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，我想基于其中一个类属性是否包含自定义字符串来过滤数组。我有一个方法，传递我想要搜索的属性（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，它总是在
抛出异常displayProviders = [[temp filteredArrayUsingPredicate：query] mutableCopy];
说这个类不是密钥值编码兼容的密钥[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].

任何想法我做错了什么？

Any ideas what I am doing wrong?

推荐答案

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

当您使用％@ 替换时谓词格式字符串，结果表达式将是常量值。听起来你不想要一个恒定的价值;相反，您希望将属性的名称解释为关键路径。

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，如果您这样做：

In other words, if you're doing this:

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

这相当于：

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

与以下内容相同：

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

这显然不是你想要的。你想要的等价物这个：

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 。％K 是谓词格式字符串的唯一说明符，它表示替换字符串应解释为键路径（即属性名称），不作为文字字符串。

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];

---

使用 @％K包含％K也不起作用，因为它与以下内容相同：

---

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

[NSPredicate predicateWithFormat:@"name contains Dave"]

与以下内容相同：

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

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