使用 NSPredicate 搜索/过滤自定义类数组 [英] Searching/Filtering a custom class array with a NSPredicate

问题描述

我有一个包含自定义类对象的数组，我想根据类属性之一是否包含自定义字符串来过滤该数组.我有一个方法，它传递了我想要搜索的属性(列)和它将搜索的字符串(searchString).这是我的代码:

I have a array that contains objects of a custom class, and I would like to filter the array based on if one of the classes attributes contains a custom string. I have a method that is passed the attribute that I want to be searched (column) and the string that it will search for (searchString). Here is the code I have:

NSPredicate *query = [NSPredicate predicateWithFormat:@"%K contains %K", column,   searchString];
NSMutableArray *temp = [displayProviders mutableCopy];
[displayProviders release];
displayProviders = [[temp filteredArrayUsingPredicate:query] mutableCopy];
[temp release];

然而，它总是抛出异常displayProviders = [[temp filteredArrayUsingPredicate:query] mutableCopy];说这个类不是键值编码兼容的键 [无论 searchString 是什么].

However, it always throws an exception on displayProviders = [[temp filteredArrayUsingPredicate:query] mutableCopy]; saying this class is not key value coding-compliant for the key [whatever searchString is].

知道我做错了什么吗?

推荐答案

[NSPredicate predicateWithFormat:@"%@ contains %@", column,      searchString];

当您在谓词格式字符串中使用 %@ 替换时，结果表达式将是一个常量值.听起来你不想要一个常数值；相反，您希望将属性名称解释为键路径.

When you use the %@ substitution in a predicate format string, the resulting expression will be a constant value. It sounds like you don't want a constant value; rather, you want the name of an attribute to be interpreted as a key path.

换句话说，如果你这样做:

In other words, if you're doing this:

NSString *column = @"name";
NSString *searchString = @"Dave";
NSPredicate *p = [NSPredicate predicateWithFormat:@"%@ contains %@", column, searchString];

这将等同于:

p = [NSPredicate predicateWithFormat:@"'name' contains 'Dave'"];

与以下相同:

BOOL contains = [@"name rangeOfString:@"Dave"].location != NSNotFound;
// "contains" will ALWAYS be false
// since the string "name" does not contain "Dave"

这显然不是您想要的.你想要相当于这个:

That's clearly not what you want. You want the equivalent of this:

p = [NSPredicate predicateWithFormat:@"name contains 'Dave'"];

为了得到这个，你不能使用 %@ 作为格式说明符.你必须使用 %K 代替.%K 是谓词格式字符串独有的说明符，表示被替换的字符串应该被解释为关键路径(即属性的名称)，而不是作为文字字符串.

In order to get this, you can't use %@ as the format specifier. You have to use %K instead. %K is a specifier unique to predicate format strings, and it means that the substituted string should be interpreted as a key path (i.e., name of a property), and not as a literal string.

所以你的代码应该是:

NSPredicate *query = [NSPredicate predicateWithFormat:@"%K contains %@", column, searchString];

<小时>

使用 @"%K contains %K" 也不起作用，因为这与:

---

Using @"%K contains %K" doesn't work either, because that's the same as:

[NSPredicate predicateWithFormat:@"name contains Dave"]

与以下相同:

BOOL contains = [[object name] rangeOfString:[object Dave]].location != NSNotFound;

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