Objective-C中的抽象类

Objective-C 没有抽象类，只是指语言的语法构造上没有C++/Java那样专门的abstract class定义，这并不妨碍我们在用Objective-C写程序的时候使用"abstract class"这一通用的OOP概念，即类的部分实现需要subclass来完成。

代码如下：

//  创建类Person 声明如下方法
@interface Person : NSObject
- (void)love;

- (void)coding;

- (void)travelling;

- (void)home;
@end

//  在.m文件中最好做如下处理
#define MethodNotImplemented() \
    @throw \
    [NSException exceptionWithName:NSInternalInconsistencyException \
                            reason:[NSString stringWithFormat:@"You must override %@ in a subclass", NSStringFromSelector(_cmd)] \
                          userInfo:nil]

@implementation Person

- (instancetype)init {
    self = [super init];
    if (!self) return nil;
    return self;
}

- (void)work { MethodNotImplemented(); }

- (void)coding {MethodNotImplemented(); }

- (void)love { MethodNotImplemented(); }

- (void)travelling { MethodNotImplemented(); }

- (void)home { MethodNotImplemented(); }

@end

这个声明的其实只是一个普通的基类，而且可以重写-init方法，那哪里抽象？和普通的类有什么区别？如果这个类需要有协议（代理），需要额外的属性，以及必须实现的的抽象方法，又该怎么处理？
下面，我又创建了一个Person+Private的类，这只是一个.h文件，并未实现任何的方法。

代码如下：

#import "Person.h"

@protocol PersonDelegate;

@interface Person ()

@property (nonatomic, copy) NSString *name;

@property (nonatomic, assign) int age;

@property (nonatomic, weak) id<PersonDelegate> delegate;

- (void)eat;

@end

@interface Person(Abstruct)

- (void)work;

@end

@protocol PersonDelegate <NSObject>

- (void)run;

@end

Snip20171015_2.png Snip20171015_1.png

配合 MethodNotImplemented() 宏，在OC中的抽象基类就创建好了。在子类中调用对应的方法之前实现该抽象类的方法即可。

抽象类在第三方框架中的应用分析：

Masonry

在Masonry中的MASConstraint就是一个抽象类，上例中的Person也是遵照MASConstrain的写法写的一个示例。
在MASConstraint中作者这样写的:

@interface MASConstraint : NSObject
- (MASConstraint * (^)(MASEdgeInsets insets))insets;
......
- (MASConstraint *)left;
- (MASConstraint *)top;
- (MASConstraint *)right;
- (MASConstraint *)bottom;
- (MASConstraint *)leading;
- (MASConstraint *)trailing;
- (MASConstraint *)width;
- (MASConstraint *)height;
- (MASConstraint *)centerX;
- (MASConstraint *)centerY;
- (MASConstraint *)baseline;

- (MASConstraint *)firstBaseline;
- (MASConstraint *)lastBaseline;
@end

@implementation MASConstraint
- (MASConstraint * (^)(id))equalTo {
    return ^id(id attribute) {
        return self.equalToWithRelation(attribute, NSLayoutRelationEqual);
    };
}

- (MASConstraint * (^)(id))mas_equalTo {
    return ^id(id attribute) {
        return self.equalToWithRelation(attribute, NSLayoutRelationEqual);
    };
}

- (MASConstraint * (^)(id))greaterThanOrEqualTo {
    return ^id(id attribute) {
        return self.equalToWithRelation(attribute, NSLayoutRelationGreaterThanOrEqual);
    };
}

- (MASConstraint * (^)(id))mas_greaterThanOrEqualTo {
    return ^id(id attribute) {
        return self.equalToWithRelation(attribute, NSLayoutRelationGreaterThanOrEqual);
    };
}

- (MASConstraint * (^)(id))lessThanOrEqualTo {
    return ^id(id attribute) {
        return self.equalToWithRelation(attribute, NSLayoutRelationLessThanOrEqual);
    };
}

- (MASConstraint * (^)(id))mas_lessThanOrEqualTo {
    return ^id(id attribute) {
        return self.equalToWithRelation(attribute, NSLayoutRelationLessThanOrEqual);
    };
}

#pragma mark - MASLayoutPriority proxies

- (MASConstraint * (^)(void))priorityLow {
    return ^id{
        self.priority(MASLayoutPriorityDefaultLow);
        return self;
    };
}

- (MASConstraint * (^)(void))priorityMedium {
    return ^id{
        self.priority(MASLayoutPriorityDefaultMedium);
        return self;
    };
}

- (MASConstraint * (^)(void))priorityHigh {
    return ^id{
        self.priority(MASLayoutPriorityDefaultHigh);
        return self;
    };
}

@end

MASConstraint+Private.h

@protocol MASConstraintDelegate;

//  延展类
@interface MASConstraint ()

@property (nonatomic, assign) BOOL updateExisting;

@property (nonatomic, weak) id<MASConstraintDelegate> delegate;

- (void)setLayoutConstantWithValue:(NSValue *)value;

@end


@interface MASConstraint (Abstract)

- (MASConstraint * (^)(id, NSLayoutRelation))equalToWithRelation;

- (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute;

@end


@protocol MASConstraintDelegate <NSObject>

- (void)constraint:(MASConstraint *)constraint shouldBeReplacedWithConstraint:(MASConstraint *)replacementConstraint;

- (MASConstraint *)constraint:(MASConstraint *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute;

@end

具体请参看Masonry源码。