通过以编程方式设置键来动态创建NSPredicate [英] Creating NSPredicate dynamically by setting the key programmatically

问题描述

为什么以下代码段之前的代码段不工作，而不是后者？

Why does the former of following snippets work while not the latter ?

代码段1

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(coin_unique == %@)", [NSNumber numberWithInt:species]];

程式码片段2

// Does NOT Work
NSString *predicateText = @"coin_unique";
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(%@ == %@)", predicateText, [NSNumber numberWithInt:species]];

我必须动态创建谓词，取决于我的方法中接收的参数。

I have to dynamically create predicate depending upon the argument received in my method.

推荐答案

coin_unique 是一个键，所以需要％K 格式说明符：

coin_unique is a key, so it needs the %K format specifier:

NSString *predicateText = @"coin_unique";
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(%K == %@)", predicateText, [NSNumber numberWithInt:species]];

格式语法描述得相当不错这里。

The format syntax is described quite well here.

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