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Android app not restart from mainactivity while after open background

Hello our application works perfect but when our users move the app to background, after a while coming back to app its been crashing.
For example i open the app then change three pages then move the app to background. after open from background 30 minutes its crash because its try to load activity when i resumed
Another applications do this from mainactivity, for example: instagram,twitter vs vs.
another applications not to try load resume activity, they are trying to main activity
how can i start my app from mainactivity when user come back to app from background ?

well, the better way would be to identify the null objects and reassign or repopulate them in the onResume() method so that the user can actually return to what they were doing.
if you can't or it's not an option, then try this code in every activity that you do not want to return to.
override fun onTrimMemory(level: Int) {
this.finishAffinity()
}
this code closes the active activity as soon as the app goes into background so when returned, the app is forced to start from the launcher activity.
Although I am not convinced that this is the best approach. there might be something better.

Connection between activity and layout? How to change layout? How to start and destroy an activity?

I'm a total beginner with Android and Eclipse and I have few questions that will help me understand the philosophy of Android:
The activity does all the work "behind the scenes". Activities are connected to a layout in the XML file. When the activity is started, if declared in setContentView method, the connected layout will be shown. Activity can be independent, without any layout, it can be invoked by another activity and will do all the work without showing any layout.
Activity is something like a php file which is invoked by my submit button in HTML, and the layout is .HTML which shows elements.
Am I right with this one?
For example, if I want to change the layout of my app, I want to show Layout2.xml when clicking button in Layout1.xml. Then I have to destroy the activity which is connected with Layout1.xml and start the activity which is connected with Layout2.xml? Is this correct? Is there any better way to do this?
How can I (by which method) destroy/stop a certain activity?
Thank you in an advance.

The best bet is to read the Android documentation regarding activites at http://developer.android.com/reference/android/app/Activity.html
I will answer your specific questions here though
An Activity is a window that the user can see (or a hidden window if there is no layout defined). It deals with the logic of a part of the app that the user can see and interact with. If we take the MVC model (Model View Controller), the Activity is the controller, in terms of it controls which data from the Model is shown on the View (the xml layout).
If you want to show a new window/screen/activity you do not need to destroy the current one. You can open a new activity whilst keeping the old one in the background (in the back stack). With the use of fragments, you can have multiple fragments in an activity so rather than changing activities, you can change fragments in a single activity. For more information about fragments take a look at http://developer.android.com/reference/android/app/Fragment.html.
This point relies heavily on the activity lifecycle. When an activity is destroyed, it means it is finishing and this can be done by the user pressing the back button whilst on the activity, the activity calling finish() on itself or by the Android operating system destroying the activity because memory is required elsewhere (this can happen when the app is in the background).
When we say an activity is stopped, it means that the activity is no longer visible to the user. This can be the case where the activity is in the back stack (another activity is in front of it) or if the app has been put into the background.
This is a brief answer to your questions but I highly recommend you read the Android documentation to gain better knowledge.

Android application with more than one screen

I'm currently trying to develop an android application in eclipse(java) which shows some jokes downloaded from a database. The user is able to vote on each joke once, and to make sure they only do that once, I have made a table in the database that contains three columns.
E-mail
Username (Used when a user publishes a joke)
Encrypted Password
I have two "screens" right now:
Login screen
Main screen
At every single start up the application checks the SharedPreferences for a file containing some information, and if there is some information it should load the Main screen, but if there is no account information the Login screen should be loaded.
Any idea on how I can use different screens, and how should it be coded?

Two options:
In your Activity, check if there is account info. If there isn't then setContentView to the Log in screen. Otherwise, setContentView to your other content. If you go this route, you'll have to have the logic of both the login Activity and the other in the same Activity. Shouldn't be too bad if the logic is relatively uncomplicated.
Have two activities. The default Activity can be the login Activity, but in onCreate() you can check if the info already exists and if it does, simply start the other Activity right away and return from onCreate(). Otherwise, continue with setContentView, etc.

You can have a single top layout (e.g. FrameLayout) that contains two overlapping layouts (one for the login and one for the main). Use a single activity. When the activity starts do setContentView() to the top layout. Then define a method selectScreen(boolean isMain) that based on the argument sets the main layout on and the login off (or vice versa). You turn screens on/off using the setVisibility() method in class View. You can switch screen any time by calling that method. If you want to be extra fancy you can use standard animations when flipping screens.

In Android, how would you develop this? (activity and views question)

When the user opens the application, there is a screen with a button on it, which says "login."
The user clicks on the button, and a webview pops up to allow him to log in to the website.
After logging in (the app would need to know somehow), the webview would disappear, and then a list of usernames will pop up. (ListView?)
When the user clicks on one of the usernames, a webview of the username's profile will pop up. Of course, when the user pushes "back", it goes back to the list of usernames.
Can someone explain this to me in terms of Activity and Views? Am I using two activities to do this? Do I hide webview or listview when the user clicks between them?
I did the tutorial (notepad tutorial), but I'm still confused as to what is the best way to develop this.
Thanks

When the user opens the application, there is a screen with a button on it, which says "login." The user clicks on the button, and a webview pops up to allow him to log in to the website. After logging in (the app would need to know somehow),
Yo can do this with two separate Activity classes. I would put the WebView in its own Activity. This is easier than managing lots of different View objects yourself. Also, you'll get transitions between different things if you put each part in its own Activity.
You'll can launch the login Activity with the startActivityForResult() method, allowing it to return if the login was successful or not.
If you want to detect the login, you can monitor events in a WebView using a WebViewClient. You set the WebViewClient of your WebView using the setWebViewClient() method.
the webview would disappear,
Simply start the next Activity using an Intent and call the finish() method on your first Activity. If you do this then the use won't come back to login button Activity if they click back as it won't be on the stack any more.
What I'm not clear on is how long the login at the website will be valid for. You may need to set the flags on the Activities in your Manifest, to ensure the user has to log in again if they leave and then return to your application.
and then a list of usernames will pop up. (ListView?)
Use a ListActivity. This is an Activity which comes with the API designed for displaying a single ListView.
When the user clicks on one of the usernames, a webview of the username's profile will pop up. Of course, when the user pushes "back", it goes back to the list of usernames.
So use the onListItemClick() method in ListActivity to detect the touch and launch a new Activity containing the WebView to show the profile. As this is in a new Activity the back handling is all automatic.

Essentially it could all be done within one activity. I can try to easily demonstrate how it could be done.
When you run your app, Activity (AndroidManifest.xml) is called.
View (could be called, main.xml) is shown displaying a login button
Upon clicking the button, you launch your WebView
Successful login, switches the View to a custom view displaying usernames on a ListView (might be a predefined XML file you create named userlist.xml)
Using onClick on a specific username will launch a WebView regarding that specific username.
To return back to your application when the user hits the Back button, you might need to utilize onPause and onResume but I am not sure.
This may not be the best approach, or even a good one, but it might help clear up any confusion.
EDIT:
An Activity is what gets bound to the AndroidManifest.xml as the main entry point into an application. A View contains user interactive components within your Activity, such as a login button, displaying username's, viewing contents on the web (WebView), etc.

First of all I'd consider a different way of approaching this problem.
You could create your own login layout in Andoid and send your login data to the website. After doing this you should create a ListView of all user names. If the user selects one of these user names you should open a WebView.
The problem would be, that you would have to check whether the login was successful or not! Basically you need to do a HttpRequest, parse the output and check it.
If it's your website you want to login you could write a small wrapper for your login which returns true/false for your login. (php wrapper which returns xml or plaintext)
Anyway, if you really want to do the login via WebView you can check it via:
private void initWebView(final WebView mWebView) {
mWebView.setWebViewClient(new WebViewClient() {
public boolean shouldOverrideUrlLoading(WebView view, String url) {
Page = new WebView(YourClass.this);
Page.getSettings().setJavaScriptEnabled(true);
Page.loadUrl(url);
if(url.contains("OkDoLogIn.php")) {
/* Check if login was successful if true start new Activity */
}
return true;
}
});
}
With url.contains("OkDoLogIn.php") you basically check which url is about to open. In your case it would be the url or a part of the url of your login button.
Nevertheless you would have to check if your login was successful or not!
Edit: See 1st comment!

Saving Android application state

I understand how to save an application's state by using SharedPreferences, onSavedInstanceState() & onRestoreInstanceState(), etc as outlined in a similar post ( Saving Android Activity state using Save Instance State ), but how do I save the last activity?
To be more specific, my application starts up and goes to a login screen. Once a user logs in and navigates through several activities, lets say he or she leaves the app using the home button or in some other way. Next time the user starts the app, it will go back to the login screen and do a login again. Instead, I want the app to start up and go to the last activity that was on top of the stack when the user left the app in the previous session.
How is the last activity saved so that it can be restored on app startup?

I believe Android does this automatically. We have an app that we are working on. When I click the home button and them come back to the app, it starts in the activity where I left off. We have not written any code to make this happen. It just seems to work.

My colleague has written an article on Android application state including details on getLastNonConfigurationInstance() which retrieves the instance that was last stored. Take a look here: http://www.eigo.co.uk/Managing-State-in-an-Android-Activity.aspx

Please note that the above answer is correct only on one case: If your process does not get killed by android because the resources (memory, ...) are needed for a different reason.
To get what you describe, I would write a custom parent activity and override the correct life-cycle methods to store your application state and read it and act accordingly. Then let all your activities inherit from MyActivity (instead of android.app.Activity)
public MyActivity extends android.app.Activity {
...
#Override
public onCreate(...) {
// Read the Application state,
// check if the currently launching Activity is the right one,
// if not, start the last Activity and finish the current one.
}
#Override
public onDestroy(...) {
// Store the current Activity ID in the SharedPreferences
}
...
}
Take care to call the super.onDestroy and super.onCreate methods in all your Activites (like you should do anyways).
Happy coding and have fun with Android!