NSString 类别中的未知类方法

Question

我有一个定义的类别：

#import <Foundation/Foundation.h>

@interface NSString (MyApp)

+ (UIColor *)colorFromHexString;

@end


#import "NSString+MyApp.h"

@implementation NSString (MyApp)

+ (UIColor *)colorFromHexString
{
    self = [self stringByReplacingOccurrencesOfString:@"#" withString:@""];
    unsigned rgbValue = 0;
    NSScanner *scanner = [NSScanner scannerWithString:hexString];
    [scanner scanHexInt:&rgbValue];
    return [UIColor colorWithRed:((rgbValue & 0xFF0000) >> 16)/255.0 green:((rgbValue & 0xFF00) >> 8)/255.0 blue:(rgbValue & 0xFF)/255.0 alpha:1.0];
}

@end

我在self = [self stringByReplacingOccurrencesOfString:@"#" withString:@""]; 行收到一条错误消息："No know class method for selector stringByReplacingOccurrencesOfString:withString" and "cannot assign self in a class method"。

我很困惑为什么我不能在NSString Category 中做到这一点。我在NSString Category 中看到examples online 与'self' 做的事情非常相似，所以我不确定为什么它在这里不起作用。

有人知道我做错了什么吗？

Answer 1

您的代码中有多个错误。

使用实例方法（-）代替静态方法（+）

// INCORRECT. self in a static method points to class object
// + (UIColor *)colorFromHexString

// CORRECT. self is instance method points to instance of class.
- (UIColor *)colorFromHexString

同样在第 1 行分配 hexString 而不是 self。

- (UIColor *)colorFromHexString
{
    NSString *hexString = [self stringByReplacingOccurrencesOfString:@"#" withString:@""];
    unsigned rgbValue = 0;
    NSScanner *scanner = [NSScanner scannerWithString:hexString];
    [scanner scanHexInt:&rgbValue];
    return [UIColor colorWithRed:((rgbValue & 0xFF0000) >> 16)/255.0 green:((rgbValue & 0xFF00) >> 8)/255.0 blue:(rgbValue & 0xFF)/255.0 alpha:1.0];
}