FIRAggregateQuery
@interface FIRAggregateQuery : NSObjectA query that calculates aggregations over an underlying query.
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                  The query whose aggregations will be calculated by this object. DeclarationObjective-C @property (nonatomic, readonly) FIRQuery *_Nonnull query;
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                  Executes this query. DeclarationObjective-C - (void)aggregationWithSource:(FIRAggregateSource)source completion: (nonnull void (^)(FIRAggregateQuerySnapshot *_Nullable, NSError *_Nullable))completion;ParameterssourceThe source from which to acquire the aggregate results. completiona block to execute once the results have been successfully read. snapshot will be nilonly if error isnon-nil.