I read somewhere that each time you call startActivity() to transition to a new screen, you are creating a new instance of that activity. This immediately raised a red flag to me.
In search of a way to prevent this issue, I read about using FLAG_ACTIVITY_REORDER_TO_FRONT. However I'm a little alarmed that this method is not used in any of the tutorials I've seen for opening a new screen in your app. So I think I might be missing something. I mean wouldn't you always want to make sure you're not creating a duplicate Activity? isn't this kind of a big deal and shouldn't tutorials address it?
I just want to make sure that I am understanding, and dealing with, this issue correctly, and using the commonly practiced way to transition between screens.
I understand in some cases you open a screen, do something, then close it using finish() and go back. But if an app has a complex 4 level hierarchy , and the user needs to be able to jump around between the screens?
I mean wouldn't you always want to make sure you're not creating a duplicate Activity
Most of the time you probably won't want to have two instances of the same Activity but I suppose there are situations where someone might
isn't this kind of a big deal and shouldn't tutorials address it
It is definitely something that Android developers need to know about and understand how to use. I would imagine that you may not see a lot of it in tutorials because most of them teach you the basics to get you started. When a developer is new to Android, they typically have enough to learn about the framework. And they normally show you how to do a few screens (which is often all someone may need). They expect you to learn more of the details by reading through the docs and using websites such as SO
I understand in some cases you open a screen, do something, then close it using finish() and go back. But if an app has a complex 4 level hierarchy , and the user needs to be able to jump around between the screens?
Yes, a lot of times you won't be getting too deep because the deeper the easier it is to get a "lost" feeling. This is why I like using Activities with a Dialog Theme when possible. It gives the feeling that you aren't actually leaving the screen you are on. However, that doesn't really answer your question. There are many flags that can be used with Intents to keep the stack from growing. The one you mentioned works and I also use Intent.FLAG_ACTIVITY_CLEAR_Top quite often if I need to clear all Activities from the stack and get back to one single Activity. There can be so many different situations between apps, users, and developers that how you transition and work the flow or navigation just depends on what you (really, the user) needs.
It seems like you are on the right track by asking these questions. Think about what will give the user the best and most natural experience and find the right flags in the Intent Docs for your situation
I hope this cleared things up for you a little. If you don't understand something, feel free to ask
You usually want to avoid creating a duplicate activity, and IMHO, it's a bug in the Android system that the default behavior is to allow it.
(Sometimes you do want to allow it, e.g. you've written a "get filename" activity, and more than one application is likely to call it.)
You control activity creation in two places: In the manifest, and in the flags of the intent that launches it.
In the manifest, the <activity> tag has the attribute android:launchMode, which can be one of:
"standard"
Default. Can be instantiated multiple times, can belong to any task, and can appear anywhere on the stack. Normally part of the task that called startActivity() unless the FLAG_ACTIVITY_NEW_TASK was used. A new instance of the class is created to respond to each new intent.
"singleTop" Identical to standard, except that if the target task already has an instance of this activity at the top of the stack, a new activity will not be created. Instead, the existing activity will receive a call to onNewIntent().
"singleTask" There can be only one. It's the first activity of a new stack, and is thus the root of an activity stack. If there are more intents, they'll be sent to onNewIntent().
"singleInstance" Identical to singleTask, except that it's the only activity in its stack. If it tries to launch a new activity, the new activity will start a new task. Same as if FLAG_ACTIVITY_NEW_TASK was in the intent.
I personally find the history back stack in android more confusing than helpful to end users. The use of back buttons can often not do what the user expects.
There are a few options available to you if you want to use activities as the main construct.
For workflow type activities (e.g. capture forms) start the first activity with Intent.FLAG_ACTIVITY_NEW_TASK and at the end use FLAG_ACTIVITY_CLEAR_TASK
For top level activities that are often a returning point start the activity with Intent.FLAG_ACTIVITY_CLEAR_TOP. This basically checks if the activity is already somewhere in the stack and if it is pops off every activity above it in the stack and resumes the original activity.
Another possible method for the top level activities is to use one top level activity and use fragments for moving around, e.g. tab browser etc. Then just use activities for actual separate tasks.
Related
If we look at one activity as a graph node, transition/calling from one activity to another as a graph edge, one android app can be conceptually converted into a graph. I am trying to see if there is any way to create such a graph starting from the first page/activity of an app.
Let us say we can use android UI test automator to help if needed.
I am trying to see if there is any way to create such a graph starting from the first page/activity of an app.
It would be exceptionally difficult.
I am assuming that you are trying to hack... er, I mean, "analyze"... somebody else's app. In that case:
You have no idea how many activities there are, unless you are reverse-engineering the APK. And even then, you have no idea how those activities will be used (e.g., legacy app that is putting activities in tabs and not using them as an ordinary activity).
You have no way to know what triggers the activities to appear. For example, a certain activity might only appear via an "Easter egg"-style bizarre set of inputs. Some might be triggered by things outside the app itself, like a Notification or a third-party invocation of ACTION_VIEW for some MIME type the app supports. And some of the triggers may be dependent upon other inputs (e.g., a disabled action bar item that becomes enabled only if you have set up an account elsewhere in the app).
With full source code, you could do static analysis to find all startActivity() and startActivityForResult() calls, and for simple variations you might be able to divine which activity starts which. Even that could get tricky in some cases, for complex Intent construction.
I need general advice.
I'm building an app that can be started via the icon (normal way) but also with an intent triggered by a click in the notification area (the app places an icon in the notification area). The is awaken when a time event / alarm occurs.
The app has 5 to 10 views. Is it better to open a new activity for each view? How can I be sure not to have mutliple tasks open? Say if my app is Activity-A, Activity-B, Activity-C and can be started from Activity-A and Activity-B (depending on if it's opened via the icon or the notification area) ... how do I clean everything upon app close?
Or is it better to have one activity and just switch views (xml layouts)?
I'll start of with what you should watch and read. It should be clear that navigation in Android is very easy to do wrong. Google has a history of doing differently from app to app, but they are getting better. This said, if you are to do an app you should know how it is intended to work, and the best way to do that is to read the docs. As mentioned, there was an excellent talk at IO'12. There is also a very good section on the design site, and finally there is a good section in the API Guides.
To summarize: a full screen is an activity (which in turn can be constructed of fragments). The activities should in virtually all cases be structured as a tree with the root being the activity that are launched from home. For every activity you should have an "up" activity that takes you up in the hierarchy. Note that this is different from back which should take you to the last full screen state you were in. Also note that full screen means that for example tabs should not be recorded in the "back history", since they provide navigation within a screen.
When it comes to tapping a notification it should be equivalent to: pressing home, remove the task from the recents view (clearing the task), opening the task, and finally take the shortest path to get to the activity presenting the info that the notification told you about. Complicated, yes indeed... But at least there is a helper class in JB and in the support library called TaskStackBuilder.
The key to all this is a UI design that follow the Android design guidelines. Take your time to make the design for your app, and make sure to separate up (static) and back (temporal).
It might be as simple as using a different launchmode, which you can define in the manifest or I think in the inent you are using. It takes some experimenting but SingleTask or SingleInstance may be the right choices for you,
Your activity should support onNewIntent in this case, to reuse existing Activities if that is the desired effect.
In this case I better use one activity or use fragments.
Under some circumstances I need to be able to tear down all my activities that are in my application stack and recreate them all due to configuration changes. I have accomplished this by first calling finish for each activity and then recreating the stack.
To recreate, I relauch my root activity. And within its onStart I have it create my second activity. Within my second activities onStart I have it create my third Activity. This does work but the problem that I am having is that when watching the screen you see each of the three activities created and animate into the next activity. I want to have this rebuilding invisible to the user and hide these transitions. Does anyone know how to accomplish this?
Android already takes care of restarting activities when there is a configuration change.
If you are saying you want all of your activities to be restarted, even if they aren't currently visible (Android will do this lazily as the user returns them and they become visible, if the configuration is still different at that point), then no there is no simple way to do this. I can't imagine you coming up with anything that isn't going to be hideously ugly, because to get the platform to restart your activity you will need to make it visible, and then you are going to have flicker up the wazoo.
Things just aren't intended to work that way. This isn't how pretty much any other application you run on Android will operate, so if you deeply feel like it is something you need to do then it will be useful to explain why that is so we can tell you a better way to accomplish what you want. :) For example, if you have a bunch of activities whose state is fundamentally tied together to require this, consider using fragments instead (or cleaning that up).
On the other hand, if you just have some internal concept of a configuration and want to get your activity to be restarted (say for example to switch between themes), there is an API for this but it only was added in Android 3.0: http://developer.android.com/reference/android/app/Activity.html#recreate()
Does anyone know of a way to show another class without creating a new instance?
It seems a bit crazy from a memory management point of view that each time you want to display a different form / page you need to use StartActivity which then creates a new instances of the class instead of reusing instances previously created.
Thanks in advance
I guess from what has been said - there is no real way to do it which won't hinder the "Back" functionality of the OS?
I'm building an app which is linear except on each screen it has a home button which then makes it possible to countermand this functionality and end in a loop - is there anyway you know of to destroy all over views and reset back to the main class? (IE prevent a memory leak from becoming a problem but also not damaging OS functionality)
Consider it a "clear history" without restarting the app
Not sure if this would work for Android (coming from a MS/C# background), but conceptually one option is to iterate through open forms looking for one with a specific handle. Then, once you find it, simply call the method to show that form. This would depend on there being a Java equivalent to the Application.OpenForms property in .NET.
It seems a bit crazy from a memory management point of view that each time you want to display a different form / page you need to use StartActivity which then creates a new instances of the class instead of reusing instances previously created.
Tactically, you are welcome to add FLAG_REORDER_TO_FRONT to bring an existing activity back to the foreground, so long as you understand the ramifications from a navigation standpoint.
However, your question is rather curious. How are you accessing StackOverflow?
Clearly it's not via a Web browser. Web browsers use the exact mechanism that you feel is "crazy", rendering Web pages even if that Web page had been viewed previously. They have been doing so for over 15 years, and we've been doing OK by it.
The Android navigation model is designed to approximately mirror that of the Web:
Users click on things to move forward
Users click on a BACK button to move to the previous thing they were looking at
Users click on a HOME button when they want to switch to some other major thing to go look at
By "reusing instances previously created", you're circumventing that navigational model. For example, let's suppose your activity stack were A-B-C-D, and you call startActivity() with an Intent for B and FLAG_REORDER_TO_FRONT. Now, the activity stack is A-C-D-B. When the user presses BACK times, they no longer are on the B they were looking at originally, but are back at A. In a browser, this would be rather strange behavior.
There are other flags on Intent, or attributes on <activity> in the manifest, that offer "reusing instances previously created". However, they are not there "from a memory management point of view". They are there where the traditional Web BACK-heavy navigation pattern does not fit your needs.
Assuming you aren't screwing up anywhere, Android will destroy under-utilized activities, garbage collect that memory, and even return that memory to the OS.
I have developed some apps for Android, and this questions stays always:
How should I structure my UI? Should I launch activity after activity and leave the phone to make the "back" button, or should I choose more optimized, but more complex to implement, way with switching manually Views and then manually doing the "Back" button functionality?
What do you think (or know) is the better practice?
I would say that multiple Activities almost always makes more sense. I just don't think Android is designed for constantly switching its own views - you miss out on so much. You have to implement Back yourself, you don't get any inter-Activity transitions, you have to implement a lot of internal logic to resume an application in the correct state. If you don't partition your app into Activities, it makes it a lot more difficult later on to change the flow of your application. It also results in one mega-Activity that can be a lot harder to handle than a lot of smaller pieces of code.
I have trouble imagining that speed is really an issue; if it is then there's something wrong with the way you're initializing each Activity. For example, I used try to pass Serializable objects between Activities, and that proved to be incredibly slow; when I switched to a faster method of passing objects, the speed of launching Activities increased immensely.
Also, I think it's telling that the Android guidelines for Activity and Task Design don't mention switching Views at all; it's centered around an Activity-as-View design.
I'd like to point out some instances when a single activity might be better design for an Android application that has more than one full screen View:
If the application screens are tightly coupled and share a common Object that they are all operating on. In this case passing around the Object may require a Bundle and can be error prone since there will be copies of it. A good example might be a wizard. Yes you could use static's to access the common Object but static can be dangerous in Android (think configuration changes!)
If you want some really cool animations in between screens. Maybe you want a bird to take off in one screen and land in another screen. Try doing that when each screen is an activity!
On the other hand if one of your screens is designed to be shown by any number of other applications then that screen should be its own Activity.
UPDATE March 2014:
At this point the question should now include the choice of Fragments. I think that Views are probably the least likely choice of the 3: Activity, Fragment, View. If you want to implement screens that make use of the back button then it should be either Activties or Fragments because both handle the back button natively. Fragments will need to be added to the FragmentManager back stack for the back button to work. Managing fragments, dialogs and the back stack can be a bit of an annoyance though!
UPDATE Sept 2018:
Some devs at Google are recommending single activity apps using the new navigation architecture component.
Also keep in mind that implementing your app with multiple Activities will give the user a more coherent experience with the platform as a whole. Part of the experience will be shaped by using the built-in Google apps, so users will probably have an easier time using your application if it behaves similarly to the ones that are already installed on the phone.
Different from others I use a mixture of both, for example,
1. There is a main menu when the application starts
2. You click on search, takes you to search activity
3. Then there's a filter button, which just switches view and shows you filter options
4. There are two buttons at the end of the filter view, You hit "Search" or "Cancel" and you are back to the Search View again (without switching activity)
5. Now if the user hits the phone back button he's taken back to the main menu instead of the search filter options. Which I guess is the correct behavior.
Use it the way user will feel natural. And keeping everything in one activity will make it complex.
It all depends on application, what are you trying to achieve better performance, smoother UI. IMHO I prefer the second approach of controlling the Activities manually even that it is more complex as you have stated. This is a approach I have used in my android tabs project, also you might want to take a look at a class called ActivityGroup (not sure the package) it allows you to have multiple activities that you can switch between, good thing about this class is that your activities are not unloaded when you switch but a bad thing is it takes longer to load your main app.
Just my opinion.
The problem with switching views, that I stumbled upon, is also caused by garbage collector. Seems that GC is triggered when you leave activity and not the view. So, changing tabs with a fairly complex children views, for instance, will almost inevitably lead to stack overflow exception..
I've experienced so many problems with multiple activity layout that I strongly discourage it, unless there's good reason to pick it.
Disadvantage of multiple activities
Using multiple activities it is much hard to refactor code to return data from activity.
If you call a 'sub'-activity then the main activity may be killed. But you never experience that while debugging on a decent device, hence you need to handle always saving state and correctly recovering state. That is a pain. Imagine calling a method on a library (ie. another activity), and you would have to be ensure that when that method returns your app must be able to recreate its state completely with all fields on all objects in the VM (ie. activity.restoreIntance). Its insane.
Also the other way round, when you open a subactivity the VM might have been killed since the subactivity was first spawned, such as when app is minimized while subactivity is displayed.
Its so much cleaner to just have one place to store the relevant app-state, and in my case, most often if VM is killed, I want to return user to main-screen, and let them do their stuff again, because I don't spend 30-50 hours coding save/resume functionality that 0.1% of users will ever experience.
Alternative
Fragments or just manage you activity views yourself. Managing views manually, requires coding some view-switching alternative to activities/fragments with transitions if desired.
And no it does not mean one mega-activity, as suggested in the accepted answer, in any other way than its one mega-app. It just requires a bit more design of the codebase into fitting pieces, because there's slightly more work managing views, though much less work managing activity-state and other weirdness.
Possibly relevant: Reddit: It's official : Google officially recommends single activity app architecture