by 2/20/2008 02:52:00 PM 0 comments

Learing Cocoa Notes

I'm working through the tutorials on Cocoa at MacResearch

Notes from 'Living Objects'

  • Cocoa is a framework built upon conventions. If you stick to the conventions, you will be fine; if you flout them, it will hurt.
  • Typical init block:
    -(id)initWithLeftValue:(double)lv andRightValue:(double)rv {
        if ( self = [super init] ) {
            leftValue = lv;
            rightValue = rv;
        return self;
  • RULE: You should check the return value to see if it is nil after you call an initializer.
  • RULE: Cocoa has the concept of a designated initializer, which is the initializer you should generally chain to from a subclass. There is no language support for the designated initializer; it is purely a convention, and, as such, typically involves no more than a statement to the effect in the comments or documentation.
  • In Cocoa, when an object is ready to disappear, the method dealloc gets called.
  • Typical pattern:
    -(id)init... {
        // Chain to designated initializer of superclass
        if ( self = [super init] ) {
        return self;
    -(void)dealloc {
        [super dealloc];

Notes from 'Good References'

  • Cocoa uses a system of memory management called reference counting
  • Whenever an object is allocated, it automatically has a retain count of one. If you do nothing more with that object, it will remain in existence until the program exits. Other methods for creating objects, such as copy and new, also leave the object with a retain count of one.
  • Often, some part of your program will want to make use of an object that it did not create itself. In such cases, you want to ensure that the object in question does not disappear when it is still needed. To achieve this, the object's retain method can be called, which increases the retain count by one.
  • When an object is no longer needed, you call either the release method, or the autorelease method. Both methods decrease the retain count by one, but they differ in the timing of the change. release decreases the count immediatly.
  • When you use autorelease, you are saying you are finished with the object, but maybe some other part of the program would like to make use of it for a bit longer. An example of this is when you need to return an object from a method, and you don't need the object anymore. If you call release, it will disappear before you can return it; if you call autorelease, you have time to return the object, and the calling code has a chance to retain it if it wants to keep it around.
  • RULE: Whenever you call one of the retain count increasing methods, like alloc, retain, or copy, you should balance that with a call to a retain count decreasing method like release or autorelease.

Notes from 'All in the Genes'

  • You can actually apply constraints on the accessibility of instance variables in Objective-C, as in most object-oriented programming languages. The default, which we have been using to this point, is @protected, which means that an instance variable can be accessed from within the class in which it is defined, and from any descendent (eg subclass). The other two options are @private, which restricts access to the defining class, excluding its descendent classes, and @public, which allows free-for-all access from anywhere in the program.
  • In Objective-C, all methods are effectively public
  • You can fake a private method by declaring it not in the public header file, but in the .m implementation file.
  • Objective-C does not support multiple inheritance.
  • Protocols are blocks that declare one or methods that a class must implement. They are basically the same as interfaces in the Java language.
  • Protcol syntax:
    // Declaration
    @protocol Add
    // Inclusion
    @interface AdditionOperator : Operator <Add> {
    // Implementation
    @implementation AdditionOperator
    -(id)add:(id)toAdd {
        // Implementation here
  • Objective-C allows overriding methods by using a method of the same name in a subclass

Brian Schlining


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