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165 lines
4.5 KiB
Markdown
165 lines
4.5 KiB
Markdown
# Understanding Initializers
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## Initializers
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* The internal implementation of the new method is to call alloc first, then call init.
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* alloc: Sends the alloc message to a class; alloc returns an instance of that class.
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* init: An instance method whose role is to initialize the object.
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* Steps to create an object: first use alloc to create an object, then use init to initialize it before using the object.
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* Using an uninitialized object is dangerous.
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* init method: Purpose — initialize the object and set initial values; this is called a constructor (initializer).
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## Overriding init
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* If you want the created object's properties to have values other than the default initialization values, you need to override init.
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* Rules for overriding init:
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* You must first call the superclass's init method (because initialization of the current class proceeds through [super init]), and assign the returned value to self.
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* Calling init may fail; if it fails, return nil.
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* Check whether the superclass initialized successfully. If self != nil, initialization succeeded.
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* If initialization succeeded, initialize the current class's properties.
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* Finally return self.
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#### Clarifications:
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1. Why call the superclass init method?
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1. The current object has an isa pointer; the isa pointer is set through the superclass's init.
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2. You need to ensure that the superclass's properties are also initialized.
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2. Pattern for overriding init:
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```
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-(instancetype)init{
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if (self = [super init]) {
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// todo: custom property initialization
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}
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return self;
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}
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```
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```objc
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// Person
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#import <Foundation/Foundation.h>
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@interface Person : NSObject
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@property NSString* name;
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@property int age;
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-(void)sayHi;
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@end
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#import "Person.h"
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@implementation Person
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-(void)sayHi{
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NSLog(@"Hi");
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}
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-(instancetype)init{
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self = [super init];
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if (self) {
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self.name = @"杭城小刘";
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self.age = 22;
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}
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return self;
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}
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@end
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// Test
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Person *p1 = [[Person alloc] init]; // p1.name = "杭城小刘", p1.age = 22;
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Person *p2 = [Person new]; // p2.name = "杭城小刘", p2.age = 22;
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```
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If two classes have a composition relationship and one property is an object of another class, that property defaults to nil when the containing class is initialized. How to initialize it?
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```
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-(instancetype)init{
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self = [super init];
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if (self) {
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self.name = @"lbp";
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self.age = 22;
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self.pig = [[Pig alloc] init];
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}
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return self;
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}
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// Test
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Person *p1 = [[Person alloc] init]; // p1.pig != nil
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```
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## Custom initializers
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* Current situation: Although each created object's properties are not the default values, every initialized object has the same values.
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* Requirement: Allow callers to decide the property values when instantiating.
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* Solution: Custom initializer.
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* Rules for custom initializers:
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* The return type should be instancetype.
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* The method name must begin with initWith.
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* The implementation is similar to init.
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* Note: You cannot use new to call a custom initializer. (new performs alloc then init; default init assigns the default property values.)
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```
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-(instancetype)initWithName:(NSString *)name andAge:(int)age{
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if (self = [super init]) {
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self.name = name;
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self.age = age;
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}
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return self;
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}
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```
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```objc
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// Person
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#import <Foundation/Foundation.h>
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@interface Person : NSObject
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@property NSString* name;
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@property int age;
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-(instancetype)initWithName:(NSString *)name andAge:(int)age;
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@end
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#import "Person.h"
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@implementation Person
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-(instancetype)init{
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self = [super init];
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if (self) {
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self.name = @"lbp";
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self.age = 22;
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}
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return self;
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}
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// You cannot assign to self outside of an initializer.
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// The compiler treats only methods starting with initWith as initializers.
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-(instancetype)initWithName:(NSString *)name andAge:(int)age{
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if (self = [super init]) {
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self.name = name;
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self.age = age;
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}
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return self;
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}
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@end
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// Test
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Person *p1 = [[Person alloc] init];
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Person *p2 = [Person new];
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Person *p3 = [[Person alloc] initWithName:@"杭城小刘2号" andAge:23];
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```
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An experiment about "custom initializers must start with initWith"
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The error message is clear: you cannot assign to self outside of an initializer.
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Because the compiler considers only methods starting with initWith as initializers. |