链接替换在NSPredicate中不起作用 [英] Chaining replacements don't work in NSPredicate

问题描述

以下代码提出了一个问题

The following code raises a question

NSPredicate * myPred = [NSPredicate predicateWithFormat:@"$my_var.employees.@count == 3"];

NSExpression * myExpr = [NSExpression    expressionForKeyPath:@"$my_var.test"] ;

 NSPredicate * myPredBis = [myPred predicateWithSubstitutionVariables:@{@"my_var" : myExpr}] ;



NSExpression * myExpr2 = [NSExpression    expressionForKeyPath:@"self"] ;

NSPredicate * myFinalPred = [myPredBis predicateWithSubstitutionVariables:@{@"my_var": myExpr2}];

NSLog(@"%@", myFinalPred) ;

它打印

$my_var.test.employees.@count == 3

但是我们想要

self.test.employees.@count == 3

为什么会这样?

推荐答案

这只是部分答案，但这也许有助于您走上正确的道路.

This is only a partial answer, but perhaps it helps to put you on the right path.

首先，将 $ my_var 替换为 [NSExpression expressionForKeyPath:@"$ my_var.test"] 不起作用，因为后一个表达式将 $ my_var 视为纯键，而不是变量.

First, substituting $my_var by [NSExpression expressionForKeyPath:@"$my_var.test"] does not work, because the latter expression treats $my_var as a plain key, not as a variable.

但是您可以使用 expressionForVariable 将一个变量替换为另一个变量:

But you can substitute one variable by another variable, using expressionForVariable:

NSPredicate * myPred = [NSPredicate predicateWithFormat:@"$my_var.employees.@count == 3"];
NSExpression * myExpr = [NSExpression expressionForVariable:@"another_var"] ;
NSPredicate * myPredBis = [myPred predicateWithSubstitutionVariables:@{@"my_var" : myExpr}] ;

第二， [NSExpression expressionForKeyPath:@"self"] 将"self"视为普通键，而不是"the" self ，即对象谓词被应用.你可能想要

Second, [NSExpression expressionForKeyPath:@"self"] treats "self" as an ordinary key, not as "the" self, i.e. the object onto which the predicate is applied. You probably want:

NSExpression * myExpr2 = [NSExpression expressionForEvaluatedObject];

相反.现在您可以链接替换了:

instead. Now you can chain the substitution:

NSPredicate * myFinalPred = [myPredBis predicateWithSubstitutionVariables:@{@"another_var": myExpr2}];
NSLog(@"%@", myFinalPred);

输出: employees.@ count == 3 ("self"部分只是不打印).

Output: employees.@count == 3 (the "self" part is just not printed).

好的，这是您解决问题的方法.棘手的部分是第二步.我没有找到构建包含以下内容的键路径表达式的直接方法:变量( $ my_var )与普通键( test )组合.但这是可以完成的通过获取谓词的左侧:

OK, here is the solution to your problem. The tricky part is the second step. I did not find a direct way to build a key-path expression that consists of a variable ($my_var) combined with a normal key (test). But it can be done by taking the left-hand side of a predicate:

NSPredicate * myPred = [NSPredicate predicateWithFormat:@"$my_var.employees.@count == 3"];
NSLog(@"%@", myPred) ;
// $my_var.employees.@count == 3            

NSExpression *myExpr = [(NSComparisonPredicate *)[NSPredicate predicateWithFormat:@"$my_var.test == 0"] leftExpression];
NSPredicate * myPredBis = [myPred predicateWithSubstitutionVariables:@{@"my_var" : myExpr}] ;
NSLog(@"%@", myPredBis) ; 
// $my_var.test.employees.@count == 3           

NSExpression * myExpr2 = [NSExpression expressionForEvaluatedObject];
NSPredicate * myFinalPred = [myPredBis predicateWithSubstitutionVariables:@{@"my_var": myExpr2}];
NSLog(@"%@", myFinalPred) ;            
// test.employees.@count == 3

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