This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Caret in objective C
I recently came across some line of code and found out that it has ^ sign in front of an object like this one:
typedef void (^AnimatedViewBlock)(CGContextRef context, CGRect rect, CFTimeInterval totalTime, CFTimeInterval deltaTime);
#interface AnimatedView : UIView
Can anyone explain it with simple example about the same.
It denotes a block object.
Read Apple's documentation here.
It is so called block.
Objective-C kind of closure.
Docs here
The symbol you are referring to denotes the start of a block in objective-c. They are primarily used in Grand Central Dispatch in ios but you can use them elsewhere as well.
As John Muchow writes:
A block is really nothing more than a chunk of code. What makes them
unique is that a block can be executed inline as well as passed as an
argument into a method/function. Blocks can also be assigned to a
variable and called as you would a C function.
Related
Closed. This question needs to be more focused. It is not currently accepting answers.
Want to improve this question? Update the question so it focuses on one problem only by editing this post.
Closed 8 years ago.
Improve this question
Assume that I have a subfunction seen in the below. What is the difference between these two
function a=b(x,y)
.
.
.
a=output
and
function b(x,y)
......
if I write it in second form how can I define it main function and how can I see its outputs.
Another question,
I found a code from here (http://www.mathworks.com/matlabcentral/fileexchange/21443-multiple-rapidly-exploring-random-tree--rrt-) including a function like:
%% SetObstacleFilename
function SetObstacleFilename(self,value)
if isa(value,'char')
self.obstacleFilename = value;
self.GenerateObstacles();
end
end
how can I use it in my main function? Moreover what is self.GenerateObstacles() command? There is no equality on it?
I think I see how both of your questions are related to the same thing. You really should've asked something along the lines of:
I always saw MATLAB functions written in the form function a=b(x,y), however recently I came across some code which included functions in the form function b(x,y) (e.g. function SetObstacleFilename(self,value)).... so what's up with that?
In order to understand the 2nd type of functions, you need to consider object-oriented programming (OOP).
The code example you found is taken from within a MATLAB class. Class-related functions are known in OOP as "methods", and this specific code in another programming language would take the shape of a void return type function\method.
Now consider the term object that refers to an instance of a class.
Traditionally, methods are limited to a single output. For this reason, some methods are designed to operate on objects (actually pointers, AKA "passing by reference") such that returning a value is not necessary at all, because the input objects are directly manipulated. Other cases when methods don't need to return anything may include functions that have some "utility" functionality (e.g. initialize something, plot something, output something to the console etc. - just like the self.GenerateObstacles() method you mentioned).
Regarding your other questions:
The self in SetObstacleFilename(self,value) looks like an instance of the considered class.
Usually to use class methods you need to instantiate an object using a constructor (which should be a function with the same name of the class), unless these methods are static.
To conclude - above are just the most fundamental basics of OOP. I won't attempt to teach you the whole OOP Torah while standing on one leg, so I am providing some additional materials below, should you be interested to further your understanding of the topic.
Hopefully, what's going on is a bit clearer now!
Here are some resources for you:
MATLAB's OOP Manual.
MATLAB's documentation on OOP.
I am learning Objective C and noticed this funky quirk while reading up on methods.
Like Java and C++, Obj.C can take in multiple parameters, which is fine, however it states that objective C methods can have multiple names which does not seem to register to well with me.
For instance:
-(NSArray *)shipsAtPoint:(CGPoint)bombLocation withDamage:(BOOL)damaged;
In the above example, there are two parameters, bombLocation (return type CGPoint) and damaged (return type BOOL) and alongside the method name seems to be split as shipsatpoint:withDamage
I don't understand what's up with this...
What does it signify when it states that a method can have multiple names?
Is this applicable only for methods that require multiple parameters? Alternately, say I want to name my method with a single name but provide it with multiple parameters, is that possible or I must provide it with multiple names each of which correspond to a parameter? If yes, then why?
Thanks for jumping in with my confusion!!! :)
The reason is to make it easier to understand.
With your example, the method would be something like this in C++:
int shipsAtPointWithDamage (CGPoint bomb, BOOL damage) //I don't really know C++
OK, so the first parameter is the ship's point, and the damage is the second. It's easy enough to figure out, but that's the thing, you have to FIGURE it out, you have to look at the method to try and figure out what each thing is.
In Objective-C you have
-(NSArray *)shipsAtPoint:(CGPoint)bombLocation withDamage:(BOOL)damaged;
Each parameter is clearly defined, the first is the ship's point, the second is damage. It reads like a sentence, whereas with C++ (and almost every other language) it doesn't.
If you want a method to have multiple parameters in Obj-C you have to write it this way:
-(returnType)paraOne:(type*)name paraTwo:(type*)name
It's something that just takes getting used to, every language is different. Once you get used to the way Objective-C does things, you'll think it's absolutely fantastic.
EDIT: and as filipe pointed out, because the method as multiple parameters it doesn't mean it has multiple names, in the example I gave above, the method name would be paraOne:paraTwo, NOT paraOne:
Objective-C uses a system of message passing based on selectors. This is not quite the same thing as method calling. When you see code like this:
[world shipsAtPoint:point withDamage:YES];
That is converted into the following C call (in the most common case):
objc_msgSend(world, #selector(shipsAtPoint:withDamage:), point, YES);
The #selector() construct returns a unique identifier. The exact format of that identifier is an internal implementation detail.
objc_msgSend includes quite a lot of logic in it's few dozen bytes of assembler. But in simplest case, it looks up the class for world, walks through a table of selectors until it finds the one that matches shipsAtPoint:withDamage: and then grabs the function pointer at that slot. It then jumps to that function pointer, leaving the rest of the parameters alone (in registers or on the stack as appropriate for the processor). The function at that location is your method, and it knows the order and types of its parameters based on your declaration.
What's important in all this for you is that the selector is shipsAtPoint:withDamage:. This is generally the one-and-only name of the method. There are not "multiple names" as you suggest. (Usually.... the Objective-C runtime is very powerful and it's possible to point multiple selectors to the same implementation.)
As Joe points out, a selector can be in the form foo::. This would represent a method that took two parameters and would be called like [world foo:point :YES]. You should never do this. It's incredibly confusing to read. But it's legal.
Here is the best explanation i've ever seen. It includes comparisons with C++/C as well as lots of other good info.
I think you are confused. A method cannot have multiple names, but the argument may be named differently in the header then they are in the implementation.
The name of that method is shipsAtPoint:withDamage:. This is also known as a selector.
This method returns an instance of NSArray, and accepts a CGPoint as the first argument, and a BOOL as the second argument.
The names of the arguments may differ, however. This is totally valid:
// .h file
-(NSArray *)shipsAtPoint:(CGPoint)bombLocation withDamage:(BOOL)damaged;
// .m file
-(NSArray *)shipsAtPoint:(CGPoint)loc withDamage:(BOOL)dmg {
// ...
}
Lastly, ObjC is mainly some nice syntax sugar. You should know that any method invocation really just boils down to some C that looks more or less like this:
objc_msgSend(receiverObj, #selector(shipsAtPoint:withDamage:), point, damage);
So at the end of the day, you have a receiver, a selector, and your arguments. But the ObjC syntax is much nicer than that.
It is possible provide a method without labeled parameters but it is obviously discouraged.
-(void)badmethod:(id)obj1:(id)obj2:(id)obj3
{
}
//...
//Usage
[self badmethod:nil :nil :nil];
SEL sel = #selector(badmethod:::);
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Placement of the asterisk in Objective-C
What is the differenct between the following:
(Assume we have a class calculator)
Calculator* calc;
Calculator *calc;
Also what is the need for pointers? Is it not possible to omit the star?
There is no difference between those two declarations, you could also do Calculator * calc if you're feeling adventurous.
As far as if you can omit the star, no, you cannot. It is a carry over from C and shows that calc is a pointer to a Calculator, and not a Calculator itself.
There is no difference between the 2 declaration is just an preference for typing.
Every thing on computers uses pointers. You need pointers java, c#, etc use pointers.
No is not posible to omit the star.
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
What is Method, Property and Function ?
Can anyone tell me what is function and what is property?
Just a basic explanation.
Property denotes the object's state, Method denotes the object's behavior. Function is like method except for that it's not dependent upon an object (I'm guessing you don't really mean that).
For example, if you have a Car class, a property may be it's model, it's year, it's current speed etc., a method may be Stop, Drive etc.
see msdn: Properties, Methods
Additionally, pragmatically, properties will 'interact' with some tools. For instance, they will show up in the Properties window of VS.
This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
List selectors for obj-c object
Does anybody know how to get all selectors that a instance respond to during runtime in objective C?
As answered here, #import < objc/runtime.h > and use class_copyMethodList().
In general, this is not possible. "The selectors an instance responds to" may be an infinite set. For example, it is possible to implement a class that is sent Roman numerals as messages and returns the corresponding integer value. If you want to know the precise set of instance methods implemented by a class at a given time (which is a different question), you can just use the Objective-C runtime functions to get a class's instance method list and walk up the class tree to find the ones it inherits from superclasses. Again, though, these are two totally different things. A class might have a method for a message that it chooses not to respond to and it might respond to messages for which it does not have a directly corresponding method.
dapptrace (Dtrace) is your friend.
on the man page (man dapptrace):
dapptrace prints details on user and
library function calls
dapptrace is written for the Dtrace scripting language (D). This means you can adjust dapptrace or pull ideas from it's script file to do many things. For instance:
wait for myFunctionWhichCreatesSpecialObject to be called. Store the object address that it returns (the special object). Print out any selectors invoked on that object.
You can also invoke dtrace directly to write simple single-line spells. I'll let you go search for those.
During runtime you would use
the class method "+ (BOOL)instancesRespondToSelector:(SEL)aSelector"
provided you know the selectors you want to check on.