【Collection Operators】
Collection operators
are specialized key paths that are passed as the parameter to
the valueForKeyPath: method. The operator
is specified by a string preceded by an at sign (@). The key path on the
left side of the collection operator, if present, determines the array or set,
relative to the receiver, that is used in the operation. The key path on the
right side of the operator specifies the property of the collection that the
operator uses. Figure
1 illustrates the operator key path format.
All the collection operators, with the exception
of @count, require a key path to the right of the
collection operator.
Note: It is not currently possible to define your own collection operators.
1 NSNumber *transactionAverage = [transactions valueForKeyPath:@"@avg.amount"]; 2 NSNumber *numberOfTransactions = [transactions valueForKeyPath:@"@count"]; 3 NSDate *latestDate = [transactions valueForKeyPath:@"@max.date"]; 4 NSDate *earliestDate = [transactions valueForKeyPath:@"@min.date"]; 5 NSNumber *amountSum = [transactions valueForKeyPath:@"@sum.amount"];
1 NSArray *payees = [transactions valueForKeyPath:@"@distinctUnionOfObjects.payee"]; 2 NSArray *payees = [transactions valueForKeyPath:@"@unionOfObjects.payee"]; 3 // Important: These operators raise an exception if any of the leaf objects is nil.
1 NSArray *payees = [arrayOfTransactionsArrays valueForKeyPath:@"@distinctUnionOfArrays.payee"]; 2 NSArray *payees = [arrayOfTransactionsArrays valueForKeyPath:@"@unionOfArrays.payee"]; 3 @distinctUnionOfSets 4 Important: These operators raise an exception if any of the leaf objects is nil.
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原文：http://www.cnblogs.com/tekkaman/p/3587721.html