NSPredicate predicateWithFormat:(NSString *)不一致? [英] NSPredicate predicateWithFormat:(NSString*) inconsistency?

问题描述

在根据用户输入过滤我的NSMutableDictionary时，我创建了以下代码:

While working on filtering my NSMutableDictionary based on user input, I created the following code:

NSString *predicateString = [NSString stringWithFormat:@"SELF beginsWith[cd] %@", searchString];
NSPredicate *pred = [NSPredicate predicateWithFormat:predicateString];
NSArray *filteredKeys = [[myMutableDictionary allKeys] filteredArrayUsingPredicate:pred]; 

"searchString"通过以下定义传递到方法中:

"searchString" is passed into the method with this definition:

(NSString*) searchString

但这导致以下异常:

... raised [valueForUndefinedKey:]:此类是 不符合该键的键值编码...

...raised [ valueForUndefinedKey:]: this class is not key value coding-compliant for the key...

原来的解决方法是:

NSPredicate *pred = [NSPredicate predicateWithFormat:@"SELF beginsWith[cd] %@", searchString];
NSArray *filteredKeys = [[myMutableDictionary allKeys] filteredArrayUsingPredicate:pred]; 

我不明白的是，后者为何起作用，而前者引发了例外.我已经阅读了键值编码，但我不明白在这里如何应用. (即仅通过更改NSPredicate的定义方式)有人可以启发我吗?

What I don't understand, is why the latter worked, and the former threw the exception. I have read a little bit on key value coding, but I don't understand how it applies here. (i.e. only by changing how the NSPredicate is defined) Can someone enlighten me?

更新: 为了回应jtbandes的评论，我继续创建了一个TestApp项目来演示此问题. http://dl.dropbox.com/u/401317/TestApp1.tar.gz

Update: In response to jtbandes comment, I went ahead and created a TestApp project to demo this problem. http://dl.dropbox.com/u/401317/TestApp1.tar.gz

推荐答案

答案是

字符串常量必须在表达式中加引号-单引号和双引号均可接受，... 如果您使用％@ ...进行变量替换，则会自动为您添加引号 .如果您在格式字符串中使用字符串常量，则您必须自己用引号

String constants must be quoted within the expression—single and double quotes are both acceptable, ... If you use variable substitution using %@ ..., the quotation marks are added for you automatically. If you use string constants within your format string, you must quote them yourself

[我的重点]

predicateWithFormat为您添加引号，但stringWithFormat则不.如果您这样做，第一个示例可能会起作用:

predicateWithFormat puts the quotes in for you, but stringWithFormat doesn't. Your first example would probably work if you did this:

NSString *predicateString = [NSString stringWithFormat:@"SELF beginsWith[cd] '%@'", searchString];
//                                                                           ^  ^ single or double quotes

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