I'm trying to minimize memory usage in my app, and one of the things I'm doing is calling finish() in the onPause method (which I know is not the best way to do things). For the most part, it seems to be working well, but when the user clicks the back button from the next activity, it logically skips over the finished activity and goes back further. Is it possible to have that activity in the back stack and just get recreated if the user presses back?
No. This conclusion comes from the task and backstack documentation as well as the activity documentation and a general understanding of how a stack data structure works.
A stack data strucure only has 2 possible operations push/put, which adds something to the collection, and pop, which removes it. Stacks folow a last in first out model, or LIFO, where by last thing added - in your case an activity - is the first thing removed when pop is called.
Within the android lifecycle activities are generally popped from the stack when the back button is pressed. At that point onDestroy() is called and the activity is removed (you can verify this by overriding the onDestroy() method and logging the results if you want to check). Alternativly you can force onDestroy() to be called by calling finish() as you are. Finishing an activity effectivly does the same thing as pressing back. The activity is destroyed and must be recreated before it can be added to the stack.
For what you're trying to do the stack would have to incorporate some intermediate state in which an activity does not exist but rather something akin to a reference is held that, when moved to the top, would indicate that the corresponding activity should be recreated. Since this is not how the sack works - it only holds activities - that state cannont exist and so the result you are talking about is not possible.
Your Goal is to minimize memory usage,Just make use of activity life cycle, You can do this alternative(if you need)
-Just leave onCreate() method blank.(only do setContentView(layout))
-Override onResume();
-whatever you were doing in onCreate just copy paste to onResume().
-and In onPause(), Recycle your all bitmaps and set them to null(I think you are using Bitmaps thats why you are very cautious about it ). and remove your views.
Now what will happen, when you launch your new activity, onPause() would be called. that will remove your all bitmap and views. and when you come back, onResume() will be call.(onCreate will not be called). and that will again initialize your view and bitmaps.
No, i don't think that is possible. Once you finish the Activity it's gone. You could, however, implement and handle your own stack. On back pressed, you would just start the closed Activity again.
Related
I'm studying with AOSP, and I found ActivityStack and TaskRecord in "ActivityStack" Class. There is explanation In https://developer.android.com/guide/components/tasks-and-back-stack , Back-stack(=Activity Stack) And Task seems similar to me... What is the the difference between ActivityStack and TaskRecord?
In this https://developer.android.com/guide/components/tasks-and-back-stack,
Focus on below lines, For Task
A task is a cohesive unit that can move to the "background" when users begin a new task or go to the Home screen, via the Home button. While in the background, all the activities in the task are stopped.
For back stack
The back stack for the task remains intact—the task has simply lost focus while another task takes place. A task can then return to the "foreground" so users can pick up where they left off.
For info you can refer below links:-
What is the relationship between Task and Back stack
https://blog.mindorks.com/android-task-and-back-stack-review-5017f2c18196
https://medium.com/google-developers/tasks-and-the-back-stack-dbb7c3b0f6d4
If you think of activity back stack as two levels, it might be easier to understand the purpose of ActivityStack. Android supports launchMode and taskAffinity to put activities into different TaskRecords. But even activities are put into different TaskRecords, it keeps supporting the back button to switch back to previous activity. So when you launch activity that needs to be in new TaskRecord, then back button is pressed, it switches to top activity of previous TaskRecord. So ActivityStack is more like a TaskRecord stack, and TaskRecord is more like the activity stack inside the TaskRecord, however, general speaking, ActivityStack controls the pop up sequence, and you can say it's an indirect activity stack.
And creating ActivityStack seems to make management easier logically in multiple window environment. If you enable free form in Android, each launched window mode app has its own free form stack, and each stack has its own back stack.
My 2c.
I'm still pretty new to scripting in Unity3D, and I'm following along with a tutorial that uses GUI.Button() to draw a button on the screen.
I am intrigued by how this function works. Looking through the documentation, the proper use of GUI.Button is to invoke the function in an if statement and put the code to be called when the button is pushed within the if statement's block.
What I want to know is, how does Unity3D "magically" delay the code in the if statement until after the button is clicked? If it was being passed in as a callback function or something, then I could understand what was going on. Perhaps Unity is using continuations under the hood to delay the execution of the code, but then I feel like it would cause code after the if statement to be executed multiple times. I just like to understand how my code is working, and this particular function continues to remain "magical" to me.
I don't know if it's the right term, but I usually refer to such system as immediate mode GUI.
how does Unity3D "magically" delay the code in the if statement until
after the button is clicked?
GUI.Button simply returns true if a click event happened inside the button bounds during last frame. Basically calling that function you are polling: every frame for every button asking the engine if an event which regards that button (screen area) is happened.
If it was being passed in as a callback function or something, then I
could understand what was going on
You are probably used to an MVC like pattern, where you pass a controller delegate that's called when an UI event is raised from the view. This is something really different.
Perhaps Unity is using continuations under the hood to delay the
execution of the code, but then I feel like it would cause code after
the if statement to be executed multiple times.
No. The function simply returns immediately and return true only if an event happened. If returns false the code after the if won't be executed at all.
Side notes:
That kind of system is hard to maintain, especially for complex structured GUI.
It has really serious performance implications (memory allocation, 1 drawcall for UI element)
Unless you are writing an editor extension or custom inspector code, I'd stay away from it. If you want to build a menu implement your own system or use an external plugin (there are several good ones NGUI, EZGUI,..).
Unity has already announced a new integrated UI System, it should be released soon.
Good question. The unity3d gui goes through several event phases, or in the documentation
Events correspond to user input (key presses, mouse actions), or are UnityGUI layout or rendering events.
For each event OnGUI is called in the scripts; so OnGUI is potentially called multiple times per frame. Event.current corresponds to "current" event inside OnGUI call."
In OnGUI you can find out which event is currently happening with >Event.current
The following events are processed link:
Types of UnityGUI input and processing events.
-MouseDown
-MouseUp,mouse button was released
-MouseMove,Mouse was moved (editor views only)
-MouseDrag,Mouse was dragged
-KeyDown, A keyboard key was pressed
-KeyUp A keyboard key was released.
-ScrollWheel The scroll wheel was moved.
-Repaint A repaint event. One is sent every frame.
-Layout A layout event.
-DragUpdated Editor only: drag & drop operation updated.
-DragPerform Editor only: drag & drop operation performed.
-DragExited Editor only: drag & drop operation exited.
-Ignore Event should be ignored.
-Used Already processed event.
-ValidateCommand Validates a special command (e.g. copy & paste).
-ExecuteCommand Execute a special command (eg. copy & paste).
-ContextClick User has right-clicked (or control-clicked on the mac).
Unity GUI has much improved lately and is quite usefull if you want to handle things programmatically. If you want to handle things visually, i recommend looking at the plugins heisenbug refers to.
If you decide to use unity gui, i recommend using only one object with ongui, and let this object handle all your gui.
I have a Page View Controller that's implemented in 3 files. One of them, the ModelController.m implements "generateData" method, which basically just builds and formats all the pages for this Page View Controller. Inside this method, the first thing I do is I create a Progress Bar popup with "Cancel" button. Then, inside the while() loop, I keep building the pages and at the same time, every 10th page, I update the progress bar for the user to see that the application is still generating the output.
Once the "generateData" method completes, i.e. builds all the pages (and there may be over 1,000 or even 10,000 pages depending on user input), it returns "self" to a method inside RootViewController.m, which in turn passes that generated data in "dataObject" to "viewWillAppear" in DataViewController.m, which finally displays pages in that object to the user.
It all works great, except the "Cancel" button. Because "generateData" method runs on the main thread, it blocks and the "Cancel" button becomes totally unresponsive. By "blocks" I of course mean it takes all CPU cycles, not allowing anyting else to execute, and it may take over a minute to generate 10,000 pages so I really want to allow the user to cancel the operation.
I tried to create a separate thread for the Progress Bar popup which contains Cancel button, but that won't work because all UI operations need to be performed on the application's main thread.
What almost seems to work, is when I put the "generateData" method on a separate thread and keep the Progress Bar on the main thread. This would work just fine, except that now "generateData" is put on another thread to execute in the background and returns immediately, hence returning empty "self" or rather empty "dataObject", causing a crash because there is nothing in that object yet to display.
So how can I check if the "generateData" thread is done, and "return self" ONLY when it's done? I can't really have some BOOL true/false variable and do it in a loop, because that loop on the main thread would be again blocking the "Cancel" button.
Or, alternatively, what is the correct way to implement the "Cancel" button on a lengthy method in iOS? Maybe I'm doing it all wrong, but for the life of me I can't find the right recipe for this.
Thank you in advance for the help, the code is rather extensive and that's why I didn't include any, but I hope my description gives you a good idea of what's going on. I can provide code if that would help.
The correct way to do this is to load the data asynchronously (on a background thread) and update the main (UI) thread to update the progress bar, etc... But you probably don't need a progress bar since you can probably load those pages faster than a user can flip them, so load 100 and let the user play while you continue to load the next 99,900... Except those may not be the exact numbers you should use....
If you continue to use the progress view and cancel button, then you would cancel the background thread when the user presses "Cancel", which would respond since your data generation is on a background thread...
Look into either Grand Central Dispatch or NSInvocationOperations
Here's a good tutorial by Ray Wenderlich to get you started.
I'm currently working on a GWT application and I'm still a little fuzzy on how it all fits together (the joy of having to make changes without first coming to an understanding of the whole framework, although that might be difficult anyway).
We have a few Activities that all correspond to parts of the UI (e.g. main content, a toolbar and a list of things). I'm not really sure whether that's even how Activities are intended but I guess I can't really change that easily now. My problem now is that the list of things holds state (the current selection) that the main content also needs and in a way the toolbar too (at least the toolbar currently has it – I'm beyond asking).
But what would actually be an appropriate place to store that? I guess coupling the actual view implementations together and storing the selection only in the list isn't such a bright idea.
I see two main solutions here:
keep the state within each activity and keep them synchronized through events (on the EventBus). That is: the "list of things" has a current selection, main view has one too, so does the toolbar; and each time that value changes, the activity that's making the change fires an event on the event bus so that the other activities can update their state, so that all activities have the same value in their own state.
use a singleton object (if you're using GIN and dependency injection, simply annotate the object with #Singleton and inject it in all the activities) to keep the state at a central place. The activities register event handlers on the state holder object to be notified when it changes. That is, each time an activity calls setCurrentSelection (for example), an event is fired (for example a ValueChangeEvent), and because all activities listen for it, they can update their view or whatever depending on the new value. You can choose to either dispatch the event on the event bus (similar to the PlaceController) or have the state holder implement HasValueChangeHandlers. Just make sure to unregister the handlers when the activities stop to avoid memory leaks (dispatching on the event bus makes it easier: simply register the handlers on the bus passed as argument to the start method and they'll be unregistered automatically when the activity stops, and you don't even have to think about it).
Actually, PlaceController is a good example of such shared state (the current place).
I have been trying to design the Undo/Redo functionality in one of my app using NSUndoManager class and seeing some design difficulties.
In my app, I just have one method
-(IBAction) addLastBall:(Ball *)ball
So, when a user chooses a button in the UI, I add a ball object to my array list. However, I don't have any action button to remove a ball, this is the design of the UI workflow and can't be changed.
So, to implement a undo I called the following piece of code inside the addLastBall method
[undoManager registerUndoWithTarget:self selector:#selector(removeBall:) object:ball];
After doing this, removeBall method gets called with the ball object when the user tried to perform the undo. All is good for now.
But, I am not sure how to handle the redo part now, when the user tries to redo, I am not sure which method would get called, because I need to add the ball object the user just removed using the undo operation.
Any insights would be very helpful.
Thanks so much.
A redo simply performs the undo actions registered while the corresponding undo was executing. Therefore, the most straightforward solution is to simply to register an undo action in the removeBall: method.
- (void)removeBall:(Ball *)ball {
[undoManager registerUndoWithTarget:self
selector:#selector(addLastBall:)
object:ball];
...
}
Note that it is sometimes clearer to useprepareWithInvocationTarget: to register undo actions:
[[undoManager prepareWithInvocationTarget:self] addLastBall:ball];
By the way, be sure to keep the retain count of the ball above zero in the removeBall: method — the undo manager will not retain the ball for you, and if it gets deallocated, then undoing the remove will crash.
No method get's called.
The NSUndoManager simply saves the reverse of the undo operation (redo), that will be done, if the user choses redo.
It's all implemented in NSUndoManager, so you don't have to worry about it.
If you want your method to be called on redo, you should think about creating your own undo manager, but I wouldn't advice you to. It can get very complicated and besides, the NSUndoManager takes care of everything perfectly.