the the Android Home screen ).
The "Recent Tasks" factor is irrelevant and misleading, it's just another way of starting a new Activity.

What's the difference between opening an app from the applications screen and the recently used apps list? (android

Can anyone tell me what the difference is between opening an app from the applications screen and opening it from that recently used apps list that pops up when you long-press the home button?
I didn't even know that recently used list existed until a friend managed to break my app by launching it from there. He tried twice and got the same force quit, but when he launched it from the applications screen it opened fine.
The error log told me that a nullPointerException occurred in the getCount method on my ArrayAdaptor for my ListView.
Anyway I just wondered if there was a difference that I need to know about and adapt my code to deal with?
AFAIK, If your application is completely shutted down, launch from applications screen and recently used apps list should have no difference, both refresh start your application and open your application's MainActivity (by stack-push your application's MainActivity into a newly created task)
However, as Android is multi-task OS, your application can be put into background in standby mode i.e. open your application then short-press home button, this is not same as press back button. If you haven't override these key pressed in your application, press back button several times with pop all your activities off from activity stack and finally kill your application, whereas press home button will bring System's HomeActivity into foreground hence flip your application (AKA. task with activity stack) into background.
Things becomes more interesting here, depend on which value your configure your activity's android:launchMode in AndroidManifest.xml, if you use standard or singleTop:
1. launch app from recently used apps list always bring your standby activity back to foreground, i.e. re-order activity stack.
2. launch app from applications screen will create a new instance of your MainActivity and open it, i.e. push a newly created MainActivity into activity stack, so now you have two instances in your application's activity stack
If you use singleTask or singleInstance:
2. launch app from applications screen will use the standby MainActivity (if exist) in your application's activity stack and re-open it, i.e. re-order activity stack.
Checkout Tasks and Back Stack to see how different configurations may affect your application's activity stack behaviour.
I believe there should be no difference. These are the lifecycle methods I typically see when pressing the home button from an activity, on android 2.3.4
onPause
onStop
then when I use either the icon or previous applications to navigate back, I see
onRestart
onStart
onResume
Now, in some cases the system will tell your activity to finish while you are away (or immediately when you return if an orientation change occurred). Then you will see onDestroy, and the following when you navigate back
onCreate
onStart
onResume
I don't think there is anything mysterious going on here. According to the Activity documentation, there are only four states that a process can be in, and both of these fall under background activity.
There shouldn't be any difference in how the activity is launched from history, apart from the fact that the launching Intent will have the FLAG_ACTIVITY_LAUNCHED_FROM_HISTORY set.
Here's an easy way to think about it. All of your activities are launched form Intents. Holding down the home button allows you to open that activity using the last intent that launched it. This can give you some unexpected results however. For instance, if you are able to launch your activity from something special like a widget.

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