iOS：当迁移coredata关系时，什么是值表达式函数？ [英] iOS: what is the value expression function when migrating coredata relationship?

问题描述

关系的函数如下： FUNCTION（$ manager，destinationInstancesForEntityMappingNamed：sourceInstances：，employeesToEmployees，$ source.employees）

这个函数是什么？如何调用？是否有任何指南介绍？

阅读Apple的

核心数据模型版本化和数据迁移计划指南

但我还是没有得到。

解决方案

这是一个任意方法调用的函数表达式知道是 NSExpression类参考中的一个段落：

函数表达式

在OS X v10.4上， NSExpression 只支持
预定义的函数集： sum ， count ， min ， max 平均。这些
预定义函数在谓词语法中使用
自定义关键字（例如 MAX（1,5,10））访问。

在OS X v10.5及更高版本中，函数表达式还支持任意的
方法调用。要使用这个扩展功能，现在
对
使用语法 FUNCTION（receiver，selectorName，arguments，...） / p>

FUNCTION（@/ Developer / Tools / otest，@lastPathComponent）=> @otest

示例代码中的引用似乎不正确。但是下面的代码在iOS 5/6上编译和运行：

  NSExpression * expr = [NSExpression expressionWithFormat：@FUNCTION / Developer / Tools / otest'，'lastPathComponent'）]; 
 id result = [expr expressionValueWithObject：nil context：nil]; 
 NSLog（@result：％@，result）; 
 //输出：
 // otest 
  

是一个函数表达式，当被评估时调用

  [$ manager destinationInstancesForEntityMappingNamed：@employeesToEmployees
 sourceInstances：$ source .employees] 
  

其中 $ manager code> $ source 替换为迁移管理器和源对象，如映射模型对象。 p>

The function for a relationship is like: FUNCTION($manager, "destinationInstancesForEntityMappingNamed:sourceInstances:","employeesToEmployees",$source.employees")

What is this "Function"? How will it be called? Is there any guide introducing to this?

I've read Apple's

Core Data Model Versioning and Data Migration programming guide

but I still don't get this.

解决方案

This is a "function expressions with arbitrary method invocations" which seem to be very poorly documented. The only reference that I know of is one paragraph in the NSExpression Class Reference:

Function Expressions

On OS X v10.4, NSExpression only supports a predefined set of functions: sum, count, min, max, and average. These predefined functions were accessed in the predicate syntax using custom keywords (for example, MAX(1, 5, 10)).

On OS X v10.5 and later, function expressions also support arbitrary method invocations. To use this extended functionality, you can now use the syntax FUNCTION(receiver, selectorName, arguments, ...), for example:

FUNCTION(@"/Developer/Tools/otest", @"lastPathComponent") => @"otest"

The quoting in that sample code seems be incorrect. But the following code compiles and runs on iOS 5/6:

NSExpression *expr = [NSExpression expressionWithFormat:@"FUNCTION('/Developer/Tools/otest', 'lastPathComponent')"];
id result = [expr expressionValueWithObject:nil context:nil];
NSLog(@"result: %@", result);
// Output:
// otest

So in your case, it is a function expression which calls, when evaluated

[$manager destinationInstancesForEntityMappingNamed:@"employeesToEmployees"
                                    sourceInstances:$source.employees]

where $manager and $source are replaced by the migration manager and the source object, as described in Mapping Model Objects in the "Core Data Model Versioning and Data Migration Programming Guide".

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