I'm new with Xamarin and I'm tring to create an Android App.
I have created a 2 Views related to 2 different Activities. The first Activity, name it A, has a button that launches the second Activity, name it B.
B has an EventHandler that is connected, in the OnCreate method, to A's Event. The EventHandler print on console a string, that's it.
if I launch the app and press the A's button, the B activity appears. Now, if I press the Back button the A Activity appears again, after that I press the button again and the B Activity appears but this is a new B Activity istance and not the previous one. I can see that because the OnCreate method is called twice and because I can see on console that the event is called twice.
If I repeat this many times I can see the same string printed on console repeated many times. I would like to have only one instance of any activity, so I need to change view without creating more istances of one activity that was already created or destroy the B Activity when the Back button is pressed.
How can I do that? is it the right way to do it?
You can't link the two activities, or better, you really shouldn't. If you need shared data then go through static properties (perhaps in a separate static class).
Read about activity lifecycle and activity launch modes. Note that you are probably creating a wrong flow though.
I think in order to get what you want (having two "Activities" of which the instance is always the same when you are switching between them), you basically need to use Fragments instead of Activities, in a FragmentPagerAdapter.
http://developer.xamarin.com/guides/android/platform_features/fragments/part_1_-_creating_a_fragment/
FragmentPagerAdapter Exists Only In Android.Support.V4.App (and not Android.App)
Related
I have a MainActivity with a container FrameLayout in which I change multiple Fragments (Fragment A, Fragment B etc).
In one of this fragments let's say Fragment A I have to open another activity (Activity X).
The problem is that from this activity when I press a button I have to change Fragment A with Fragment B (in the background somehow) and after that, slideout Activity X (with translate animation), and slidein Fragment B ,all this without restarting the MainActivity because I have to keep the state.
How can I do this?
Thanks
Android uses loosely coupled components as its main building blocks. As you know, Activities are one of the main Android building blocks. Thus, interacting between activities are very restricted to a few ways.
Passing data via Intents by startActivity(), startActivityForResult() etc. This way is useful whenever you are starting new activities.
Sending broadcast Intents. This could be useful once you want to send a signal to your another app's component.
Utilizing shared Application object.
Java static fields and some other ways.
In your case I would recommend you to use a Dialog Fragment instead of your second activity, if your second activity is just a login activity or something like that.
UPDATE #1:
If you really would like to keep your second activity, so I would personally recommend using local broadcast mechanism.
Also there are another way to get this done. You could start your second activity as startActivityForResult and then whenever user gets back from your second activity to your first one, your first activity can get informed by its onActivityResult method. There you could switch those fragments.
I have two activities A and B. The A has a ListFragment which uses LoaderManager, whereas B activity shows a details about the item selected in the A's ListFragment. I've just noticed that when I use a back button to get from the B back to the A, the position in the ListFragment preserve, but when I use the up button (left-point caret) in the action bar, the A activity is recreated and thus position in list view is lost.
I would like fix this issue, but I am not sure about the best way how to do it right.
I come up with this solutions:
a) Use onBackPressed()
Replace the default implementation for the android.R.id.home (the up action bar button) in the B activity, and instead of the NavUtils.navigateUpFromSameTask(this) function call the onBackPressed() activity method. I've tested it and it works.
b) Keep use NavUtils.navigateUpFromSameTask(this)
But implement the onSaveInstanceState and restore listView position during onCreate method of the ListFragment used by the A activity. (I've not tested this approach yet)
Which of this solutions is better? Or is there any other (much more better) solution?
Solution a) is pretty simple and straight forward, but b) is probably better because the default implementation of the up caret is used.
Any ideas are welcome. Thanks.
Solution c is the correct option. First, though, an explanation of the problem with solution a.
There is absolutely no point in having two back buttons in your Activity. Furthermore, option a actually breaks the up button. The point of the up button is to provide a way for users to stay within your app when they have landed in your app from an outside source. For example, if you land on activity B from an outside activity C and if you are using your option a, then pressing "up" in activity B will result in activity C being shown. The behavior you would want would be for activity A to be shown.
As you can see, solution b is on the right track. You definitely want to go up to A and not back to C. However, simply storing the state in onSaveInstanceState will not cause the state to be retained. This is because onSaveInstanceState only gets called if your application may be killed by the system. It is not guaranteed to be called if your application was destroyed manually, and it certainly won't be called when a new instance of your Activity is created. If the Intent starts a new activity, then it will not have its state restored from the other activity.
There solution, then, is that you must write anything persistent to a shared preference file (or a custom persistent alternative). When doing this you can guarantee that all instances of an Activity share the same state across multiple tasks so long as their onResume (or wherever you restore state) is called. OR:
If you know exactly how you want your navigation to work, you can avoid writing everything to persistent state by using a combination of Intent flags and Activity task affinities. If you want to use the same activity as up even if you navigate into the application from an outside source, then you can leave your Activity A's affinity as default (linked to the application) and use something like Intent.FLAG_ACTIVITY_CLEAR_TOP.
Personally, I'd try the Intent flag approach first and failing that fall back to writing the state persistently. You just don't really want scroll location sitting on persistent storage if you can avoid it..
check out this presentation: https://speakerdeck.com/jgilfelt/this-way-up-implementing-effective-navigation-on-android. It answers to all of your problems.
My Project
It's my project, use Google Map API V2, have 2 Activity A & B .
I have activity A - a list of area, if I choose one of them, Activity B show Google Map with a Marker .
I want when I press back button, I return a list, choose anyone without re-created Activity B
Please help me ...
I have posted one question like this, but I really need the answer now :(
basically when you app return to activity A, the onResume() part is invoked, but you can block the recreation by simply declaring a static variable, declare it before oncreate as inited to false, then in oncreate make it true, and for recreation process use this as an if statement,
if(!myVar==true)
// your coding goes here
I have activities in sequence as Activity A calling Activity B,When I am on Activity B I press Home button and start another Application (for example Map). If i stay on this second application for a long time say 5-10 minutes and then press Home Button again and choose to Start my application again, then Activity B is started again and works fine. But when i Press Back key - I return to my Activity A again (which is also correct) but it shows a Blank Screen. Ideally in correct version it should show me Acitivty A with the XML data is ListView form.
Alternatively, in the above description, when the other Map Application is started and if the user stay on it only for 1-2 minutes then the above problem doesnt not occur.
Can anyone suggest a solution on the same.
Is it that i need to check in Activity B whether Activity A is still there in Activity Stack (How do i do the same) and If its not in my Activity stack then re-create it.
I tried doing alwaysRetainTaskstate in my Android manifest file for Activity A. But it doesnt work at all
You don't have to do any of that, Android takes care of the technicalities for you - it will recreate your Activity A.
You just need to remember to save A's state when B is opened (take a look at Activity.onSaveInstanceState). When A is restarted, your saved state will be passed to onCreate.
You should really read about Activity Lifecycle
This should help.
How do you handle multiple screens in an android application? I have developed with the tab bar at the bottom without problem, however what I am wanting to do is replace all the content on the screen with the content from a new .xml layout file I have created in the project. Additionally, how would I tie the back end code to the new layout file? I'm sure this question probably exists already and is googleable (may have made up a new word). However, I don't know exactly what it is that I am looking for. Thanks in advance for your help.
What you need to do is, create a new Activity and add it to the AndroidManifest.xml:
<activity android:name="ActivityClassName" android:label="Label for the Activity"></activity>
and can be called in a method:
public void startActivity() {
Intent someName = new Intent(CurrentClass.this, ActivityClassName.class);
startActivity(someName);
}
Android applications generally use a separate Activity for each screen, and switch between them using Activity.startActivity and Activity.startActivityForResult. You can pass arbitrary data to an Activity via Intent.putExtra.
Hope this helps,
Phil Lello
It really depends on how you want your application to flow.
Let's consider the scenario where a user does the following:
Starts your first activity
Presses the 2nd tab
Presses the 3rd tab
Presses the back button
If you use a separate activity for each screen, then the following would happen
Activity 1 is started
Activity 2 is started
Activity 3 is started
Activity 3 is closed, user returns to Activity 2
(in this case pressing the back button again would you take you back to Activity 1, and pressing it again would exit your application)
If you used one activity for all the tabs, then the following would occur
Activity 1 is started
Activity 1 sets tab content to tab 2 content
Activity 1 sets tab content to tab 3 content
Activity 1 is closed, user returns to home screen
If you are using a screen with tabs, then the second method (a single Activity with a TabHost or similar) is the preferred method, otherwise the user will end up making a large activity-stack just switching between tabs (meaning if they switch between tabs a lot they'll have to press the back button a lot of times to exit).
If you want to go for the single activity approach, then do some research on TabHost and TabContentFactory. In the createTabContent method of your factory you can inflate a View/layout from XML to set as the tab content using View.inflate. Look those up and come back ask another question if you get stuck ;)
i think you may want to play with more than one activity.... you can have multiple activities and one xml for each of them... in this way you can have different screens... check these links. Multiple Activities, Creating an Activity.... hope this helps...