I have an objective C method which passes in a string and an integer as its arguments. Inside of this method, I want to use a selector - the name of which is based upon the integer value I am passing in. For example, if the integer argument is 5, I want the selector to be named "buildXArrayIndex5" or if the integer argument is 3, I want the selector to be named "buildXarrayIndex3." I am really at a loss for how to do this or if it is possible/reasonable. I am new to objective C, so I wrote out what I want to happen, but it is not working/valid code. But here it is:
- (void) startBuildingXArray:(int)senderID:(NSString *)moveTrackerObject {
NSString *methodNamePrefix = #"buildXArrayIndex";
NSString *realMethodName = [[NSString alloc]initWithFormat:#"%#%d",methodNamePrefix,
senderID];
SEL realSelector = NSSelectorFromString(realMethodName);
[self realSelector: moveTrackerObject];
}
In the interface, I declared SEL realSelector; but I am getting an error without running this that says"no visible #interface declares the selector realSelector." But I'm sure that this is not the only problem with this code. Can anyone tell me how to create the proper code for this or highlight a better approach?
You need to call your selector using -performSelector: method, e.g.:
[self performSelector:realSelector];
Also if methods you want to call accept parameter you need to add a colon to selector name:
NSString *realMethodName = [[NSString alloc]initWithFormat:#"%#%d:",methodNamePrefix, senderID];
and call it:
[self performSelector:realSelector withObject:moveTrackerObject];
Related
As my title states, I'm wondering if I can pass a method signature or an #selector as a parameter? I'm asking because I'm creating a framework and I want to be able to pass instances of a certain class within it a method name.
You can pass the selector itself of use the name of the method as a string:
- (void)myMethod:(SEL)selector
{
[aClass performSelector:selector];
}
or
NSString *myMethodName = NSStringFromSelector(#selector(myMethod));
NSLog(#"The name of the method is: %#", myMethodName);
Actually, you cannot not pass a selector to a method.
In Objective-C, every method gets two implicit arguments, passed as normal parameters: The instance pointer self and the target selector _cmd. They are present in each and every method. The _cmd parameter is of type SEL. It is used by the runtime to look up the method implementation (this is the core of objc's dynamism).
You can, of course, add additional parameter of SEL type.
I have a small piece of code using MapBox framework:
if (tmpItem.itemTyype > 0 && tmpItem.itemTyype <= 12)
marker = [[RMMarker alloc] initWithUIImage:[UIImage imageNamed:[NSString stringWithFormat:#"PoiSmall%d",tmpItem.itemTyype]]];
else
marker = [[RMMarker alloc] initWithUIImage:[UIImage imageNamed:#"PoiSmall"]];
This code leads to an exception:
-[RMQuadTreeNode itemTyype]: unrecognized selector sent to instance 0xe9aaf00
The RMQuadTreeNode class is a MapBox' class and never be used by myself in my code. Note the unrecognized selector name - this is the name of my tmpItem.itemTyype property!
If I replace this code with the following:
if (tmpItem.itemTyype > 0 && tmpItem.itemTyype <= 12)
image = [UIImage imageNamed:[NSString stringWithFormat:#"PoiSmall%d",tmpItem.itemTyype]];
else
image = [UIImage imageNamed:#"PoiSmall"];
marker = [[RMMarker alloc] initWithUIImage:image];
then error is gone. It's not a problem to replace one fragment of code with another one, but I want to know HOW the 'itemTyype' property became a selector name and later was called somethere inside MapBox framework. The property name went thru NSString call, UIImage call, RMMarker call and do-not-know-how-many another calls to reach RMQuadTreeNode. HOW THIS CAN BE?
This knowledge is not necessary for me to solve this particular problem, but it is necessary to know Objective C better.
EDIT. I have to change the question above. Adjusted question is: I want to know HOW the 'itemTyype' property was passed not only to [NSString ...] call but also somethere inside MapBox framework.
In other words, should we see something like
-[UIImage itemTyype]: unrecognized selector sent to instance 0xe9aaf00
or
-[RMMarker itemTyype]: unrecognized selector sent to instance 0xe9aaf00
?
Answer is NO because the [tmpItem itemTyype] is a parameter of
[NSString stringWithFormat:#"PoiSmall%d",tmpItem.itemTyype]
(ok, just rewriting it as)
[NSString stringWithFormat:#"PoiSmall%d",[tmpItem itemTyype]]
but it's not a parameter of [UIImage ...] nor [RMMarker ...] nor something else like [RMQuadTreeNode ...].
So how it was passed thru the chain of all these calls above and reached [RMQuadTreeNode ...] call? How the parameter of [NSString ...] call magically turned into a parameter of [RMQuadTreeNode ...] call?
In Objective-C dot notation for properties is just a 'shortcut' for writing accessor methods. So when you write
object.property
This is equivalent to writing:
[object property]
And
object.property = ?
is equivalent to:
[object setProperty:?]
So, back to your example,
[[RMMarker alloc] initWithUIImage:… ,tmpItem.itemTyype]]];
is the same as:
[[RMMarker alloc] initWithUIImage:[UIImage imageNamed:…,[tmpItem itemTyype]]]];
This is why you're seeing method itemTyype being called.
So object tmpItem is an instance of RMQuadTreeNode, which doesn't declare method itemTyype (or the equivalent property).
These Apple docs give a fuller explanation.
Simple explanation:
If you declare a property setter and getter methods to access this property are generated automatically (different for older Xcode versions) as well as an instance variable for this property. A setter is used to set the value of this variable, a getter is used to get the value, both so KVO can be used as well.
If you declare a property itemType like for example
#property (strong) NSNumber *itemType;
you also get in your class
- (void)setItemType:(NSNumber*)newValue;
- (NSNumber*)itemType;
as well as a variable
NSNumber *_itemType;
When you use the dot notation of Objective-C 2.0 like self.itemType these getter/setter methods are actually called.
This is a short overview, you should definitely invest time and read up on Objective-C like for example this here
I have problem that im trying to get solve for like week.
My goal is to get variable out of my IBAction, to use for example in -(void)viewDidLoad..
But as far as I am now I can use my variable only in my IBAction..
- (IBAction) changeLat:(NSNumber *)str {
longi = str;
double lop = longi.doubleValue;
NSLog(#"%f",lop);
}
- (void)viewDidLoad
{
[super viewDidLoad];
NSLog (#"%#",lop);
}
It NSLog shows everything fine in action, but in view did load it doesn't even recorganize it.
If you create a variable inside of -IBAction, the scope of that variable is only that method, so you cannot access to that variable outside it.
If you want your variable to be global to your class, you have to create it in the declaration of your class, like this:
#interface MainViewController () {
#private
double lop;
}
Put this at the beginning of your .m file, and then lop would be accesible in all your class.
You can read more about the scope of the variables here:
http://www.techotopia.com/index.php/Objective-C_Variable_Scope_and_Storage_Class
Actually, IBAction is converted to void by the preprocessor. It's used by Interface Builder as a label that identifies this method as an action able to be related from an IB Object.
There's no way (AFAIK) to use two return types in a function (for example `(IBAction double)´, equivalent to ´(void double)´), but a good practice could be something like this:
- (IBAction)changeLatAction:(id)sender {
NSNumber *str = <get the NSNumber from a valid place>;
[self changeLat:str];
}
- (double) changeLat:(NSNumber *)str {
longi = str;
double lop = longi.doubleValue;
NSLog(#"%f",lop);
return ????;
}
Your first declaration of changeLat seems to be wrong, because as a first parameter you'll always get the "sender" or "caller" object, related from IB (when called from an action, of course), so, you need to get the str value from a valid place.
Cheers.
I'll provide a simple method and then explain how I see it, if this is incorrect, please let me know and correct me. I feel like I understand 'self' but still doubt my self.
-(NSString *)giveBack {
NSString *string = [NSString stringWithFormat:#"Hi there!"];
return string;
}
-(IBAction)displayIt {
NSString *object = [self giveBack];
[myView setText:object];
}
the "myView" is a UITextView object.
Now as for the 'self'..
I'm basically saying in my -displayIt method that I'm creating a NSString object called 'object' and storing within it a method that returns a string which says "Hi there".
And this method (named 'giveBack') is performed ON the name of my class (whatever I named the project). Is this correct?
No, you are not creating an object called object and then storing a method within it etc. You are creating a variable which can hold a reference to an object and storing within it a reference to an object obtained by calling a method.
[Note: The following assumes you are using automatic memory management (ARC or garbage collection), no mention will be made of reference counts. If you are using manual memoery there is more to consider...]
Adding line numbers to your sample:
1. -(NSString *)giveBack
{
2. NSString *string = [NSString stringWithFormat:#"Hi there!"];
3. return string;
}
4. -(IBAction)displayIt
{
5. NSString *object = [self giveBack];
6. [myView setText:object];
}
Declares giveBack as an instance method of the class, to be invoked it must be called on a particular instance.
The RHS ([NSString stringWithFormat:#"Hi there!"]) calls a class method which creates an object of type NSString and returns a reference, of type NSString *, to that object. The LHS declares a variable (string) which can hold a reference to an NSString object. The assignment (=) stores the reference returned by the RHS into the variable declared by the LHS.
Return the value in string as the result of the method
Declare an instance method called displayIt
RHS: call an instance method (giveBack) on the object instance self - self is a reference to the current object instance when within an instance method (in this case displayIt). LHS: declare a variable, object of type NSString *. Assignment: store the reference to an NSString returned by the method call on the RHS into the variable declared on the LHS.
Call the instance method setText: on the object instance referenced by the variable myView passing it the reference to an NSString found in variable object.
I think, you are generally correct.
But in below mention:
And this method (named 'giveBack') is performed ON the name of my class (whatever I named the project)
I can't understand your meaning.
A class name is just a symbol (that is text for human readers).
Methods of an Objective-C class are indicated by - notation in the beginning of method declaration.
In other words, all method declarations start with - within #implementation CLASS_NAME ... #end block are instance method of CLASS_NAME class.
When we call another instance methods (within a instance method) we use self keyword. Because all Objective C method call must designate target object and, in this case, we are calling ourselves (current CLASS_NAME instance itself). So we use self keyword.
Sorry for my confusing words.. It's harder to explain I thought :-(
you're storing the string returned by 'giveBack', not the method itself. the method is part of the class. 'self' is the instance of the object that you're calling 'giveBack' (and 'displayIt' for that matter) on.
I'm trying to implement a dispatch table, so that I can call a selector with the following example code:
NSInteger i = 2;
[myObject performSelector:selectors[i]];
I'm trying to store user preferences which affect which method of an API gets called. Right now, I use the string name of the selector and use NSSelectorFromString, but that's a bit messy. If I use a dispatch table, then I can store an enum instead.
How can I make an array of selectors, or a dispatch table in Objective-C?
Edit:
The compiler complains when I try to set an array of selectors as a property. #property SEL[] won't compile.
Edit2:
I'm using my KosherCocoa API library and I want to call a single method at once, based on a saved user setting. I'm saving to and reading from a Plist file.
You can use the SEL type to hold selectors. Simply:
SEL dispatchTable[3] = { #selector(doThis:),
#selector(doThat:),
#selector(doTheOther:)
};
To your edit, use an NSArray/NSDictionary/etc of selectors as your property instead. You are not allowed to use C arrays as properties in Objective C; they are not one of the supported types (which are ObjC objects, CF types and basic C 'Plain Old Data' types.)
OK, on our comments below, you need to wrap the selector in an NSValue to allow you to use it in an objc container (because SEL is a C pointer type):
NSMutableArray * dispatchTable2 = [[NSMutableArray alloc] initWithCapacity:3];
SEL selIn = #selector(doThis:);
// Wrap the selector in an NSValue instance
[dispatchTable2 addObject:[NSValue valueWithPointer:selIn]];
// On extracting:
NSValue * valOut = [dispatchTable2 objectAtIndex:0];
SEL selOut = [[dispatchTable2 objectAtIndex:0] pointerValue];
[anObject performSelector:selOut];
So now your table is an objc container stored as a property or ivar, and you use NSValue to wrap SEL pointers with valueWithPointer: and get the SEL out with pointerValue.
I would recommend using NSInvocation instead of selectors. They are far more flexible, as you can send the same invocation to many objects and you can change its properties as you go.
One way to do this is using an array of NSStrings, then converting those to SELs at runtime, if that increases readability for you..
NSString *selectors[] = { ... }
[myObject performSelector:NSSelectorFromString(selectors[i])];
To use this as a property, use
#property(nonatomic, assign) NSString **selectors;