I noticed some code example in Apple's documentation shows the following style when declaring the property:
#property (nonatomic, getter=isActivated) BOOL activated;
I understand it allows you to specify a certain name for your getter method. I'd like to know what is the reason and advantage to use this style.
Will I be able to use the dot notation to get the value (e.g. BOOL aBool = someObject.isActivated)? Or should I use
[someObject isActivated];
to access the property? Thanks!
No, the getter keyword only changes the method name. The idea is that you'll access the property just like a variable:
if (self.activated) { ... }
self.activated = YES;
But when you're sending a message to the object, it's readable code: if ([self isActivated]) { ... }.
Kind of the latter. You don’t have to use the method—calling someObject.activated will still work—but it lets you improve the semantics of your class’s interface. A method called -activated could return the value of the ivar activated, or it could do something more esoteric (like activating the object); isActivated clearly returns a Boolean value for whether or not the object is “activated”.
Related
I'd like to define a member in objective C class that can only be read outside the class (public getter). The writing (setter) however shell remain private.
I've read that It's possible to conceal object's setter using the readonly property while exposing the getter using #synthesize syntax but I'm not sure how it works exactly.
Base on this information here's what I did, and I wonder what's happening here under the hood, and if this is the proper way of doing so ?
#interface MyObject : NSObject
//This line suppose to conceal both getter and setter.
#property (readonly) MyCppBaseObject *myCppBaseObject;
- (void)setMyCppBaseObject:(NSString *)SomeInput;
#end
// This line suppose to tell the compiler that the getter is exposed
#synthesize myCppBaseObject = _myCppBaseObject;
#implementation MyObject
-(void)setMyCppBaseObject:(NSString *)SomeInput {
if (someCondition) {
self.myCppBaseObject = new myCppObjectDerive1(...);
} else {
self.myCppBaseObject = new myCppObjectDerive2(...);
}
}
#end
P.S. I've seen a different approach explained in the following link, but I wish to understand the above implementation.
First, you should use the private extension described in the link you provide. That's the correct way to do this.
But to your question, what you've written here is not quite correct.
#property (readonly) MyCppBaseObject *myCppBaseObject;
This line makes a promise to implement -myCppBaseObject. That's all it does. It's just a promise. If you fail to live up to your promise, the compiler will auto-generate (synthesize) one for you using a backing ivar.
- (void)setMyCppBaseObject:(NSString *)SomeInput;
This line is not correct for your purposes. It's making a public setter. But you said you don't want the setter to be public. You could put this in a private extension, however.
#synthesize myCppBaseObject = _myCppBaseObject;
This asks the compiler to create a backing ivar _myCppBaseObject for the property myCppBaseObject. This is the default behavior, however, and so isn't required. (There was a time when it was, but that was a very long time ago.)
-(void)setMyCppBaseObject:(NSString *)SomeInput {
if (someCondition) {
self.myCppBaseObject = new myCppObjectDerive1(...);
} else {
self.myCppBaseObject = new myCppObjectDerive2(...);
}
}
This code is completely incorrect. It is an infinite loop, since self.x =... is syntactic sugar for [self setX:...]. What you mean is:
_myCppBaseObject = ...
You're going to create a lot of headaches having the name of the custom setter be exactly the expected name of the default setter, but with a different type. Don't do this. In theory it could work most of the time, but don't. Especially when there's dot-syntax involved. Especially since one of your objects does not appear to be ARC-compatible (i.e. a C++ object), this is going to really be a trap for really confusing problems. Name your setter differently.
If all i have is a list of keys, is there an elegant way to test an object for read-only vs. read/write properties? I realize I could string-bang the key:
NSString *setterString = [#"set" stringByAppendingString:[someKeyString capitalizedString]];
BOOL isReadWrite = [myObject respondsToSelector:NSSelectorFromString(setterString)];
Or better yet try to set a value for the key and check for an NSUndefinedKeyException - but using exceptions for non-exceptional behavior seems like poor form.
To be clear, i'd like to programmatically audit an object's properties and tell the difference between, for instance,
#property (readonly) NSString *someReadOnlyKey
#property NSString *someReadWriteProperty
edit: To be more clear, it shouldn't matter if the keys are implemented as #propertys or manual getter/setter. Only concerned about public interface. And thanks for asking what I'm trying to accomplish - that's probably more important to get right in the first place.
I'm trying to sketch out some code that generates graphic representations of an object's keys. All the keys are known beforehand - but I won't always know which keys will be sett-able (it's up to the particular subclass to implement)
You have two valid approaches:
the respondsToSelector: method
the runtime "trick" as exposed in Tommy's answer
I'll try to summarize the implication of both approaches, along with their drawbacks. You can then choose the approach more suitable to your needs.
Case 1
//MyClass.h
#property (readonly) NSString *someReadOnlyKey;
runtime => readonly
respondsToSelector => NO
Ok, everything works as expected.
Case 2
//MyClass.h
#property (readonly) NSString *someReadOnlyKey;
///////////////////////////////////////////////////////////////////////////
//MyClass.m
#property (readwrite) NSString *someReadOnlyKey;
runtime => readwrite
respondsToSelector => YES
If the property definition has been overridden, you'll get the actual property attribute being used. It doesn't matter whether you query about it from where the setter is visible, i.e. inside the class definition, or from outside. You will get the actual definition that has been used to synthesize the accessor methods.
Case 3
//MyClass.h
#property (setter=myCoolSetter) NSString *someReadOnlyKey;
runtime => readwrite
respondsToSelector => NO
With a custom setter name, the respondsToSelector trick won't work anymore, whereas the runtime is still providing the correct information.
Case 4
//MyClass.h
#property (readonly) NSString *someReadOnlyKey;
///////////////////////////////////////////////////////////////////////////
//MyClass.m
- (void)setSomeReadOnlyKey:(NSString *)someReadOnlyKey {
_someReadOnlyKey = someReadOnlyKey;
}
runtime => readonly
respondsToSelector => YES
This time the runtime is failing, since the setter is there, but the property definition "doesn't know" about it.
Assuming you're fine with asking the metaclass (ie, you don't want to allow for potential instance-specific patches to the dispatch table), you can get property attributes from the runtime. Specifically:
// get the property; yes: that's a C string. This can see only things
// declared as #property
objc_property_t property =
class_getProperty([instance class], "propertyName");
/* check property for NULL here */
// get the property attributes.
const char *propertyAttributes = property_getAttributes(property);
You can then check those attributes for read-only:
NSArray *attributes =
[[NSString stringWithUTF8String:propertyAttributes]
componentsSeparatedByString:#","];
return [attributes containsObject:#"R"];
If you want to be completely thorough, you should also check protocols via class_copyProtocolList and protocol_getProperty, in order to catch any #propertys that are incorporated into the class that way and — as noted below by Gabriele — some caveats apply around class extensions.
This might be one of the few times I might consider the exception route - that is of course if you need handle KVC.
The reason being that just checking for a set<key>: style method name is not really enough to determine if something is really settable, especially when you bring KVC into it.
KVC has well defined search patterns for when you try to set values, which can be seen in the Key Value Coding Programming Guide. This search path has many stops and even allows objects a last chance "are you sure you don't want to handle this setter" method with setValue:forUndefinedKey:, which can be used to prevent the NSUndefinedKeyException exception being thrown
You can use the following:
if ([obj respondsToSelector:#selector(setSomeReadOnlyKey:someReadOnlyKey:)]) {
// Not a readonly property
}
else
// Readonly property
This should work. Essentially, I am checking if the setter method exists.
I am doing the 'Your Second App' tutorial and it has me add the line of code below that is a setter for the masterBirdSightingList property. I just have a basic questions on it:
Is this line the same as if I were to synthesize it? If not, what makes it different?
- (void)setMasterBirdSightingList:(NSMutableArray *)newList
{
if (_masterBirdSightingList != newList) {
_masterBirdSightingList = [newList mutableCopy];
}
}
If the property is defined as:
#property (nonatomic, copy) NSMutableArray *masterBirdSightingList;
then implementing this method is not the same as simply using #synthensize masterBirdSightingList;.
Defining a property with copy semantics for a mutable container type doesn't actually work as expected using the default synthesized setter.
Without the explicit method, you actually end up with the property referencing an immutable copy of the array.
By using the code you posted, instead of relying on the synthesized method, you get the proper and expected behavior of having a mutable copy of the original array.
Another way to look at this is that calling copy on an NSMutableArray returns an NSArray, not an NSMutableArray. This is why the default synthesized property setter doesn't work as expected (when dealing with a mutable container property). So you must implement the setter yourself and call mutableCopy on the parameter.
In my implementation I have getters and setters like below. I want to use properties and synthesize the getters and setters but have a few questions.
- (NSString *)title {
return title;
}
- (void)setTitle:(NSString *)value {
if(title != value) {
[title release];
title = [value retain];
}
}
If I was to convert that to a property, what attributes would I use? Am I right in thinking:
readwrite so both getters and setters are present
retain so that it increase the retain value of the value string so the object don't lose it.
Am I right with the above?
One final thing. I have the method below ...
- (void)setReleaseDate:(NSString *)value {
// YYYY-MM-DD HH:MM:SS +HHMM
if([releaseDate description] != value) {
[releaseDate release];
releaseDate = [[NSDate alloc] initWithString:value];
}
}
Am I right in thinking I still have to include that method because it contains code that the synthesized getter would not include?
Thanks.
For your title property, you can declare it in your class interface as follows:
#property (nonatomic, retain) NSString* title;
Which is the same as the following:
#property (readwrite, nonatomic) NSString* title;
readwrite is a default setting. Most of the time you will want setters for your properties, so for the times when you don't you would use the non-default readonly to specify this.
The nonatomic part basically means that the accessors will be faster, and is typically used. You can find out more information about this here: What does the property "Nonatomic" mean?.
For your second question, you can implement your own accessors if you wish. If you do this then it kind of 'overrides' the accessor that would be generated by Objective-C. Remember that you have to keep to the naming conventions. So in your example, the "setReleaseDate:" method you've defined would be used for the setter method for the property "releaseDate" - which is completely correct! :) The problem you have though is that you're passing an *NSString** to set the date, which means that this method won't override the default setter that would be used if you synthesized the property. You have to pass a value of the same type as the one you're setting as the single argument, so for this case you would have to pass an *NSDate**.
You must also ensure that if you provide your own implementation of an accessor that it does what is declared in the interface. I presume your releaseDate property should be declared as retain.
Your assertion about using readwrite as well as retain is correct as it would create semantically equivalent code to what you have posted.
The releasedate property setter can't be synthesized as you're transforming a NSString into a NSDate to store it, that also avoids common issues with NSString properties, for which you'd better use copy to avoid problems with NSMutableString.
Other than that, your code is fine, except that for string comparison you may want to replace the simple pointer check != with isEqualToString, see Comparing Strings in Cocoa.
It is common, though not always required, to use copy semantics for NSString properties, to avoid issues with NSMutableString objects being changed behind your back.
Otherwise, you seem to be pretty much on top of it.
Can custom property attributes be created in Objective-C just like in VB.NET? For example, in VB.NET you can create the "Browsable" attribute and read it at runtime to determine whether you should display a property or not.
Public Class Employee
<Browsable(True)> _
Public Property Property1() As String
Get
End Get
Set(ByVal Value As String)
End Set
End Property
<Browsable(False)> _
Public Property Property2() As String
Get
End Get
Set(ByVal Value As String)
End Set
End Property
End Class
I would like to do the same in Objective-C, even if it is a fixed attribute that can only be set at compile time and cannot be changed at all.
What I'm trying to do is to add an attribute to properties of my class to determine whether the properties should be serialized or not.
I know the standard Objective-C attributes (readonly, nonatomic, etc.), but those don't help me... unless you have a creative way of using them. I also looked into using C attributes with the __attribute__(("Insert attribute here")) keyword, but C has specific attributes that serve specific purposes, and I'm not even sure you can read them at runtime. If I missed one that can help me, let me know.
I tried using typdef. For example:
typdef int serializableInt;
serializableInt myInt;
and use the property_getAttributes() Objective-C runtime function, but all it tells me is that myInt is an int. I guess typedef is pretty much like a macro in this case... unless I can create a variable of type serializableInt at runtime. Anyhow, here's Apple's documentation on the values you get from property_getAttributes().
The other requirement is that this attribute has to work with NSObject sub-classes as well as primitive data types. I thought about the idea of adding to the class a black lists or white lists as an ivar that would tell me which properties to skip or serialize, which is basically the same idea. I'm just trying to move that black/white list to attributes so it's easy to understand when you see the header file of a class, it's consistent across any class I create and it's less error prone.
Also, this is something to consider. I don't really need the attribue to have a value (TRUE or FALSE; 1, 2, 3; or whatever) because the attribute itself is the value. If the attribute exists, then serialize; otherwise, skip.
Any help is appreciated. If you know for sure that this is not possible on Objective-C, then let me know. Thanks.
If you want to add attribute to property, class, method or ivar, you can try to use github.com/libObjCAttr. It's really easy to use, add it via cocoapods, and then you can add attribute like that:
#interface Foo
RF_ATTRIBUTE(YourAttributeClass, property1 = value1)
#property id bar;
#end
And in the code:
YourAttributeClass *attribute = [NSDate RF_attributeForProperty:#"bar" withAttributeType:[YourAttributeClass class]];
// Do whatever you want with attribute, nil if no attribute with specified class
NSLog(#"%#", attribute.property1)
unless i've missed your point…
i'd recommend declaring a protocol. then using instances of objc objects as variables in your objc classes which adopt the protocol.
#interface MONProtocol
- (BOOL)isSerializable;
- (BOOL)isBrowsable;
/* ... */
#end
#interface MONInteger : NSObject <MONProtocol>
{
int value;
}
- (id)initWithInt:(int)anInt;
#end
#interface MONIntegerWithDynamicProperties : NSObject <MONProtocol>
{
int value;
BOOL isSerializable;
BOOL isBrowsable;
}
- (id)initWithInt:(int)anInt isSerializable:(BOOL)isSerializable isBrowsable:(BOOL)isBrowsable;
#end
// finally, a usage
#interface MONObjectWithProperties : NSObject
{
MONInteger * ivarOne;
MONIntegerWithDynamicProperties * ivarTwo;
}
#end
if you want to share some implementation, then just subclass NSObject and extend the base class.
you'd then have a few variants to write for the types/structures you want to represent.
The deficiency with the other answers I've seen so far is that they are implemented as instance methods, i.e., you need to have an instance already before you can query this metadata. There are probably edge cases where that's appropriate, but metadata about classes should be implemented as class methods, just as Apple does, e.g.:
+ (BOOL)automaticallyNotifiesObserversForKey:(NSString*)key { }
We could imagine our own along similar lines:
+ (BOOL)keyIsBrowsable:(NSString*)key { }
or
+ (NSArray*)serializableProperties { }
Let's imagine our class is called FOOBar, and we want to know whether the baz key is browsable. Without having to create a FOOBar we can just say:
if ([FOOBar keyIsBrowsable:#"baz"]} { ... }
You can do pretty much anything with this technique that can be done with custom attributes. (Except for things like the Serializable attribute which require cooperation from the compiler, IIRC.) The nice thing about custom attributes, though, is that it is easy to distinguish at a glance what is metadata and what is intrinsic to that class's actual functionality, but I think that's a minor gain.
(Of course, you may have to check for the existence of the keyIsBrowsable: selector, just as you'd have to check for the existence of a specific custom attribute. Again, custom attributes have a slight leg up here, since we can tell the .NET runtime to give them all to us.)
I've come across a similar issue whe serializing objects. My solution is to add a #property (nonatomic, readonly) NSArray *serialProperties; which has a custom getter that returns the names (as NSString*) of the properties of this (sub-)class that should be serialized.
For example:
- (NSArray *)serialProperties {
return #[#"id", #"lastModified", #"version", #"uid"];
}
Or in a subclass:
- (NSArray *)serialProperties {
NSMutableArray *sp = [super serialProperties].mutableCopy;
[sp addObject:#"visibleName"];
return sp;
}
You can then easily get all properties and their values via [self dictionaryWithValuesForKeys:self.serialProperties].
You can't add custom properties other than what sdk has provided..
.
But there is a work around to attain your objective...
#interface classTest:NSObject
#property(strong,nonatomic)NSString *firstName;
#property(strong,nonatomic)NSString *lastName;
#property(strong,nonatomic)NSMutableDictionary *metaData;
#end
#implementation classTest
- (id) init
{
self = [super init];
//Add meta data
metaData=[[NSmutableDictionary alloc]init];
//
if( !self ) return nil;
return self;
}
#end
so use the dictionary to add and retrieve meta data...
i hope it helps....