可可 – 解析和改变NSPredicate

2019年7月21日 218次阅读

我必须将数据从以前的应用程序版本迁移到新版本.这也会影响用户保存的一些谓词(NSPredicate实例),这意味着我必须以编程方式更改它们.

目前我尝试用[NSPredicate predicateFormat]解析我得到的字符串并手动更改一些表达式.例如oldKeyPath ==“something”to newKeyPath ==“something”.但感觉就像一个黑客,我很好奇,如果有更好的方法？

我阅读了Apple关于使用NSPredicate和NSExpression进行编程的文档.有很多方法可以从NSExpressions中组合NSPredicate对象.我希望找到从NSPredicate获取NSExpression对象的相反方法.我错过了什么吗？

谢谢你的提示.

解

感谢Martin R,我能够在NSPredicate上创建一个类别,允许我对表达式进行修改.

@implementation NSPredicate (ExtractComparisions)

- (NSPredicate *)predicateByChangingComparisionsWithBlock:(NSPredicate *(^)(NSComparisonPredicate *))block {
  if ([self isKindOfClass: [NSCompoundPredicate class]]) {
    NSCompoundPredicate *compPred = (NSCompoundPredicate *)self;
    NSMutableArray *predicates = [NSMutableArray array];
    for (NSPredicate *predicate in [compPred subpredicates]) {
      NSPredicate *newPredicate = [predicate predicateByChangingComparisionsWithBlock: block];
      if (newPredicate != nil)
        [predicates addObject: newPredicate];
    }
    return [[[NSCompoundPredicate alloc] initWithType: compPred.compoundPredicateType
                                       subpredicates: predicates] autorelease];
  } if ([self isKindOfClass: [NSComparisonPredicate class]]) {
    return block((NSComparisonPredicate *)self);
  }
  return self;
}

@end

以下是有关如何使用它的示例代码

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
  NSPredicate *predicate = [NSPredicate predicateWithFormat: @"key.path like %@ and count > 17", @"Hello"];
  NSPredicate *newPredicate = [predicate predicateByChangingComparisionsWithBlock:
                               ^NSPredicate *(NSComparisonPredicate *cp) {
    NSExpression *left = [cp leftExpression];
    NSExpression *right = [cp rightExpression];
    if ([[cp leftExpression] expressionType] == NSKeyPathExpressionType) {
      NSString *keyPath = [[cp leftExpression] keyPath];
      if ([keyPath isEqualToString: @"key.path"])
        left = [NSExpression expressionForKeyPath: @"key.new.path"];
      return [NSComparisonPredicate predicateWithLeftExpression: left
                                                rightExpression: right
                                                       modifier: cp.comparisonPredicateModifier
                                                           type: cp.predicateOperatorType
                                                        options:cp.options];
    }
    return cp;
  }];
  NSLog(@"Before: %@", predicate);
  NSLog(@"After: %@", newPredicate);
}

最佳答案 (这只是一个想法.)每个谓词都是子类的实例

NSCompoundPredicate或NSComparisonPredicate.所以你可以查看实际情况

谓词的类,将其强制转换为该类的对象然后

检查其属性.如有必要,请使用子谓词或重复该过程

表达式.

以下只是一个简单的复合谓词如何的例子
以这种方式“解剖”.检查所有案例不是一般解决方案,
但可能会指出你正确的方向.

NSPredicate *p0 = [NSPredicate predicateWithFormat:@"foo = 'bar' AND count > 17"];

if ([p0 isKindOfClass:[NSCompoundPredicate class]]) {
    NSCompoundPredicate *p0a = (NSCompoundPredicate *)p0;
    NSCompoundPredicateType type0 = p0a.compoundPredicateType;  // NSAndPredicateType
    NSPredicate *p1 = p0a.subpredicates[0];                     // foo = 'bar'
    NSPredicate *p2 = p0a.subpredicates[1];                     // count > 17
    if ([p1 isKindOfClass:[NSComparisonPredicate class]]) {
        NSComparisonPredicate *p1a = (NSComparisonPredicate *)p1;
        NSPredicateOperatorType type1 = p1a.predicateOperatorType;  // NSEqualToPredicateOperatorType
        NSExpression *e3 = p1a.leftExpression;                  // foo, NSKeyPathExpression
        NSExpression *e4 = p1a.rightExpression;                 // "bar", NSConstantValueExpression
    }
    if ([p2 isKindOfClass:[NSComparisonPredicate class]]) {
        NSComparisonPredicate *p2a = (NSComparisonPredicate *)p2;
        NSPredicateOperatorType type2 = p2a.predicateOperatorType;  // NSGreaterThanPredicateOperatorType
        NSExpression *e5 = p2a.leftExpression;                  // count, NSKeyPathExpression
        NSExpression *e6 = p2a.rightExpression;                 // 17, NSConstantValueExpression
    }
}
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