method syntax in objective-c [duplicate]

This question already has answers here: Method Syn开发者_如何学Gotax in Objective-C (4 answers) Closed 3 years ago.

I'm a .NET programmer new to objective-c, and I'm struggling to understand some nuts and bolts syntax. For example, how should I parse this method signature:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath     *)indexPath {

I understand what the "-" char means, and (UITableViewCell *) defines the return type. But the rest has me confused.

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- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath 

1. "-" Defines an instance method
2. Returns UITableViewCell pointer
3. First part of the method signature named "tableView"
4. Takes a UITableView pointer
5. With the local variable name "tableView"
6. Second part of the method signature "cellForRowAtIndexPath"
7. Takes a NSIndexPath pointer
8. With the local variable name "indexPath".

The actual method signature is: tableView:cellForRowAtIndexPath:.

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Read Apple's documentation, like Objective-C: A Primer. It's explained right there. You know, the maker (Apple or Microsoft) has a lot of documentation on their site ...

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Objective-C uses named, inline parameters for methods. (As bblum points out in the comment below, this style of parameters are sometimes called "interleaved".) This is a reflection of it's heratage as a mix of C and SmallTalk syntax. The trailing colons denote the names of the parameters to the method. For your method, the full name of the method is referred to as tableView:cellForRowAtIndexPath:. It takes two parameters, a pointer to a UITableView, and pointer to a NSIndexPath. In a java-like language, this method signature would look something like:

   public UITableViewCell cellInTableViewForRowAtIndexPath(UITableView tableView, NSIndexPath indexPath);

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Every foo:(bar)baz defines a parameter, for example

- (id)initWithTitle:(NSString *)title
            message:(NSString *)message
           delegate:(id)delegate
  cancelButtonTitle:(NSString *)cancelButtonTitle
  otherButtonTitles:(NSString *)otherButtonTitles, ... {

defines a method with five* parameters.

The stuff before the : is part of the name of the method. In this example, the method's name is

initWithTitle:message:delegate:cancelButtonTitle:otherButtonTitles:

The stuff between the (…) is the type of that argument. Here, we see that the first argument must be an NSString*.

Finally it's the name of the parameter.

(*: Sometimes there is sometimes a , ..., like in here, indicating it's a variadic method.)

The method is called in the syntax

id result = [theAllocedAlertView initWithTitle:@"title"
                                       message:@"message"
                                      delegate:someDelegate
                             cancelButtonTitle:@"cancel button title"
                             otherButtonTitles:@"other", @"button", @"titles", nil];

So the name of the method is repeated (in order!), and the parameter names are substituted by the actual arguments.

In C#, the corresponding function signature would look like

object InitWithTitleAndMessageAndDelegateAndCancelButtonTitleAndOtherButtonTitles(
        string title,
        string message,
        object delegate,
        string cancelButtonTitle,
        params string[] otherButtonTitles);

and called like

object result = theAllocedAlertView.InitWithBlahBlahBlahAndOtherButtonTitles(
                   "title",
                   "message",
                   someDelegate,
                   "cancel button title",
                   "other", "button", "titles");

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The method selector is:

tableView:cellForRowAtIndexPath:

where each value after the colon is a parameter. The signature is meant to read like an English sentence, i.e. "The TableView's cell for a row at this index".

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If this were written in another language it might look like this

// @param (UITableView *) tableView
// @param (NSIndexPath*)indexPath
// @return UITableViewCell
- (UITableViewCell *) someFunctionName(tableView, indexPath) {

}

Thats roughly speaking of course. It would not be written like this in objective-c. However I believe it is possible to write a good chunk of your program in c++