iphone - 在 Objective-C 中确定属性是否为 int、float、double、NSString、NSDate、NSNumber 等

Question

我需要确定一个对象的属性(按名称传递)类型，以便从 XML 执行反序列化。我有一些通用的伪代码(但是我不确定如何在 Objective-C 中执行这些比较):

id object = [[[Record alloc] init] autorelease];

NSString *element = @"date";
NSString *data = @"2010-10-16";

objc_property_t property = class_getProperty([object class], [element UTF8String]);
const char *attributes = property_getAttributes(property);

char buffer[strlen(attributes) + 1];
strcpy(buffer, attributes);

char *attribute = strtok(buffer, ",");
if (*attribute == 'T') attribute++; else attribute = NULL;

if (attribute == NULL);
else if (strcmp(attribute, "@\"NSDate\"") == 0) [object setValue:[NSDate convertToDate:self.value] forKey:element];
else if (strcmp(attribute, "@\"NSString\"") == 0) [object setValue:[NSString convertToString:self.value] forKey:element];
else if (strcmp(attribute, "@\"NSNumber\"") == 0) [object setValue:[NSNumber convertToNumber:self.value] forKey:element];

我已经查看了 class_getProperty和 property_getAttributes , 但是我仍然不确定如何进行上述比较。

Answer 1

@Ahruman 的回答是正确的，如果你正在处理对象。让我提出一些替代方案:

1. valueForKey::如果您使用[myObject valueForKey:@"myPropertyName"]，它将返回一个对象。如果该属性对应于某种原始类型(int、float、CGRect 等)，那么它将被装箱到一个 NSNumber 或 NSValue(视情况而定)。如果它作为 NSNumber 返回，您可以轻松提取 double 表示 (doubleValue) 并将其用作 NSTimeInterval 以创建 NSDate。我可能会推荐这种方法。

2. 每种类型的特例。 property_getAttributes() 返回表示属性所有属性的 char*，您可以通过执行以下操作提取类型:

   const char * type = property_getAttributes(class_getProperty([self class], "myPropertyName"));NSString * typeString = [NSString stringWithUTF8String:type];NSArray * attributes = [typeString componentsSeparatedByString:@","];NSString * typeAttribute = [attributes objectAtIndex:0];NSString * propertyType = [typeAttribute substringFromIndex:1];const char * rawPropertyType = [propertyType UTF8String];if (strcmp(rawPropertyType, @encode(float)) == 0) {  //it's a float} else if (strcmp(rawPropertyType, @encode(int)) == 0) {  //it's an int} else if (strcmp(rawPropertyType, @encode(id)) == 0) {  //it's some sort of object} else ....

   This is pedantically more correct than Louis's answer, because while most types have a single-character encoding, they don't have to. (his suggestion assumes a single-character encoding)

3. Finally, if you're doing this on a subclass of NSManagedObject, then I would encourage checking out NSPropertyDescription.

From these alternatives, you can probably see that letting the runtime box the value for you is probably simplest.

edit extracting the type:

From the code above, you can extract the class name like so:

if ([typeAttribute hasPrefix:@"T@"] && [typeAttribute length] > 1) {
  NSString * typeClassName = [typeAttribute substringWithRange:NSMakeRange(3, [typeAttribute length]-4)];  //turns @"NSDate" into NSDate
  Class typeClass = NSClassFromString(typeClassName);
  if (typeClass != nil) {
    [object setValue:[self convertValue:self.value toType:typeClass] forKey:element];
  }
}

然后不使用类方法类别来进行转换(即 [NSDate convertToDate:])，而是在 self 上创建一个方法来为您完成转换，并且接受所需的类型作为参数。你可以(现在)这样做:

- (id) convertValue:(id)value toType:(Class)type {
  if (type == [NSDate class]) {
    return [NSDate convertToDate:value];
  } else if (type == [NSString class]) {
    return [NSString convertToString:value];
  } ...
}

不过，我内心的一部分在想:你究竟为什么需要以这种方式做事？你在做什么？