I have a table that is usingNSMutableArray`. When I refresh the table, I need to reload all the items in array.
I have 2 options
a) Recreate the new NSMUtableArray and throw away the old one
b) Remove all items from existing array and reload that again
From the performance/memory etc point of view what is the most efficient option in iOS world?
From performance perspective I think that the best bet would be instantiate a new NSMutableArray object, and just free the old one, or if you have ARC let the compiler to free the old object.
From the memory occupation perspective it would be better to clear the instance and reload it again, since you don't have two instances in any time in the application, however it could be useful only for large arrays.
The best choice is relative to the context as in most case ...
Reusing the old NSMutableArray will be more efficient as that will reuse the same memory (memory is not released when you remove all elements from an array). It is unlikely this will affect performance in a notable way in your situation though, as you would need to do it thousands of times for it to have an impact on performance.
Related
I've discovered Helm (http://helm-engine.org/) the other day and I've been playing a bit with it. I like Elm, so Helm has been great to use thus far.
Simply put, the update function gets called every tick and get passed the model, and has to return an updated version of that model. Another function then gets called to render that model on screen.
For small games with not much in the model it seems ideal, but I've been thinking about something a bit bigger, for which a HashMap (or a lot of them) would be ideal, and I'm wondering about the performances of that.
I'm no expert but I believe using the Data.HashTable.IO should modify the hashtable in ram instead of creating a new copy on change, but that seems complicated to interface with Helm. That would mean using a Cmd for each lookups and each changes and returning that to Helmn and then get passed the result from a new call to update, a nightmare to use if you have more than one or two things to lookup I think.
Data.HashMap.Strict (or Lazy ?) would probably work better, but I imagine each change would create a new copy, and the GC would free up the old one at some future point. Is that correct ?
That would potentially mean hundreds of copy then free per frame, unless the whole thing is smart enough to realise I'm not using the old hashtable again after the change and just not copy it.
So how does this work in practice ? (I'm thinking of HashMap because it seems like the easier solution, but I guess this applies to regular lists too).
I support the comments about avoiding premature optimization and benchmarking, instead of guessing, to determine if performance is acceptable. That said, you had some specific questions too.
Data.HashMap.Strict (or Lazy ?) would probably work better, but I imagine each change would create a new copy, and the GC would free up the old one at some future point. Is that correct ?
Yes, the path to the modified node will consist of new nodes. Modulo balancing, the sub trees on the left and right of the path will all be shared (not copied) by the old and new tree structures.
That would potentially mean hundreds of copy then free per frame
I'm not sure where you get "hundreds" from. Are you saying there are hundreds of updates? For some structures there are rewrite rules that allow much of the intermediate values to be used in a mutating manner. See for example, this small examination of vector.
So how does this work in practice ?
In practice people implement what they want and rework parts that are too slow. I might reach for HashMap early, instead of assuming the containers Data.Map will suffice, but I don't go beyond that without evidence.
I would like to make a backup copy of my Core Data database, without using either the File Manager to make a copy of the sqlite file, or using the Persistent Store Coordinator's migratePersistentStore method (for reasons that are too long to explain here). What I want to do is to open a new persistent store with the same MOMD as the original file, create a new Managed Object Context, then iterate over all the objects in my database and insert them into the new context.
This will work for simple entities, but the problem is that my model has about 20 entities, many of them with one-to-many and many-to-many relationships. The slightly more complex solution would be to insert every object into the new MOC, and then hold all the new Managed Objects in memory and use them to tie up all the relationships between the objects in a subsequent passes. But it seems like a really messy solution.
Is there a clean, elegant way to achieve this, that might work for any kind of data model, not just a customized solution for my own model, and without having to hold every object in memory at the same time?
Thanks.
Copying the persistent store is far and above the easiest way to do this – I'd suggest revisiting your reasons against it or explaining what they are.
Copying objects from one context to another – from one on-disk persistent store to another – doesn't necessarily hold them all in memory at the same time. Core Data can turn them into faults.
This question is mainly targeted towards Miguel as the creator of MT.Dialog but I would like to hear opinions of others as well.
I'm currently refactoring a project that has many table views. I'm wondering if I should replace All of them with MT.Dialog.
My pros are:
easy to use
simple code
hope that Xamarin will offer it cross platform one day
Cons:
my cells are complete custom made. Does it make sense in that case?
performance? Is that an issue?
breaking the MVC paradigms (source no longer separated from view and controller)
Is it in general better to just use MT.Dialog or inherit from it for specific use cases? What are your experiences?
To address some of your questions.
The major difference between MonoTouch.Dialog and UITableView is that with the former you "load" all the data that you want to render upfront, and then forget about it. You let MonoTouch.Dialog take care of rendering it, pushing views and taking care of sections/elements. With UITableView you need so provide callback methods for returning the number of sections, the titles for the sections and the data itself.
UITableView has the advantage that to render say a million rows with the same size and the same cells, you dont really have to load all the data upfront, you can just wait to be called back. That being said, this breaks quickly if you use cells with different heights, as UITableView will have to query for the sizes of all of your rows.
So in short:
(1) yes, even if you use custom cells, you will benefit from shorter code and a simpler programming model. Whether you use the other features of it or not, is up to you.
(2) For performance, the issue boils down to how many rows you will have. Like I mentioned before, if you are browsing a potentially large data set, you would have to load all of those cells in memory up front, or like TweetStation, add features to load "on-demand".
The reality is that it will consume more memory, because you need to load your data in MonoTouch.Dialog. Your best optimization technique is to keep your Elements very lightweight. Tweetstation for example uses a "TweetElement" that merely holds the ID to the tweet, and loads the actual contents on demand, to keep the size of the TweetElement in memory very small.
With UITableView, you do not pay for that price. But if you are not using a database of some sort, the data will still be in memory.
If your application calls for the data to be in memory, then you might as well move the data to be elements and use that as your model.
(3) This is a little bit of a straw man. Your data "source" is never really independent of UIKit. I know that people like to talk about these models as being reusable, but in practice, you wont ever be able to reuse a UITableViewSource as a source for anything but a UITableView. It's main use is to support scalable controls that do not require data to be loaded in memory up-front, it is not really about separating the Model from the View.
So what you really have is an adaptor class that bridges the world of the UITableView with your actual data model (a database, an XML list, an in-memory array, a Redis connection).
With UITableView, your adaptor code lives in the constructor and the UITableViewSource. With MonoTouch.Dialog your adatpro code lives in the code that populates the initial RootElement to DialogViewController.
So there are reasons to use UITableView over MonoTouch.Dialog, but it is none of those three Cons.
I use MonoTouch.Dialog (and it's brother QuickDialog for objc) pretty much every time I use a tableview. It does help a lot to simplify the code, and gives you a better abstraction of a table.
There's one exception, though, which is when the table will have thousands and thousands of rows, and the data is in a database. MT.D/QD requires you to load all the data upfront, so you can create the sections, and that's simply too slow if you don't already have the objects in memory.
Regarding "breaking MVC", I kind of agree with you. I never really use the reflection bindings in MT.D because of that fact. Usually I end up creating the root from scratch in code, or use something like JSON (in my fork https://github.com/escoz/MonoMobile.Forms), so that my domain objects don't need to know about MT.D.
I have an application where I pass a NSManagedObject with many (more 30) to UIViews.
I am doing it using assign.
I wonder if its is more expensive then passing a 2 or 3 properties (only the ones that the view needs) instead ?
I would love to get a clear explanation :).
Thanks
Shani
If I understand your question correctly you want to know if it more expensive to pass only the values of the NSManagedObject or the NSManagedObject itself. Objects are stored in the heap memory and are referenced by other objects as a memory address (a byte or two). It does not make duplicates of the object unless you tell it to. So if you use "strong", "assign", "retain", etc. you are not adding much memory. If you use "copy" then a new object is created and for the most part has everything in the original object copied as well. That would be expensive in terms of memory. So I think you're OK holding a reference to the NSManagedObject in each of the UIView without too much worry.
HTH
Multithreading is hard. The only this you can do is program very carefully and follow good advice. One great advice I got from the answers on this forum is to avoid mutable state. I understand this is even enforced in the Erlang language. However, I fail to see how this can be done without a severe performance hit and huge amounts of caches.
For example. You have a big list of objects, each containing quite a lot of properties; in other words: a large datastructure. Suppose you have got a bunch of threads and they all need to access and modify the list. How can this be done without shared memory without having to cache the whole datastructure in each of the threads?
Update: After reading the reactions so far, I would like to put some more emphasis on performance. Don't you think that copying the same data around will make the program slower than with shared memory?
Not each algorithm can be parallelized in a successful manner.
If your program doesn't exhibit any "parallel structure", then you're pretty doomed to use locking and shared, mutable structures.
If your algorithm exhibit structure, then you can express your computation in terms of some patterns or formalism (for ex., a macro dataflow graph) that makes the choice of an immutable datastruct trivial.
So: think in term of the structure of the algorithm and just not in term of the properties of the datastructure to use.
You can get a great start in thinking about immutable collections, where they are applicable, how they can actually work without requiring lots of copying, etc. by looking through Eric Lippert's articles tagged with immutability:
http://blogs.msdn.com/ericlippert/archive/tags/Immutability/default.aspx
I guess the first question is: why do they need to modify the list? Would it be possible for them to return their changes as a list of modifications rather than actually modifying the shared list? Could they work with a list which looks like it's a mutable version of the original list, but is actually only locally mutable? Are you changing which elements are in the list, or just the properties of those elements?
These are just questions rather than answers, but I'm trying to encourage you to think about your problem in a different way. Look at the bigger picture as the task you want to achieve instead of thinking about the way you'd tackle it in a normal imperative, mutable fashion. Changing the way you think about problems is very difficult, but you may find you get some great "aha!" moments :)
There are many pitfalls when working with multiple threads and large sets of data. The advice to avoid mutable state is meant to try to make life easier for you if you can manage to follow the guideline (i.e. if you have no mutable state then multi-threading will be much easier).
If you have a large amount of data that does need to be modified then you perhaps cannot avoid mutable state. An alternative though would be to partition the data into blocks, each of which is passed to a thread for manipulation. The block can be processed and then passed back, and the controller can then perform the updates where necessary. In this scenario you have removed the mutable state from out of the the thread.
If this cannot be done and each thread needs update access to the full list (i.e. it could update any item on the list at any time) then you are going to have a lot of fun trying to make sure you have got your locking strategies and concurrency issues sorted. I'm sure there are scenarios where this is required, and the design pattern of avoiding mutable state may not apply.
Just using immutable data-objects is a big help.
Modifying lists sounds like a constructed argument, but consider granular methods that are unaware of lists.
If you really need to update the structure one way to do this is have a single worker thread which picks up update requests from a fixed area prtected by a mutex.
If you are clever you can update the structure in place without affecting any "reading"
threads (e.g. If you are adding to the end of an array you do all the work to add the new structure but only as the very last instruction do you increment the NoOfMembers count -- the reading threads should not see the new entry until you do this - or - arrange your data as an array of references to structures -- when you want to update a structure you copy the current member, update it, then as the last operation replace the reference in the array)
The other threads then only need to check a single simple "update in progess" mutex only when they activly want to update.