FIRDocumentChange
@interface FIRDocumentChange : NSObjectA DocumentChange represents a change to the documents matching a query. It contains the
document affected and the type of change that occurred (added, modified, or removed).
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                  The type of change that occurred (added, modified, or removed). DeclarationObjective-C @property (nonatomic, readonly) FIRDocumentChangeType type;
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                  The document affected by this change. DeclarationObjective-C @property (nonatomic, strong, readonly) FIRQueryDocumentSnapshot *_Nonnull document;
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                  The index of the changed document in the result set immediately prior to this DocumentChange(i.e. supposing that all priorDocumentChangeobjects have been applied).NSNotFoundforDocumentChangeTypeAddedevents.DeclarationObjective-C @property (nonatomic, readonly) NSUInteger oldIndex;
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                  The index of the changed document in the result set immediately after this DocumentChange(i.e. supposing that all priorDocumentChangeobjects and the currentDocumentChangeobject have been applied).NSNotFoundforDocumentChangeTypeRemovedevents.DeclarationObjective-C @property (nonatomic, readonly) NSUInteger newIndex;