我如何通过apple(NSString和NSCFString)实现集群模式中的行为

Question

我只是编写以下代码用于测试目的:

NSString *aStr = [[NSString alloc] initWithFormat:@"Foo"];
aStr = [aStr initWithFormat:@"Bar"];//Crashed here

我收到以下错误:

*** initialization method -initWithFormat:locale:arguments: cannot be sent to an abstract object of class __NSCFString: Create a concrete instance!

如果我写下面的代码同样的事情发生

NSString *aStr = [NSString alloc];
aStr = [aStr initWithFormat:@"Foo"];
aStr = [aStr initWithFormat:@"Bar"]; //Crashed here

通过谷歌,我知道initWithFormat将返回NSCFString对象.我的问题是,如果NSCFString是派生类,NSString那么为什么我不能调用该initWithFormat方法NSCFString.如果可以停止可见性,我该如何在代码中实现.

Answer 1

让我们对NSString类集群如何在内部进行调查:

NSString *factory = [NSString alloc];
NSString *theInstance = [factory initWithString:@"I am constant"];
NSLog(@"factory class: %@, instance class: %@", [factory class], [theInstance class]);

输出是:

factory class: NSPlaceholderString, instance class: __NSCFConstantString

如您所见,该alloc方法返回一个实例NSPlaceholderString.它是一个"工厂"类,它实现了init...声明的所有方法NSString.这些方法返回的具体(私有)子类NSString.它__NSCFConstantString在此示例中返回.

如果您将第一行更改为

NSString *factory = [NSMutableString alloc];

输出将更改为:

NSPlaceholderMutableString,实例类:__ NSCFString

因此,可变和不可变字符串有不同的工厂类,并且这些工厂返回不同的子类.

您甚至可以在iOS运行时标题中检查私有子类的层次结构:此处和此处.

现在让我们看看当我们调用刚刚创建initWithString:的实例时会发生什么__NSCFConstantString.

[theInstance initWithString:@"Crash"];

如你所料 - 它崩溃了.在stacktrace中我们可以看到-[NSString initWithCharactersNoCopy:length:freeWhenDone:]调用该方法,抛出异常:

'NSInvalidArgumentException',原因:'***初始化方法-initWithCharactersNoCopy:length:freeWhenDone:无法发送到类__NSCFConstantString的抽象对象:创建一个具体的实例!'

所以我们可以猜测NSString类中的这个初始化器实际上是一个抽象方法(有点 - 在Objective-C中没有抽象方法,因此它在调用时抛出异常).

此方法在工厂类中实现NSPlaceholderString.但它并没有在所有具体的子类中实现,所以如果你调用任何init...方法,它将调用NSString抛出异常的实现.

让我们把它们放在一起,构建NSString类集群的一小部分.它真的很简单,可能与真正的实现完全不同,但我只想展示这个想法.

@interface NSPlaceholderString : NSString
@end

@interface __NSCFConstantString : NSString
@end


@implementation NSString

+ (instancetype)alloc {
    return [[NSPlaceholderString alloc] init];
}

- (instancetype)initWithCharactersNoCopy:(unichar *)characters length:(NSUInteger)length freeWhenDone:(BOOL)freeBuffer {
    [NSException raise:NSInvalidArgumentException format:@" initialization method -initWithCharactersNoCopy:length:freeWhenDone: cannot be sent to an abstract object of class %@: Create a concrete instance!'", [self class]];
    return nil;
}

- (instancetype)initWithString:(NSString *)aString {
//this method has to call the "abstract" initializer somewhere. The real implementation is probably more complex, this single line is here for simplicity
    return [self initWithCharactersNoCopy:[aString UTF8String] length:[aString length] freeWhenDone:YES];       
}

@end

@implementation NSPlaceholderString

- (instancetype)initWithCharactersNoCopy:(unichar *)characters length:(NSUInteger)length freeWhenDone:(BOOL)freeBuffer {
    __NSCFConstantString *concreteClassInstance = ...; // create the concrete instance. 
    return concreteClassInstance;
}

@end

@implementation __NSCFConstantString

//implement all the needed methods here. But do NOT implement initWithCharactersNoCopy:length:freeWhenDone:

@end