For those who have seen my other questions: I am making progress but I haven't yet wrapped my head around this aspect. I've been pouring over stackoverflow answers and sites like Cocoa With Love but I haven't found an app layout that fits (why such a lack of scientific or business app examples? recipe and book examples are too simplistic).
I have a data analysis app that is laid out like this:
Communication Manager (singleton, manages the hardware)
DataController (tells Comm.mgr what to do, and checks raw data it receives)
Model (receives data from datacontroller, cleans, analyzes and stores it)
MainViewController (skeleton right now, listens to comm.mgr to present views and alerts)
Now, never will my data be directly shown on a view (like a simple table of entities and attributes), I'll probably use core plot to plot the analyzed results (once I figure that out). The raw data saved will be huge (10,000's of points), and I am using a c++ vector wrapped in an ObjC++ class to access it. The vector class also has the encodeWithCoder and initWithCoder functions which use NSData as a transport for the vector. I'm trying to follow proper design practices, but I'm lost on how to get persistent storage into my app (which will be needed to store and review old data sets).
I've read several sources that say the "business logic" should go into the model class. This is how I have it right now, I send it the raw data, and it parses, cleans and analyzes the results and then saves those into ivar arrays (of the vector class). However, I haven't seen a Core Data example yet that has a Managed Object that is anything but a simple storage of very basic attributes (strings, dates) and they never have any business logic. So I wonder, how can I meld these two aspects? Should all of my analysis go into the data controller and have it manage the object context? If so, where is my model? (seems to break the MVC architecture if my data is stored in my controller - read: since these are vector arrays, I can't be constantly encoding and decoding them into NSData streams, they need a place to exist before I save them to disk with Core Data, and they need a place to exist after I retrieve them from storage and decode them for review).
Any suggestions would be helpful (even on the layout I've already started). I just drew some of the communication between objects to give you an idea. Also, I don't have any of the connections between the model and view/view controllers yet (using NSLog for now).
While vector<> is great for handling your data that you are sampling (because of its support for dynamically resizing underlying storage), you may find that straight C arrays are sufficient (even better) for data that is already stored. This does add a level of complexity but it avoids a copy for data arrays that are already of a known and static size.
NSData's -bytes returns a pointer to the raw data within an NSData object. Core Data supports NSData as one its attribute types. If you know the size of each item in data, then you can use -length to calculate the number of elements, etc.
On the sampling side, I would suggest using vector<> as you collect data and, intermittently, copy data to an NSData attribute and save. Note: I ran into a bit of problem with this approach (Truncated Core Data NSData objects) that I attribute to Core Data not recognizing changes made to NSData attribute when it is backed by an NSMutableData object and that mutable object's data is changed.
As for MVC question. I would suggest that data (model) is managed in by Model. Views and Controllers can ask Model for data (or subsets of data) in order to display. But ownership of data is with the Model. In my case, which may be similar to yours, there were times when the Model returns abridged data sets (using Douglas-Peucker algorithm). The views and controllers were none the wiser that points were being dropped - even though their requests to the Model may have played in a role in that (graph scaling factors, etc.).
Update
Here is a snippet of code from my Data class which extends NSManagedObject. For a filesystem solution, NSFileHandle's -writeData: and methods for monitoring file offset might allow similar (better) management controls.
// Exposed interface for adding data point to stored data
- (void) addDatum:(double_t)datum
{
[self addToCache:datum];
}
- (void) addToCache:(double_t)datum
{
if (cache == nil)
{
// This is temporary. Ideally, cache is separate from main store, but
// is appended to main store periodically - and then cleared for reuse.
cache = [NSMutableData dataWithData:[self dataSet]];
[cache retain];
}
[cache appendBytes:&datum length:sizeof(double_t)];
// Periodic copying of cache to dataSet could happen here...
}
// Called at end of sampling.
- (void) wrapup
{
[self setDataSet:[NSData dataWithData:cache]]; // force a copy to alert Core Data of change
[cache release];
cache = nil;
}
I think you may be reading into Core Data a bit too much. I'm not that experienced with it, so I speak as a non-expert, but there are basically two categories of data storage systems.
First is the basic database, like SQLite, PostgreSQL, or any number of solutions. These are meant to store data and retrieve it; there's no logic so you have to figure out how to manage the tables, etc. It's a bit tricky at times but they're very efficient. They're best at managing lots of data in raw form; if you want objects with that data you have to create and manage them yourself.
Then you have something like Core Data, which shouldn't be considered a database as much as an "object persistence" framework. With Core Data, you create objects, store them, and then retrieve them as objects. It's great for applications where you have large numbers of objects that each contain several pieces of data and relationships with other objects.
From my limited knowledge of your situation, I would venture a guess that a true database may be better suited to your needs, but you'll have to make the decision there (like I said, I don't know much about your situation).
As for MVC, my view is that the view should only contain display code, the model should only contain code for managing the data itself and its storage, and the controller should contain the logic that processes the data. In your case it sounds like you're gathering raw data and processing it before storing it, in which case you'd want to have another object to process the data before storing it in the model, then a separate controller to sort, manage, and otherwise prepare the data before the view receives it. Again, this may not be the best explanation (or the best methods for your situation) so take it with a grain of salt.
EDIT: Also, if you're looking on getting into Core Data more, I like this book. It explains the whole object-persistence-vs-database concept a lot better than I can.
Related
at the heart of it, my app will ask the user for a bunch of numbers, store them via core data, and then my app is responsible for showing the user the average of all these numbers.
So what I figure I should do is that after the user inputs a new number, I could fire up a new thread, fetch all the objects in a NSFetchDescription instance and call it on my NSManagedObjectContext, do the proper calculations, and then update the UI on the main thread.
I'm aware that the rule for concurrency in Core Data is one thread per NSManagedObjectContext instance so what I want to know is, do you I think can what I just described without having my app explode 5 months down the line? I just don't think it's necessary to instantiate a whole a new context just to do some measly calculations...
Based on what you have described, why not just store the numbers as they are entered into a CoreData model and also into an NSMutableArray? It seems as though you are storing these for future retrieval in case someone needs to look at (and maybe modify) a previous calculation. Under that scenario, there is no need to do a fetch after a current set of numbers is entered. Just use a mutable array and populate it with all the numbers for the current calculation. As a number is entered, save it to the model AND to the array. When the user is ready to see the average, do the math on the numbers in the already populated array. If the user wants to modify a previous calculation, retrieve those numbers into an array and work from there.
Bottom line is that you shouldn't need to work with multiple threads and merging Contexts unless you are populating a model from a large data set (like initial seeding of a phonebook, etc). Modifying a Context and calling save on that context is a very fast thing for such a small change as you are describing.
I would say you may want to do some testing, especially in regard to the size of the data set. if it is pretty small, the sqlite calls are pretty fast so you may get away with doing in on the main queue. But if it is going to take some time, then it would be wise to get it off the main thread.
Apple introduced the concept of parent and child managed object contexts in 2011 to make using MO contexts on different threads easier. you may want to check out the WWDC videos on Core Data.
You can use NSExpression with you fetch to get really high performance functions like min, max, average, etc. here is a good link. There are examples on SO
http://useyourloaf.com/blog/2012/01/19/core-data-queries-using-expressions.html
Good luck!
I've been using app delegate interface variables through-out my app to quickly access app wide bits of data.
I believe this is causing efficiency issues in my app. And I've finally decided to move to NSUserDefaults. Unless you suggest some other way ?
Often I will want to access the same variable through-out a view, so it doesn't make sense to access this variable using NSUserDefaults each time. I figure it will be slow.
So I'm thinking a class which will read all the values into an array of some kind, in viewDidLoad and then if a value is altered, save / synchronize and update the class variable.
But if I push to a view, I guess I'm going to have to save then too.
This is beginning to sound a bit messy.
However, I'm just wondering what approach will be efficient and easy to use ?
A shared Model object (using the MVC paradigm) is the usual recommended way to share variables to multiple other objects or classes. This Model object can be a singleton, or other objects can locally copy or be delegated a reference to a single Model object for fast access to its getters/setters.
The fastest variables access method is often just using C global variables (since regular C is a subset of Objective C), if you are willing to deal with potentially more difficult code reuse and debugging problems.
NSUserDefaults is just using the file system as a key-value dictionary global variable. Slow, and with the many of same structure problems as C global variable use.
It really depends on the size of data you're accessing. The serialization of several NSDictionary objects to disk may be enough to cause your app to lag.
If you're serious about iOS development, you should learn to use Core Data. Anything NSUserDefaults can do, Core Data does better. The only one advantage NSUserDefaults provides reduces lines of code in tiny one-off apps. Think of Core Data like laying the foundations for a house, and NSUserDefaults like pitching up a tent.
If you need to use variables like arrays or sets you'd better not use NSUserDefaults since on synchronize they save all stored values onto disk (of course if you don't need storing them to disk). If you need to store large amounts of data in object graphs with relations like this
User
-name
-lastname
-email
-messages
Message
-date
-text
Then you'd better start looking into CoreData framework, it's very easy to use and allows to store large data sets to disk and access them quickly.
If you just need some class that would, for instance, store current user profile values like username, email etc. Try creating a singleton class - which is quite the same as using AppDelegate, but it's much clearer to use when you're separating your data into smaller chunks.
Anyways I wrote this answer based on my assumptions only. It would be nice to know what data kinds you are working with and what is its lifecycle in your app.
I would like to make a data manager class that retrieves data from plist, and I wonder if I should make a class with all class methods which read plist every time when the method is called and returns requested value, or create a class initializer which initializes an array(instance variable) with the plist data and all methods are instance methods which get data from the array.
I would like to know which is more expensive:reading plist many times (like 50 times) or instantiating an object, or simply which is better.
Thank you for your help in advance.
This is one of the classical tradeoffs in programming - speed vs. memory use. The technique of reading something once and storing it on a faster medium (in this example, in memory) is called caching. It's very popular, and for good reason. Mass storage devices are still magnitudes slower than RAM, and network access is magnitudes slower than local mass storage.
If you assume the data will be asked of the manager often, and if you assume the plist won't change (or you can detect changes), then read the plist on the first access to the getter, store it in an iVar, and answer only the iVar as long as the plist hasn't changed. This uses a little more memory, but is a lot faster for subsequent accesses.
NOTE: This approach would be harmful for very, very large files. If you are concerned about memory usage, implement the - (void)didReceiveMemoryWarning method in your viewControllers, and flush the cache (delete it) when you are low on memory.
A getter method could look like this:
- (NSArray *)data
{
if (!cacheArray) {
//what we do now is called "lazy initialization": we initialize our array only when we first need the data.
//This is elegant, because it ensures the data will always be there when you ask for it,
//but we don't need to initialize for data that might never be needed, and we automatically re-fill the array in case it has been deleted (for instance because of low memory)
cacheArray = ... //read array from plist here; be sure to retain or copy it
}
return cacheArray;
}
I am creating a simple DB application for reports. According to DB design theory, you should never store the same information twice. This makes sense for most DB applications, but I need something that you can simply select a generic topic, you could then keep the new instance copy of the generic topic untouched or change the information but the generic topic should not be modified by modifying the instance copy, but the relationship needs to be tracked between the original topic and the instance copy of the topic.
Confusing, I know. Here is a diagram that may help:
I need the report to be immutable or mutable based off of the situation.
A quick example would be you select a customer, then you finish your report. A month later the customer's phone number changes so you update the customer portion of the DB, but you do not want to pull up a finished report and have the new information update into the already completed report.
What would be the most elegant solution to this scenario?
This may work:
But by utilizing this approach I would find myself using looping statements and if statements to identify the relationships between Generic, Checked Off and Report.
for (NSManagedObject *managedObject in checkedOffTaskObjects) {
if ([[reportObject valueForKeyPath:#"tasks"] containsObject:managedObject]) {
if ([[managedObject valueForKeyPath:#"tasks"] containsObject:genericTaskObjectAtIndexPath]) {
cell.backgroundView = [[[UIImageView alloc] initWithImage:[UIImage imageNamed:#"cellbackground.png"]] autorelease];
}
}
}
I know a better solution exists, but I cannot see it.
Thank you for time.
It's tricky to be very precise without knowing much about what exactly you're modelling, but here goes...
As you've noted, there's at least two strategies to get the "mutable instance copies of a prototype" functionality you want:
1) When creating an instance based on a prototype, completely copy the instance data from the prototype. No link between them thereafter.
PRO: faster access to the instance data with less logic involved.
CON 1: Any update to your prototype will not make it into the instances. e.g. if you have the address of a company wrong in the prototype.
CON 2: you're duplicating database data -- to a certain extent -- wasteful if you have huge records.
2) When creating an instance based on a prototype, store a reference to the 'parent' record, i.e. the prototype, and then only store updated fields in the actual instance.
PRO 1: Updates to prototype get reflected in all instances.
PRO 2: More efficient use of storage space (less duplication of data)
CON: more logic around pulling an instance from the database.
In summary: there's not any magical solution I can think of that gets you the best of both of these worlds. They're both valid strategies, depending on your exact problem and constraints (runtime speed versus storage size, for example).
If you go for 2), I certainly don't think it's a disaster -- particularly if you model things well and find out the best most efficient way to structure things in core data.
I'm developing an iPhone application and am new to Objective-C as well as SQLite. That being said, I have been struggling w/ designing a practical data management solution that is worthy of existing. Any help would be greatly appreciated.
Here's the deal:
The majority of the data my application interacts with is stored in five tables in the local SQLite database. Each table has a corresponding Class which handles initialization, hydration, dehydration, deletion, etc. for each object/row in the corresponding table. Whenever the application loads, it populates five NSMutableArrays (one for each type of object). In addition to a Primary Key, each object instance always has an ID attribute available, regardless of hydration state. In most cases it is a UUID which I can then easily reference.
Before a few days ago, I would simply access the objects via these arrays by tracking down their UUID. I would then proceed to hydrate/dehydrate them as I needed. However, some of the objects I have also maintain their own arrays which reference other object's UUIDs. In the event that I must track down one of these "child" objects via it's UUID, it becomes a bit more difficult.
In order to avoid having to enumerate through one of the previously mentioned arrays to find a "parent" object's UUID, and then proceed to find the "child's" UUID, I added a DataController w/ a singleton instance to simplify the process.
I had hoped that the DataController could provide a single access point to the local database and make things easier, but I'm not so certain that is the case. Basically, what I did is create multiple NSMutableDicationaries. Whenever the DataController is initialized, it enumerates through each of the previously mentioned NSMutableArrays maintained in the Application Delegate and creates a key/value pair in the corresponding dictionary, using the given object as the value and it's UUID as the key.
The DataController then exposes procedures that allow a client to call in w/ a desired object's UUID to retrieve a reference to the actual object. Whenever their is a request for an object, the DataController automatically hydrates the object in question and then returns it. I did this because I wanted to take control of hydration out of the client's hands to prevent dehydrating an object being referenced multiple times.
I realize that in most cases I could just make a mutable copy of the object and then if necessary replace the original object down the road, but I wanted to avoid that scenario if at all possible. I therefore added an additional dictionary to monitor what objects are hydrated at any given time using the object's UUID as the key and a fluctuating count representing the number of hydrations w/out an offset dehydration. My goal w/ this approach was to have the DataController automatically dehydrate any object once it's "hydration retainment count" hit zero, but this could easily lead to significant memory leaks as it currently relies on the caller to later call a procedure that decreases the hydration retainment count of the object. There are obviously many cases when this is just not obvious or maybe not even easily accomplished, and if only one calling object fails to do so properly I encounter the exact opposite scenario I was trying to prevent in the first place. Ironic, huh?
Anyway, I'm thinking that if I proceed w/ this approach that it will just end badly. I'm tempted to go back to the original plan but doing so makes me want to cringe and I'm sure there is a more elegant solution floating around out there. As I said before, any advice would be greatly appreciated. Thanks in advance.
I'd also be aware (as I'm sure you are) that CoreData is just around the corner, and make sure you make the right choice for the future.
Have you considered implementing this via the NSCoder interface? Not sure that it wouldn't be more trouble than it's worth, but if what you want is to extract all the data out into an in-memory object graph, and save it back later, that might be appropriate. If you're actually using SQL queries to limit the amount of in-memory data, then obviously, this wouldn't be the way to do it.
I decided to go w/ Core Data after all.