Most of these guidelines are to match Apple's documentation and community-accepted best practices. Some are derived some personal preference. This document aims to set a standard way of doing things so everyone can do things the same way. If there is something you are not particularly fond of, it is encouraged to do it anyway to be consistent with everyone else.

This document is mainly targeted toward iOS development, but definitely applies to Mac as well.

NSString *foo = @"bar";
NSInteger answer = 42;
answer += 9;
answer++;
answer = 40 + 2;

The ++, --, etc are preferred to be after the variable instead of before to be consistent with other operators. Operators separated should always be surrounded by spaces unless there is only one operand.

NSInteger and NSUInteger should be used instead of int, long, etc per Apple's best practices and 64-bit safety. CGFloat is preferred over float for the same reasons. This future proofs code for 64-bit platforms.

All Apple types should be used over primitive ones. For example, if you are working with time intervals, use NSTimeInterval instead of double even though it is synonymous. This is considered best practice and makes for clearer code.

- (void)someMethod {
    // Do stuff
}


- (NSString *)stringByReplacingOccurrencesOfString:(NSString *)target withString:(NSString *)replacement {
    return nil;
}

There should always be a space between the - or + and the return type ((void) in this example). There should never be a space between the return type and the method name.

There should never be a space before or after colons. If the parameter type is a pointer, there should always be a space between the class and the *.

There should always be a space between the end of the method and the opening bracket. The opening bracket should never be on the following line.

There should always be two new lines between methods. This matches some Xcode templates (although they change a lot) and increase readability.

An excerpt of a UIView:

#pragma mark - NSObject

- (void)dealloc {
    // Release
    [super dealloc];
}


#pragma mark - UIView

- (id)layoutSubviews {
    // Stuff
}


- (void)drawRect:(CGRect)rect {
    // Drawing code
}

Methods should be grouped by inheritance. In the above example, if some UIResponder methods were used, they should go between the NSObject and UIView methods since that's where they fall in the inheritance chain.

There should always be a space after the control structure (i.e. if, else, etc).

if (button.enabled) {
    // Stuff
} else if (otherButton.enabled) {
    // Other stuff
} else {
    // More stuf
}

else statements should begin on the same line as their preceding if statement.

// Comment explaining the conditional
if (something) {
    // Do stuff
}

// Comment explaining the alternative
else {
    // Do other stuff
}

If comments are desired around the if and else statement, they should be formatted like the example above.

switch (something.state) {
    case 0: {
        // Something
        break;
    }
    
    case 1: {
        // Something
        break;
    }
    
    case 2:
    case 3: {
        // Something
        break;
    }
    
    default: {
        // Something
        break;
    }
}

Brackets are desired around each case. If multiple cases are used, they should be on separate lines. default should always be the last case and should always be included.

for (NSInteger i = 0; i < 10; i++) {
    // Do something
}


for (NSString *key in dictionary) {
    // Do something
}

When iterating using integers, it is preferred to start at 0 and use < rather than starting at 1 and using <=. Fast enumeration is generally preferred.

while (something < somethingElse) {
    // Do something
}

Always use @class whenever possible in header files instead of #import since it has a slight compile time performance boost.

From the Objective-C Programming Guide (Page 38):

The @class directive minimizes the amount of code seen by the compiler and linker, and is therefore the simplest way to give a forward declaration of a class name. Being simple, it avoids potential problems that may come with importing files that import still other files. For example, if one class declares a statically typed instance variable of another class, and their two interface files import each other, neither class may compile correctly.

Adding frameworks that are used in the majority of a project to a header prefix is preferred. If these frameworks are in the header prefix, they should never be imported in source files in the project.

For example, if a header prefix looks like the following:

#ifdef __OBJC__
    #import <Foundation/Foundation.h>
    #import <UIKit/UIKit.h>
#endif

#import <Foundation/Foundation.h> should never occur in the project outside of the header prefix.

@property (nonatomic, retain) UIColor *topColor;
@property (nonatomic, assign) CGSize shadowOffset;
@property (nonatomic, retain, readonly) UIActivityIndicatorView *activityIndicator;
@property (nonatomic, assign, getter=isLoading) BOOL loading;

If the property is nonatomic, it should be first. The next option should always be retain or assign since if it is omitted, there is a warning. readonly should be the next option if it is specified. readwrite should never be specified in header file. readwrite should only be used in class extensions. getter or setter should be last. setter should rarely be used.

See an example of readwrite in the Private Methods section.

MyShoeTier.h

@interface MyShoeTier : NSObject {
    
    ...
}

@property (noatomic, retain, readonly) MyShoe *shoe;

...

@end

MyShoeTier.m

#import "MyShoeTier.h"

@interface MyShoeTier ()
- (void)_crossLace:(MyLace *)firstLace withLace:(MyLace *)secondLace;
@property (nonatomic, retain, readwrite) MyShoe *shoe;
@property (nonaomic, retain) NSMutableArray *laces;
@end

@implementation MyShoeTier

...

@end

Private methods should always be created in a class extension for simplicity since a named category can't be used if it adds or modifies any properties.

Note: The above example provides an example for an acceptable use of a readwrite property.

extern NSString *const kMyConstant;
extern NSString *MyExternString;
static NSString *const kMyStaticConstant;
static NSString *staticString;

In general, everything should be prefixed with a 2-3 letter prefix. Longer prefixes are acceptable, but not recommended.

It is a good idea to prefix classes with an application specific application if it is application specific code. If there is code you plan on using in other applications or open sourcing, it is a good idea to do something specific to your or your company for the prefix.

If you company name is Awesome Buckets and you have an application named Bucket Hunter, here are a few examples:

ABLoadingView // Simple view that can be used in other applications
BHAppDelegate // Application specific code
BHLoadingView // `ABLoadingView` customized for the Bucket Hunter application

enum {
    Foo,
    Bar
};

typedef enum {
    SSLoadingViewStyleLight,
    SSLoadingViewStyleDark
} SSLoadingViewStyle;

typedef enum {
    SSHUDViewStyleLight = 7,
    SSHUDViewStyleDark = 12
} SSHUDViewStyle;