I am having two activities, Activity A and activity B.
Activity A starts activity B.
So, the activity stack after some interactions will look like A -> B -> A -> B.
The problem:
I need to go the first activity A in the stack from activity B (last B in the stack). I am using FLAG_CLEAR_TOP as well as Intent.FLAG_ACTIVITY_NEW_TASK for achieving the same.
Right now, the activity A (stack pos 3) will be shown from activity B, but when I press back button, the activity B will be shown again (since activity B (stack pos 2) is already there in the stack).
How do I overcome this issue?
PS: I tried using launchMode singleInstance and singleTask for activities A and B, but that solution doesn't work for my app.
Thanks in advance.
Add finish (); after switching activites like
Intent intent = new Intent (activity_a.this, activity_b.class);
startActivity(intent);
finish ();
Avoid creating multiple instances of activities if possible. Android isn't designed to allow you to identify (and return to) a specific instance of an Activity in the stack, if you have multiple instances of the same Activity in the stack.
Depending on your application, there are different ways to solve the problem.
One way that may work for you is to use <activity-alias>, which allows you to reuse an existing Activity implementation by another name.
Another way would be to create another Activity class, that just inherits from the original, so that you have 2 classes with exactly the same code, but with different names.
The best way is to rearchitect your application so that you only ever have one instance of each Activity alive at any given time. You can do this by rearranging the task stack using Intent.FLAG_ACTIVITY_REORDER_TO_FRONT.
Related
Suppose I have activity instances A1, B1, A2, B2, C1 of activities A, B, C in stack. How can I traverse to Activity instance B1 from C1?
Let us generalize there will be 'n' number of activities between B1 and C1.
I don't want to create a new instance of B from C1.
This is actually a very bad architecture for Android. If you create multiple instances on an Activity, there is no way to specifically address them, for example: "Go back to the first instance of ActivityB". Android isn't designed like this.
You should avoid creating multiple instances of an Activity. It is beter to use the same instance and just create the "illusion" of multiple instances by swapping out the underlying data and maybe adding a state transition on the display so that it looks like you are starting another Activity.
Another possible solution would be to use a lot of startActivityForResult() and return information to the calling Axctivity about what to do next.
For more details see (even though these questions are specifically about using FLAG_ACTIVITY_REORDER_TO_FRONT, the problem is still basically the same):
Managing Android Activity stack: bring specific activity instance to front
Multiple activity instances and FLAG_ACTIVITY_REORDER_TO_FRONT
Bring an activity to front using FLAG_ACTIVITY_REORDER_TO_FRONT
Use Flags with Intent.
official Docs: https://developer.android.com/guide/components/tasks-and-back-stack.html
pass the flag along with Intent
FLAG_ACTIVITY_CLEAR_TOP
also you can paas multiple FLags in a single Intent according to your need.
Hope this helps.
Could someone please explain what FLAG_ACTIVITY_SINGLE_TOP do? The docs say
If set, the activity will not be launched if it is already running at
the top of the history stack.
But that statement seems to be burying a great deal of meaning beneath it. For instance someone online mentioned that the top activity may not be the same as the activity at the top of the task stack. I have no idea what all of that means. Hence my greater question: what are the implications of using FLAG_ACTIVITY_SINGLE_TOP?
It means if the activity is already up and you call it again you wont replace it with a new one and you wont create another one (which sometimes happens and is evident when you hit back and you see the same activity up), instead you will pull that one up.
So say you have 3 activities: A -> b -> c. You are in C and you came to it through A and then B. If you call to A, from C with the SINGLE_TOP filter your stack will look like A, C, B - if you started to hit the back button, you would go to C, then B. I could be wrong but thats how i believe it is. You can also pass in the CLEAR filter with it to erase the back stack and techniquely be started back at A with no stack in back of it, you would back straight out to home. - please correct if im inaccurate.
The following is an answer I read online. It is not a complete answer to the question you are asking, so I am really hoping someone else can add a lot more meat to it.
If an instance of the activity already exists at the top of the
current task, the system routes the intent to that instance through a
call to its onNewIntent() method, rather than creating a new instance
of the activity. The activity can be instantiated multiple times, each
instance can belong to different tasks, and one task can have multiple
instances (but only if the the activity at the top of the back stack
is not an existing instance of the activity).
For example, suppose a task's back stack consists of root activity A
with activities B, C, and D on top (the stack is A-B-C-D; D is on
top). An intent arrives for an activity of type D. If D has the
default "standard" launch mode, a new instance of the class is
launched and the stack becomes A-B-C-D-D. However, if D's launch mode
is "singleTop", the existing instance of D is deliverd the intent
through onNewIntent(), because it's at the top of the stack: the
stack remains A-B-C-D. However, if an intent arrives for an activity
of type B, then a new instance of B is added to the stack, even if its
launch mode is "singleTop".
Source
I'm new to Android development and I have an app with various activities. For performance reasons I'd like to properly manage the activities when users are using my app. Here's my problem.
Activity A - starting activity with map
Activity B - user navigates to Activity B (a list view) from Activity A.
The user then selects the map icon to navigate to Activity A again.
So if you can image it, my activity stack is now:
Activity A
Activity B
Activity A
So if I press the back button the device it will take forever as it scrolls through activities.
Is there a way of managing this so the old activity is destroyed and is just re-created upon choosing an activity nav icon. I've read up about onDestroy() and onStop() but I'm a little confused at their implementation.
Apologies for a poorly worded question but I'm unsure of the correct lexicon to ask about activities.
One simple solution is to kill the Activities as soon as they leave the foreground.
You could do that by calling finish() inside onPause().
You could have B finish itself and return to A instead of starting another A. Or, if your stack might be more complicated, like this:
D
C
B
A
D could start A with FLAG_ACTIVITY_CLEAR_TOP, which would cause D, C, and B to be finished, leaving A on top. (That intent flag interacts non-intuitively with a couple other flags, so read the docs.)
hi i have an application
A is launcher activity from that i create B activity. From B i want to go back to A without deleting the instance of B(so no finish) then i want to create a new instance of B lets call it B2. And from A i want to be able to show B or B2 without recreating them.
again i stress that B,B2 are the same class just diffrent instances.
from B when i want to go to A i thought of using
Intent myIntent = new Intent(getBaseContext(), MainAct.class);
myIntent.setFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP);
startActivity(myIntent);
but i think that its a mistake because it will clear other instances.
what intent must i use in the cases?
in c# windows application we would do B.hide();A.show(). then from A we could do
A.hide(); B.SHOW() or B1.show();
how can i do that in android please?
If B is an Activity, you can't have two different instances of it.
You can have 2 different Activities that extend the same class, but you've indicated you don't want that.
You need to find a way to change the appearance and behavior of B rather than creating a new instance.
Please consider reading about the Activity Lifecycle.
use FLAG_ACTIVITY_REORDER_TO_FRONT
from the doc
If set in an Intent passed to Context.startActivity(), this flag will
cause the launched activity to be brought to the front of its task's
history stack if it is already running.
I was going through Android Task and Back Stack documentation and at one point they mention this:
if your application allows users to start a particular activity from more than one activity, a new instance of that activity is created and pushed onto the stack (rather than bringing any previous instance of the activity to the top). As such, one activity in your application might be instantiated multiple times (even from different tasks), as shown in figure 3. As such, if the user navigates backward using the Back button, each instance of the activity is revealed in the order they were opened (each with their own UI state)
Let's take an example:
I have Activity A Starting Activity B which Starts Activity C which starts D.
Stack is A->B->C->D now it is possible to Start C from D so when we start C from D stack will be
A->B->C->D->C
Now instead of this standard behavior I want Activity to have only 1 instance or only 1 entry in the Back Stack. "SingleTop" will not work since Activity C was not on top when we started it from D.
I might be missing something but is there any way to achieve this making sure an activity has only 1 backstack entry?
Thanks
Pranay
Use Intent.FLAG_ACTIVITY_CLEAR_TOP, e.g.:
Intent intent = new Intent(context, <your_activity_here>);
intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
context.startActivity(intent);
So, all the activities in stack after activity C will be finished automatically. If you use the specified flag
A->B->C->D
will become
A->B->C
You can also use android:launchMode="singleInstance" in your activity tag in manifest