How can I write an object to NSUserDefaults as plain text?
I've written a unit conversion system for a scientific application that basically operates around the principal of "unit objects". These objects represent a specific unit with a specific value in a given domain. The domain is represented by the class name, since everything is organized so that a time unit is literally called "TimeUnit" and a length unit is literally called "LengthUnit".
The current value of a "unit" is represented by a double, and the "unit" itself (ie, "meters", "kelvin", "hours", "pounds", etc) is represented by an NSString.
I need to be able to write these objects to NSUserDefaults.
I've already implemented a system using NSCoding and NSKeyedArchiver. This works great, I can store and retrieve units from NSUserDefaults and everything works.
The problem is that the output from NSKeyedArchiver is too big. The base64 encoded data you get from it is this gigantic chunk of characters, which is impossible to change through a plain text editor and somewhat difficult to debug. I know you can override this to a certain extent by using NSPropertyListXMLFormat_v1_0 with NSKeyedArchiver's setOutputFormat, but even that produces a fairly verbose output with a whole bunch of stuff I don't really care about (I know all that data is there for a reason, but I'm guessing that's because NSKeyedArchiver is designed to handle a lot of different situations).
It would be much easier if I could encode these objects using my own format. Something almost like:
class_name:double_value:unit_name
So a temperature unit with a value of 22 degrees celsius would become:
TemperatureUnit:22:celsius
What is the best way of achieving something like this? Should I be subclassing NSCoder? Will this let me store the objects as a plain text string in NSUserDefaults and not a base64 encoded chunk of binary data?
For something like this you could add "serialize" and "deserialize" methods (or whatever names you want). The former returns a string and the latter takes a string and returns your object.
+ (MyUnitClass *)deserialize:(NSString *)encodedString {
// split string, create new object, assign values
}
- (NSString *)serialize {
return [NSString stringWithFormat:#"%#:%f:%#", self.classname, self.double_value, self.unit_name];
}
Now you can call serialize on one of your objects and store the string in NSUserDefaults. Use deserialize to create an object from a string you have stored in NSUserDefaults.
Related
What I want to do:
I want to implement ability for user to use CIFilters on image. So I need somehow to fix it's sequence. For example if user opens image, then applies CIGaussianBlur, and then CIColorControls and then CISepia, I need to get result like that:
On user opened image apply CIGaussianBlur -> on CIGaussianBlur output image apply CIColorControls - > on CIColorControls output image apply CISepia.
Thats OK. But what if then user turns off CIGaussianBlur? I need then to repeat this effect's sequence just without blur. It would look like this:
On user opened image apply CIColorControls -> on CIColorControls output image apply CISepia.
The question
Is it possible to do something like this:
After applying any effect, add some string in NSMutableArray or NSMutableDictionary. Then when applying another effect, check NSMutableArray or NSMutableDictionary contents like that:
if object at index 0 is equal to "blur", apply blur on source image, then take blur's output image like current effect's input image
And so on? So that effects would be re-applied every time in their sequence made by user.
If it is possible maybe someone could suggest me any solution?
I think that this is a great instance for the factory idea to be used.
You should store your array of filters to process the image as an array - that maintains sort order, and is fairly straightforward to deal with (other than something like a NSCountedSet).
The next logical question to ask, then, is how do we apply the factory pattern here? The most important thing to consider is what type should the context object be? Here are a few thoughts:
Using NSString as a constant identifier.
Probably the simplest to start, its , and easy to understand - the downside is that it's slower than other options, and can get to be quite the complex if-else block, as you cannot use a switch statement on a NSString.
NSNumber, wrapping an enum value.
This is probably one of the better options. You can convert right down to an int variable, which compares quite fast on almost any processor I can imagine, and, if you use ObjC 2.5's fancy literals, you could do something like this:
[filters addObject:#(filterType_gaussianBlur)];
Where filterType_gaussianBlur is an enum constant or something.
Another advantage to using an enum is the support for switch statements out of the box. It cleans up your code if done properly, it's faster than a large if-else block, the only thing to look out for is ensuring that you break properly!
Storing Class objects
This one may require some explaining. In objective-c, you can actually store a reference to the type of an object, CIGaussianBlur, NSString, NSData, etc.
This class "object" allows you to dynamically create an object based just on it's type, for example:
Class cls = nil;
if (stringMutable)
cls = [NSMutableString class];
else
cls = [NSString class];
NSString *mutableOrNot = [[cls alloc] initWithString:#"Hello World!"];
However, the disadvantage to this approach would be the inability to configure the objects after they are initialized, and you must use the same selector (method) to initialize each one. If that is not an issue (I do not use CoreImage), then using the Class approach should be fine.
All in all, use whatever makes sense in the situation. If these filters need no additional configuration after they have been initialized, then approach 3 makes a lot of sense. I personally wouldn't recommend approach 1, unless it really is necessary to use a string. Whenever you can, always try to control the values that an object can have. It makes your life much easier, trust me.
How does NSArchiver serialize to file? I assume it's serialized in binary format, is that correct? What if I want to store it in string so I can store into SQLite database? Do I need to write my own custom NSArchiver? If so, how do I go about doing that? Are there any tutorials out there?
p.s. I do realize Core Data can do this but let me cross that option out for now.
You can archive to an NSData object instead of to a file, if you want, with +archivedDataWithRootObject:. It won't be a "string," but that's fine, because an NSString in Cocoa represents a sequence of Unicode characters, while an NSData represents a sequence of bytes (which you could easily store wherever you want, including in a database).
Note that you really should be using NSKeyedArchiver instead:
+ (NSData *)archivedDataWithRootObject:(id)rootObject
+ (id)unarchiveObjectWithData:(NSData *)data
What is the preferred datatype to parse JSON into in Objective-C? I'm looking for a data type that mirrors the ability to use key=>value style and just array form.
Typically libraries (such as SBJson) will return their parsed results as either an NSArray or an NSDictionary, just depending on if the parsed JSON element was an object or an array.
From SBJsonParser.h:
/**
#brief Return the object represented by the given string
This method converts its input to an NSData object containing UTF8 and calls -objectWithData: with it.
#return The NSArray or NSDictionary represented by the object, or nil if an error occured.
*/
- (id)objectWithString:(NSString *)repr;
In your question you asked "I'm looking for a data type that mirrors the ability to use key=>value", that is by definition exactly what a dictionary is... so, you're probably looking for NSDictionary.
The question doesn't make any sense. The JSON string itself determines what type of object you are going to get when it's deserialized. It can be a string, number, array or dictionary. You have to be prepared to receive any of those. If you use NSJSONSerialization, you'll notice that the decoding methods return id, which means you don't know the type ahead of time. You will have to use isKindOfClass: to figure out what you actually got back.
I'm a nwebie in Core Data, i have designed a navigation based application and some of the data i use are created on run time(come from a URL via JSON). I took a few tutorials an searched for almost a day but haven't still realized how to save the incoming JSON data to the Entity (or event?) in my Core Data model. I fetch the data in the DetailViewController class and i need to save this data to Core Data(I have prepared an Entity with 7 properties). Can anyone please help?(If you know a good tutorial or sample code i will be pleased)
EDIT This may be a little specific but i really have trouble with and need just a little help.
My data comes to the app from a kind of restful server(i wrote it in PHP), firstly user enters his/her login informations(which i have saved to the database on server before) and when the response data comes i will use different elements of it in differen views(for example the user_id will be used on a view and the buttonData etc on other views). My question is, how will i save JSON data into my core data model(has tree Entities for the moment). Thanks in advance
Note: I lokked arround a lot but couldn't find any answer&tutorial about an app like mine
The best way to do that would be to create entities corresponding to JSON structure. Easiest was is when each JSON object becomes an entity, and arrays become arrays of entities. Be reasonable, however, and don't introduce too much overkill for JSON subobjects that are essentially part of its superobject.
When you have created entities, you can start off with the parsing and translation. Use some JSON framework (starting from iOS5 there's one from Apple) and parse JSON string into object tree, where root item is either an NSArray or NSDictionary, and subelements will be NSArray, NSDictionary, NSNumber, NSString or NSNull.
Go over them one by one in iterational loops and assign according values to your core data entity attributes. You can make use of NSKeyValueCoding here and avoid too much manual mapping of the attribute names. If your JSON attributes are of the same name as entity attributes, you'll be able to just go over all dictionary elements and parse them into attributes of the same name.
Example
My parsing code in the similar situation was as follows:
NSDictionary *parsedFeed = /* your way to get a dictionary */;
for (NSString *key in parsedFeed) {
id value = [parsedFeed objectForKey:key];
// Don't assign NSNull, it will break assignments to NSString, etc.
if (value && [value isKindOfClass:[NSNull class]])
value = nil;
#try {
[yourCreatedEntity setValue:value forKey:property];
} #catch (NSException *exception) {
// Exception means such attribute is not defined in the class or some other error.
}
}
This code will work in trivial situation, however, it may need to be expanded, depending on your needs:
With some kinds of custom mappings in case you want your JSON value be placed in differently named attribute.
If your JSON has sub-objects or arrays of sub-objects, you will need to detect those cases, for example in setters, and initiate new parsing one level deeper. Otherwise with my example you will face the situation that assigns NSDictionary object to an NSManagedObject.
I don't think it is reasonable to dive into these, more advanced matters in scope of this answer, as it will expand it too much.
I suggest you to use this library : https://github.com/TouchCode/TouchJSON
And then if you want to make a factory to parse json and feed your code data, you can use selectors to call methods to fill all your attributes.
Chances are your JSON data gets converted to an NSDictionary or NSArray (or some combination of the two). Simply extract the key/values from the JSON structure and add them to your entity class.
This lib helps me lot
Features
Attribute and relationship mapping to JSON key paths.
Value transformation using named NSValueTransformer objects.
Object graph preservation.
Support for entity inheritance
Works vice-versa
I have a table column that I'm binding to a value in an NSArrayController. What I'm trying to do is display only a substring of the actual value in the array controller. The way I've been trying to do that so far is by creating an NSValueTransformer subclass, and then doing the string manipulation in the transformedValue method. However, I can't figure out how to get the incoming value turned into a string (it's of type NSConcreteValue), and maybe there's an easier way to do this without value transformers.
Sounds like you're doing presentation-side formatting, in which case you should be using a formatter instead.
Then again, if this is a string containing multiple values (e.g., something like “from 42 to 100”), you should make a model object from it instead, and store those in the array controller. Then you can bind your table columns to specific properties of the model objects, and not have to worry about picking the string apart and then reassembling it (except when you load and later save the model).
Edit: Never mind; I didn't see that the object values are NSValues, not NSStrings.
You can get a string representation of any object using the -description method, but for instances of NSValue it's unlikely to print anything especially meaningful. In other words, it's up to your value transformer to interpret the passed-in object and produce a string. If it's an NSValue instance, the question is what type of data that instance contains. Once you know that, you can write code to represent it as a string (similar to NSStringFromRect()).