一个更简单的解决方案可能是在您的 Expense 类上实现另一种方法,该方法可以为您提供适当的计算金额。
E.g.
- (NSDecimalNumber *) calculatedExpenseAmount {
NSDecimalNumber *actualAmount = self.expenseAmount;
// Pseudo-code begins
if (self.expenseCategory != nil) {
actualAmount = self.expenseAmount * self.expenseCategory.unitRate;
}
return actualAmount;
}
我正在补充我之前的答案。
如果您想避免拉入每个托管对象,您可以使用 NSDictionary-result 查询来拉出expenseAmount 和expenseCategory.unitRate 值。
- (NSDecimalNumber *) totalExpenses
{
// Fetch all of the expense amounts and unit rate of any related category.
NSFetchRequest *request = ...;
[request setManagedObjectContext:<...>];
[request setEntity:<ExpenseAccountDescription>];
[request setResultType:NSDictionaryResultType];
NSArray *props = [NSArray arrayWithObjects:@"expenseAmount", @"category.unitRate", nil];
[request setPropertiesToFetch:props];
NSArray *amounts = [request executeRequest:...];
// amounts is an array of dictionaries, each hold the desired property values.
// Loop and sum the individual amounts
NSDecimal *total = [[NSDecimalNumber zero] decimalNumber];
NSAutoreleasePool *pool = nil; // contain the mess
NSCalculationError err = NSCalculationNoError;
for (NSDictionary *result in amounts)
{
pool = [NSAutoreleasePool new];
NSDecimal newTotal = [[NSDecimalNumber zero] decimalNumber];
NSDecimalNumber *expenseAmount = [result valueForKeyPath:@"expenseAmount"];
NSDecimalNumber *unitRate = [result valueForKeyPath:@"category.unitRate"];
if (unitRate != nil) {
// do the unit rate multiplication and accumulate the result in the total
NSDecimal calculated = [[NSDecimalNumber zero] decimalNumber];
err = NSDecimalMultiply (&calculated, [expenseAmount decimalNumber], [unitRate decimalNumber], NSRoundBankers);
if (err == NSCalculationNoError) {
err = NSDecimalAdd (&newTotal, total, calculated, NSRoundBankers);
}
}
else {
// just accumulate the result in the total
err = NSDecimalAdd (&newTotal, total, [expenseAmount decimalNumber], NSRoundBankers);
}
// Keep the new total
NSDecimalCopy(&total, newTotal);
[pool drain];
}
return [NSDecimalNumber decimalNumberWithDecimal:total];
}
如果您有 10000 个费用条目,则此提取和计算可能需要不到 1MB 的 RAM。仪器将是您测量这一点的朋友。