Objective-c (Syntax II)

category: You can extend some methods to a class (without modifying the code of the original class)

function : You can add some methods to the class without changing the original class content.

Use note :

1. Only add methods, cannot increase member variables

2. member variables in the original class can be accessed in the classification method implementation

3. Classification can be re-implemented in the original class method, overwriting the original class method, resulting in the original class method can no longer be used

4. Classification of the highest priority: Call a method first to the classification to find, then go to the original class, and then find the parent class

Classification (last compiled precedence)-"original Class-" Parent class

Statement:

@interface class names (category name)

@end

Realize:

@implementation class Name

@end

the nature of the class :

The class itself is also an object, a class-type object, or a class object

Definition of class type

typedef struct OBJC Class *class

The class name represents the class object, with only one class object per class

+load methods and +initialize methods:

1. When the program starts, all classes and classifications in the project are loaded, and the +load method for each class and classification is called after it is loaded. will only be called once.

2. When a class is used for the first time, the +initialize method of the current class is called, and a class invokes only one +initialize method

3. Load the parent class first, then load the subclass (call the +load method of the parent class first, and then call the +load method of the subclass)

Initializes the parent class first, initializes the subclass (calls the +initialize method of the parent class, and then calls the +initialize method of the subclass.

Description method

Modify the default output when using NSLog output properties, overriding the +description method or-desription method

+description Method:

By default, when you use the NSLog and%@ output objects, the-desription method of the calling object, the return value (NSString *) screen output is the:< class name: Memory address >

+desription Method:

By default, when you use the NSLog and%@ output class objects, you call the +desription method of the class object and get the return value (NSString *) of the screen output

Note the point:

Do not attempt to use self in the-desription method, resulting in a dead loop

NSLog Printing

1. %p

Print the address of the pointer variable p

NSLog (@ "%p", &p);

Print Object Address

NSLog (@ "%p", p);

2. %@

%@ Print < Class Name: Object address >

NSLog (@ "%@", p);

3. _line_ %d output Current line number

4. _file_ %s output file path

5. _func_ %s output Current function

SEL

Full name Selector

The SEL is actually a wrapper for the method, wrapping the method into an SEL type of data, finding the corresponding method address, and finding the method address to invoke the method

_cmd represents the current method

The message is sel.

Objective-c (Syntax II)