1. Two-Stage Allocation /
Template Methods

2. Two-Stage Creation

3. MyClass *class = [[MyClass alloc] init];
not
MyClass *class = [MyClass new];

4. Subclassing
-(id) initWithCenter:(CGPoint) aPoint {
self = [super init];
if (nil != self) {
center = aPoint;
}
return self;
}
-(id) init {
return [self initWithCenter:CGPointZero];
}

5. Each class should have one designated
initializer.
Child classes should call it.
Without Two-Stage creation, subclasses
would have to re-implement each
initializer. (Think of NSData or NSString,
which have many initializers...subclassing
each one would be a lot of work.)

6. Temporary Instances
+(id)circleWithCenter:(CGPoint) aPoint {
return [[[self alloc] initWithCenter:aPoint] autorelease];
}

7. Template Methods

8. Methods which are automatically called by the code but
may not do anything.
The programmer is expected to override them with
some functionality.

9. Can you think of any examples?

11. • Identify steps of an algorithm and one or more
methods to implement each step.

12. • Identify steps of an algorithm and one or more
methods to implement each step.
• Implement the algorithm as a sequence of calls to
the identified methods.

13. • Identify steps of an algorithm and one or more
methods to implement each step.
• Implement the algorithm as a sequence of calls to
the identified methods.
• Factor out the customizable steps of the algorithm
into Template Methods and provide a reasonable
default implementation for each Template Method.

14. • Identify steps of an algorithm and one or more
methods to implement each step.
• Implement the algorithm as a sequence of calls to
the identified methods.
• Factor out the customizable steps of the algorithm
into Template Methods and provide a reasonable
default implementation for each Template Method.
• Document whether the base class’ implementation
of each Template Method can, should or must be
called by subclasses that override them.

15. Caution

16. Caution
-(void)viewWillAppear: (BOOL) animated {

17. Caution
-(void)viewWillAppear: (BOOL) animated {
myVariable = @”NewValue”;

18. Caution
-(void)viewWillAppear: (BOOL) animated {
myVariable = @”NewValue”;
}

19. Caution
-(void)viewWillAppear: (BOOL) animated {
myVariable = @”NewValue”;
}

20. Caution
-(void)viewWillAppear: (BOOL) animated {
myVariable = @”NewValue”;
}
-(void)viewWillAppear: (BOOL) animated {

21. Caution
-(void)viewWillAppear: (BOOL) animated {
myVariable = @”NewValue”;
}
-(void)viewWillAppear: (BOOL) animated {
myVariable = @”NewValue”;

22. Caution
-(void)viewWillAppear: (BOOL) animated {
myVariable = @”NewValue”;
}
-(void)viewWillAppear: (BOOL) animated {
myVariable = @”NewValue”;
[super viewWillAppear:animated];

23. Caution
-(void)viewWillAppear: (BOOL) animated {
myVariable = @”NewValue”;
}
-(void)viewWillAppear: (BOOL) animated {
myVariable = @”NewValue”;
[super viewWillAppear:animated];
}