I have a model with a property that looks like this:
When I set its value, for example:
model.isResolved = #YES;
The NSNumber that's kept inside the model "forgets" that it's a boolean:
NSLog(#"%#", strcmp([self.isResolved objCType], #encode(BOOL)) == 0 ? #"equal" : #"different");
Prints "different". What is up with that?
What is up with that?
From the documentation:
Note that number objects do not necessarily preserve the type they are created with.
That's another inconsistency-for-optimization in Cocoa.
Core Data dynamically generates getter and setter methods for all attributes (and relationships) of managed object classes. These accessor methods are different from the "usual" #synthesized accessor methods which are backed up by an instance variable.
In particular, if you set an attribute and then retrieve the attributes value again, you can get an object that is different from the "original" object. The following test shows this, foo1 is an instance of a Core Data entity with the Boolean attribute "show":
NSNumber *yes = #YES;
NSLog(#"yes = %p, type = %s", yes, [yes objCType]);
foo1.show = yes;
NSNumber *val = foo1.show;
NSLog(#"val = %p, type = %s", val, [val objCType]);
Output:
yes = 0x16e595c, type = c
val = 0x744c150, type = i
So even if you set the attribute to a c = char encoded number, the getter method returns a i = int encoded number.
This test was done in the iOS Simulator. Interestingly the same test, running on OS X 64-bit, returns a c = char encoded number.
Therefore, the actual encoding of Boolean and other scalar attributes in Core Data objects should probably be treated as an implementation detail of Core Data.
If you need to check the Core Data type as defined in the model, you can use the objects entity description instead of objCType:
NSEntityDescription *entity = [foo1 entity];
NSAttributeDescription *attr = [[entity attributesByName] objectForKey:#"show"];
NSAttributeType type = [attr attributeType];
if (type == NSBooleanAttributeType) {
NSLog(#"Its a Boolean!");
}
Its stored as a NSNumber - by the way #YES is creating a NSNumber like
[NSNumber numberWithBool:YES]
so to get the bool back out you do:
[isResolved boolValue]
(you can avoid this by ticking the Use Scalar Properties when you create your models)
Related
I am checking if an object I am getting back from the NSURLConnectionDataDelegate is the same object that I originally created. What I have been doing is:
// TESTING TO SEE IF THE RETURNED OBJECT IS THE SAME ONE I CREATED
if(connection == [self connectionPartial]) {
But was just curious is this is the same as doing:
if([connection isEqual:[self connectionPartial]]) {
It's not the same.
if(connection == [self connectionPartial]) {
This compares the address of the objects, eg. if the pointers point to the same instance.
if([connection isEqual:[self connectionPartial]]) {
This compares the contents of the objects. For instance for two separate NSString instances, this will return YES as long as the string content is the same:
NSString *s1 = #"Something";
NSString *s2 = #"Something";
BOOL sameInstances = (s1 == s2); // will be false, since they are separate objects.
BOOL sameContent = [s1 isEqual:s2]; // will be true, because they both are "Something"
The first snippet compares the values of the pointers themselves, just as if they were any primitive type like an int. If the addresses are the same, the expression will evaluate true.
The second sends the message isEqual: to one of the connection instances. Any class can override isEqual: to define "equality" with another instance. It's entirely possible for a class's implementation of isEqual: to be:
- (BOOL)isEqual: (id)obj
{
return arc4random_uniform(2) ? YES: NO;
}
So, no, for almost all classes they are not equivalent. (NSObject, which has the "default" implementation of isEqual:, uses the objects' hashes, which, again by default, are their addresses.)
It sounds like using the equality operator, ==, is correct in your case.
How can I pass a property of a class as a parameter of a method in objective-c?
So as an example assume I have:
a CoreData managed object class MyData with dynamic properties PropA, PropB, PropC all of the same type
I have a utils method that will perform calculations and update one of these properties, which takes as input the MyData instance
how can I arrange so the utils method can accept an indication of which property to use in the calculations and updating? (e.g. PropB)
So then need:
A way to pass an indication of the property to the method (e.g. send as String?)
A way in the method to take this (from 1 above) and use this to both (a) access the value of this property in the MyData instance the method has, PLUS (b) update the property too.
A properties will have setter and getter method. In you case, I assume there are setPropA, setPropB, setPropC for setters and PropA, PropB, PropC for getters.
Then I pass string "PropA" to util, indicate I want to access property named PropA.
The util can get the value by
id val = [aObj performSelector:NSSelectorFromString(#"PropA")];
And set the property by
[aObj performSelector:NSSelectorFromString(#"SetPropA") withObject:newValue];
Or, You can pass setter and getter as parameter by NSStringFromSelector(), turn selector into a NSString. For example, I pass setter and getter by NSDictionary.
NSDictionary *userInfo = [NSDictionary dictionaryWithObjectsAndKeys:
NSStringFromSelector(#selector(setPropA:)), kSetterKey,
NSStringFromSelector(#selector(PropA)), kGetterKey, nil];
// inside myUtil
NSString *setter = [userInfo objectForKey:kSetterKey];
[aObj performSelector:NSSelectorFromString(setter) withObject:newValue];
NSString *getter = [userInfo objectForKey:kGetterKey];
id val = [aObj performSelector:NSSelectorFromString(getter)];
Hope this helps.
Yes, you can pass the property name as a String.
Then you can access the indicated property via Key-Value Coding:
http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/KeyValueCoding/Articles/KeyValueCoding.html
Example:
- (void) myUtilMethod: (MyData *) myData
forPropertyNamed: (NSString *) propName /* which property to operate on (1) */
{
id oldValue = [ myData valueForKey: propName]; // get value (2a)
id newValue = ...; // your calculation here
[myData setValue: newValue forKey: propName]; // set value (2b)
}
I won't bind the 2 classes directly. You should set up a pattern design that allows you to loosely couple them together by creating a class that will do the interface between those two.
I need an NSDictionary which has key in the form of string (#"key1", #"key2") and value in the form of a C-style two-dimensional array (valueArray1,valueArray2) where valueArray1 is defined as :
int valueArray1[8][3] = { {25,10,65},{50,30,75},{60,45,80},{75,60,10},
{10,70,80},{90,30,80},{20,15,90},{20,20,15} };
And same for valueArray2.
My aim is given an NSString i need to fetch the corresponding two-dimensional array.
I guess using an NSArray, instead of c-style array, will work but then i cannot initialize the arrays as done above (i have many such arrays). If, however, that is doable please let me know how.
Currently the following is giving a warning "Passing argument 1 of 'dictionaryWithObjectsAndKeys:' from incompatible pointer type" :
NSDictionary *myDict = [NSDictionary dictionaryWithObjectsAndKeys:valueArray1,#"key1",
valueArray2,#"key2",nil];
Is valueArray2 also an int[][3]? If so, you could use
[NSValue valueWithPointer:valueArray1]
to convert the array into an ObjC value. To retrieve the content, you need to use
void* valuePtr = [[myDict objectForKey:#"key1"] pointerValue];
int(*valueArr)[3] = valuePtr;
// use valueArr as valueArrayX.
If there's just 2 keys, it is more efficient to use a function like
int(*getMyValueArr(NSString* key))[3] {
if ([key isEqualToString:#"key1"]) return valueArray1;
else return valueArray2;
}
Rather than Adding Array Directly as a value in NSDictionary make a custom class in which create variable of NSArray ... and set this class object as value like
NSDictionary *myDict = [NSDictionary dictionaryWithObjectsAndKeys:MyClassObj1,#"key1",
MyClassObj2,#"key2",nil];
where MyClassObj1 and MyClassObj2 are member of MyClass
What kind of information is stored behind such an enum type thing? Example:
typedef enum {
UIViewAnimationCurveEaseInOut,
UIViewAnimationCurveEaseIn,
UIViewAnimationCurveEaseOut,
UIViewAnimationCurveLinear
} UIViewAnimationCurve;
I am not sure if I can safely add such an enum constant to an array. Any idea?
Enums in Objective-C are exactly the same as those in C. Each item in your enum is automatically given an integer value, by default starting with zero.
For the example you provided: UIViewAnimationCurveEaseInOut would be 0; UIViewAnimationCurveEaseIn would be 1, and so on.
You can specify the value for the enum if required:
typedef enum {
UIViewAnimationCurveEaseInOut,
UIViewAnimationCurveEaseIn = 0,
UIViewAnimationCurveEaseOut,
UIViewAnimationCurveLinear
} UIViewAnimationCurve;
This result of this would be: UIViewAnimationCurveEaseInOut is 0; UIViewAnimationCurveEaseIn is 0; UIViewAnimationCurveEaseOut is 1; and so on. However, for basic purposes you shouldn't need to do anything like that; it just gives you some useful info to toy with.
It should be noted based on the above, that an enum can't assume to be a unique value; different enum identifiers can be equal in value to each other.
Adding an enum item to a NSArray is as simple as adding an integer. The only difference would be that you use the enum identifer instead.
[myArray addObject:[NSNumber numberWithInt:UIViewAnimationCurveEaseInOut]];
You can check this out for yourself by simply outputting each enum to the console and checking the value it provides you with. This gives you the opportunity to investigate the details of how it operates. But for the most part you won't really need to know on a day to day basis.
Enums are typically int values. You can store them in an array by wrapping them in an NSNumber:
[myMutableArray addObject:[NSNumber numberWithInt:myAnimationCurve]];
... then get them back out like this:
UIViewAnimationCurve myAnimationCurve = [[myMutableArray lastObject] intValue];
Enums in Objective-C are the same as enums in vanilla C. It's just an int. If you're using an NSArray, then it expects a pointer and you'll get a warning if you try to add an int to it:
NSMutableArray *myArray = [[NSMutableArray alloc] init];
[myArray addObject:UIViewAnimationCurveEaseInOut];
// Last line results in:
// warning: passing argument 1 of 'addObject:' makes
// pointer from integer without a cast
If you're storing a large collection of 32-bit integers, consider using the appropriate CF collection type rather than the NS collection type. These allow you to pass in custom retain methods, which gets rid of the need to box every integer added to the collection.
For example, let's say you want a straight array of 32-bit ints. Use:
CFMutableArrayRef arrayRef = CFArrayCreateMutable(kCFAllocatorDefault, 0, NULL);
The last parameter tells the array to not retain/release the "addresses" you pass in to it. So when you do something like this:
CFArrayAppendValue(arrayRef, 1);
What the array thinks is that you're passing in a pointer to an object living at the memory address 0x1. But since you told it to not call retain/release on that pointer, it gets treated as a standard int by the collection.
FWIW, for educational value, standard NSMutableArrays have equivalent CF types. Through toll-free bridging you can use the CF collection as a standard Foundation collection:
CFMutableArrayRef arrayRef = CFArrayCreateMutable(kCFAllocatorDefault, 0, kCFTypeArrayCallbacks);
NSMutableArray *array = (NSMutableArray *)arrayRef;
[array addObject:#"hi there!"];
NSLog(#"%#", [array objectAtIndex:0]); // prints "hi there!"
You can apply the same tricks to dictionaries (with CFDictionary/CFMutableDictionary), sets (CFSet/CFMutableSet), etc.
I'm messing around with NSNumber for an iPhone app, and seeing what I can do with it. For most of my variables, I simple store them as "int" or "float" or whatnot. However, when I have to pass an object (Such as in a Dictionary) then I need them as an Object. I use NSNUmber. This is how I initialize the object.
NSNumber *testNum = [NSNumber numberWithInt:varMoney];
Where "varMoney" is an int I have declared earlier in the program. However, I have absolutely no idea how to get that number back...
for example:
varMoney2 = [NSNumber retrieve the variable...];
How do I get the value back from the object and set it to a regular "int" again?
Thanks!
(Out of curiosity, is there a way to store "int" directly in an Objective-C dictionary without putting it in an NSNumber first?)
You want -intValue, or one of its friends (-floatValue, -doubleValue, etc.). From the docs:
intValue Returns the receiver’s value
as an int.
- (int)intValue
Return Value The receiver’s value as
an int, converting it as necessary.
The code would be:
int varMoney2 = [testNum intValue];
NSNumber *testNum = [NSNumber numberWithInt:varMoney];
/* Then later... */
int newVarMoney = [testNum intValue];