Why does the following code not work?
[Confirm addTarget:self
action:#selector([FunctionsApp Synch_Data:CompIDS])
forControlEvents:UIControlEventTouchUpInside];
given:
FunctionsApp: its other class
Synch_Data:its function in FunctionsApp
CompIDS: its MSmutableArray
FunctionsApp *functions = [[FunctionsApp alloc] init];//of course u need to use your own init method
[Confirm addTarget:functions action:#selector(Synch_Data) forControlEvents:UIControlEventTouchUpInside];
the selector u pass is just the name of the method this is passed to the target who in turn checks if it exists and then calls it. u cant use a mutable array as a selector. since confirm is a button u should use a method with signature - (void)buttonClicked:(id)sender where sender will be the button. and also since the selector is not a class-method you do not call it like this [FunctionsApp buttonClicked:] but instead you use the FunctionsApp object reference as target ( i.e. where the method exists as target).
and please elaborate your question next time and be a bit more concrete in what things are.
Let's take this one thing at a time.
1.
-[UIControl addTarget:action:forControlEvents:] has specific requirements to what the action selector can look like.
It can be one of the following:
- (void)action;
- (void)action:(id)sender;
- (void)action:(id)sender forEvent:(UIEvent *)event;
Normally you'd use the second one.
2.
#selector takes the name of the method (selector) you'd like to have called on the target. You're specifying the return value of a method call. That's the error, you're seeing.
So to call Sync_Data: (which in objective-c would be syncData:) on FunctionsApp you'd write
[Confirm addTarget:FunctionsApp action:#selector(Synch_Data:) forControlEvents:UIControlEventTouchUpInside];
You still can't pass the CompIDS variable as a parameter, though, so I'd suggest rewriting your code, to make it an instance variable, that your action method can access.
Related
I have a method called Display. Can somebody explain me the difference of calling the same method in the following two ways.
[self Display];
[self performselector:#selector(Display)]
- (void)Display {
NSlog(#"Data");
}
both are basically the same with one minute difference.. #selector gives a name to your method which you can pass around as an attribute to other objects or in other function calls.
Like if you want to send a message to other object and you want to send display as an attribute then you will have to give it a name using #selector and thus you can send it.. its a pretty vague concept.. hope this helps.
and to quote apple documents...
"However, the performSelector: method allows you to send messages that
aren’t determined until runtime. A variable selector can be passed as
the argument:
SEL myMethod = findTheAppropriateSelectorForTheCurrentSituation();
[anObject performSelector:myMethod];
The aSelector argument should identify a method that takes no
arguments. For methods that return anything other than an object, use
NSInvocation."
[self Display] is shorter and easier to read, write and comprehend.
[self performSelector:#selector(Display)] makes it possible to execute arbitrary selectors. If you save the selector in a variable, then you can execute it later on without knowing the method you invoke. It is therefore more flexible. Even better: you can pass selectors and objects to other objects and let them invoke it for you when necessary. An example why you want to use this is the NSUndoManager which simple invokes a selector to undo an action if the user executes the Undo command.
I do not think that there is a big difference between examples you provided, but perform selector is very useful when you for instance wanna move execution of your method to the background thread.
[self Display]; is a call to a known method on a known object.
It's easy to give it some params if your want : [self DisplayWithParam1:(NSString*)aString param2:(int)aNumber param3:(NSDictionary*)aDict
[self performselector:#selector(Display)] is a call that allows you to call a possibly not known method on a possibly not known object type.
Let's imagine you have many kind of classes that all respond to a given protocol that require to have the Display method implemented. You put some objects of thoses different classes in an NSMutableArray. When parsing the array later, you will get id typed objects.
So calling[myArrayObject Display]; will work at runtime but will generate a warning at compile time as id does not support any method of course, even if you know that this object supports the method.
To prevent thoses warning, call [myArrayObject performselector:#selector(Display)];.
The problem with that call is that is harder to pass some parameters.
performSelector:withObject:withObject:
Sends a message to the receiver with two objects as arguments.
- (id)performSelector:(SEL)aSelector withObject:(id)anObject withObject:(id)anotherObject
Parameters
aSelector
A selector identifying the message to send. If aSelector is NULL, an NSInvalidArgumentException is raised.
anObject
An object that is the first argument of the message.
anotherObject
An object that is the second argument of the message
Return Value
An object that is the result of the message.
Discussion
This method is the same as performSelector: except that you can supply two arguments for aSelector. aSelector should identify a method that can take two arguments of type id. For methods with other argument types and return values, use NSInvocation.
Availability
Available in Mac OS X v10.0 and later.
http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocSelectors.html
The #select call is faster. Generally the uglier (and less dynamic) the code you have in Objective-C, the faster it runs. Here, the selector call bypasses the usual call to objc_msgSend().
I wouldn't recommend writing code like this if you can avoid it. Selectors are somewhat common in Cocoa, but if you're using it for a speedup it's really not worth it. objc_msgSend() is highly optimized and very fast.
When I call my urlconnection method from my (IBAction)buttonpressed method like this:
[self connectWeb];
I get error "request for member connectWeb in something not a structure or union"
but when I call the same method from my - (void)viewDidLoad method it works!?
Where is the connectWeb method in your implementation file?
I would guess that it is before viewDidLoad but after buttonPressed.
The reason for this is that the connectWeb method has been declared before viewDidLoad, so viewDidLoad is aware of it, whereas it's after the buttonPressed method, so the buttonPressed method isn't aware of it.
You have a couple of options.
Declare the method before any other method uses it.
- (void)connectWeb;
Then you can implement it anywhere within the implementation.
Move the implementation of connectWeb to before both viewDidLoad and buttonPressed - both the methods will then be aware of connectWeb.
A method missing from the header wouldn't cause this problem. Because Objective-C is dynamic, it will check if the instance implements that method at runtime, so as long as the method exists it will work. You would just get a compiler warning at build time.
Try deleting the line [self connectWeb] from your button delegate method and copy/pasting it from your viewDidLoad (or retyping it). From the error you're getting, it sounds like there might be an extra character in your statement. This happens to me from time to time, because I use synergy to share my keyboard & mouse between multiple computers.
I'm trying to transform one method call into another dynamically (at runtime).
For instance, I'd like the following:
[obj foo]
to delegate to:
[obj getAttribute: #"foo"]
(I'd like to do this dynamically as I don't know ahead of time what those method names or attributes are going to be).
I see that there's a hook into:
- (id) forwardingTargetForSelector: (SEL) aSelector
That only seems to work for delegation, though, I want to keep the object as "self" and transform the method arguments.
Where should I look for this sort of behavior? Is it even possible in obj-c?
You can use the method -forwardInvocation: for that. It takes a full NSInvocation object which represents the method call, and you can handle it however you wish. If you do this, you should also override -methodSignatureForSelector: to return the correct NSMethodSignature (required for -forwardInvocation: to work on unknown selectors). It's also recommended that you override -respondsToSelector: to declare that you can handle the selector in question.
I have to pass an object to an button's action... something like
[myButton addTarget:self action:#selector(sendIt:MY_OBJECT) forControlEvents:UIControlEventTouchDown];
I cannot set a variable and use that on the method because I am on a static class.
How do I do that?
You cannot, the signature of an button's action is always*) -(IBAction)methodToCall:(id)sender where sender is your button, or whatever is calling the action.
I cannot set a variable and use that on the method because I am on a static class.
I am not sure what you mean with this.
*) There are a couple of other signatures new on the iPhone compared to the Mac, but none of those allow passing objects either.
The UIButton cannot pass object to event selector. You can use the property tag example:
myButton.tag = SOMEBUTTON_ID;
Also you can cast pointer to you object to NSInteger and assign it to tag property, but this technique does not solve problem of the lifetime of the object and in the future, such code may stop working.
I discovered a way to do that.
You create a class of UIButton and inside the class you declare a public variable that corresponds to the type of object you want to pass.
When you create the button you use that class and store the object inside its property.
then when the button is clicked you retrieve the ID, retrieve the button and the object stored inside it.
Now you have the object...
Simple question, as I am coming from another programming language. In Objective-C, lets say in a controller class I want to separate certain code into its own method, how do I call that method let's say, from viewLoad. As an example, let's say I create a method:
(void)checkIfInputCorrect
{
NSLog(#"text");
}
Now, i wanted to have in a delegate method, call this method. I tried [self checkIfInputCorrect] and get a warning saying Controller may not respond to -CheckIf...
I thought something like checkIfInputCorrect() would work that gives an error as well.
Basically how do you call a method?
Add this to your .h file
- (void)checkIfInputCorrect;
Call it with:
[self checkIfInputCorrect];
You need to list the method in the interface (ideal) or list the method implementation before the calling method (less ideal) so that the compiler can know that the class responds to the selector before it compiles the calling line.
To paraphrase Martin,
In your .m file, make sure your method -checkIfInputCorrect is placed so that it's physically above the method that has the line: [self checkIfInputCorrect];