vb.net barcode maker Circle class in Objective-C

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0, 0, 10, 30 Red (Circle)
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- (void) draw { draw a circle in the bounds filled with fillColor } // draw - (float) area { calculate the area of the circle } // area
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- (BOOL) hitTestWithPoint: (Point) point { see if point is inside the circle } // hitTestWithPoint:
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Figure 3-3. A circle and its class
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What s the point of having class objects Wouldn t it be simpler just to have each object point directly to its code Indeed, it would be simpler, and some OOP systems do just that. But having class objects is a great advantage: if you change the class at runtime, all objects of that class automatically pick up the changes (we ll discuss this more in later chapters). Figure 3-4 shows how the draw message ends up calling the right function for the circle object.
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CHAPTER 3: Introduction to Object-Oriented Programming
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- (void) draw { draw a circle in the bounds filled with fillColor } // draw
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Circle class
code
- (float) area { calculate the area of the circle } // area
[shape draw];
- (BOOL) hitTestWithPoint: (Point) point { see if point is inside the circle } // hitTestWithPoint:
Figure 3-4. A circle finds its draw code.
Here are the steps illustrated in Figure 3-4: 1. The object that is the target of the message (the red circle in this case) is consulted to see what its class is. 2. The class looks through its code and finds out where the draw function is. 3. Once it s found, the function that draws circles is executed. Figure 3-5 shows what happens when you call [shape draw] on the second shape in the array, which is the green rectangle.
- (void) draw { draw a rectangle in the bounds filled with fillColor } // draw
30, 40, 50, 60 Green (Rectangle)
Rectangle class
code
- (float) area { calculate the area of the rectangle } // area
[shape draw];
- (BOOL) hitTestWithPoint: (Point) point { see if point is inside the rectangle } // hitTestWithPoint:
Figure 3-5. A rectangle finds its draw code.
CHAPTER 3: Introduction to Object-Oriented Programming
The steps used in Figure 3-5 are nearly identical those in the previous image: 1. The target object of the message (the green rectangle) is consulted to see what its class is. 2. The rectangle class checks its pile of code and gets the address of the draw function. 3. Objective-C runs the code that draws a rectangle. This program shows some very cool indirection in action! In the procedural version of the program, we had to write code that determined which function to call. Now, that decision is made behind the scenes by Objective-C, as it asks the objects which class they belong to. This reduces the chance of calling the wrong function and makes our code easier to maintain.
Time Out for Terminology
Before we dig into the rest of the Shapes-Object program, let s take a moment to go over some object-oriented terminology. We ve already talked about some of these terms; others are brand new.
A class is a structure that represents an object s type. An object refers to its class to get various information about itself, particularly what code to run to handle each action. Simple programs might have a handful of classes; moderately complex ones will have a couple of dozen. Objective-C style encourages developers to capitalize class names. An object is structure containing values and a hidden pointer to its class. Running programs typically have hundreds or thousands of objects. Objective-C variables that refer to objects are typically not capitalized. Instance is another word for object. For example, a circle object can also be called an instance of class Circle. A message is an action that an object can perform. This is what you send to an object to tell it to do something. In the [shape draw] code, the draw message is sent to the shape object to tell it to draw itself. When an object receives a message, its class is consulted to find the proper code to run. A method is code that runs in response to a message. A message, such as draw, can invoke different methods depending on the class of the object. The method dispatcher is the mechanism used by Objective-C to divine which method will be executed in response to a particular message. We ll get out our shovels and dig a lot more into the Objective-C method dispatch mechanism in the next chapter.
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