***的方法是使用 获取特定值 并提供带有 的 NSExpressionDescriptionsum: 函数.

The best way is to use a fetch for specific values and supply a NSExpressionDescription with a sum: function.

当您执行 fetch 时，您会得到一个单元素数组，其中包含一个字典，其键与表达式描述匹配，其值是表达式的结果.在这种情况下，您将获得一个 sum 键，其值将是给定表达式的属性总和.

When you execute the fetch you get a one element array containing a dictionary whose keys match the expression descriptions and whose values are the results of the expressions. In this case, you would get a sum key whose value would be the sum of the attributes given the expression.