FSTSpecTests.mm: implement parseQueryPurpose

Author: dconeybe

Firestore/Example/Tests/SpecTests/FSTSpecTests.mm

@@ -327,6 +327,20 @@ - (SnapshotVersion)parseVersion:(NSNumber *_Nullable)version {
   return Version(version.longLongValue);
 }
 
+- (QueryPurpose)parseQueryPurpose:(NSNumber *)value {
+  switch ([value intValue]) {
+    case 0:
+      return QueryPurpose::Listen;
+    case 1:
+      return QueryPurpose::ExistenceFilterMismatch;
+    case 3:
+      return QueryPurpose::LimboResolution;
+    default:
+      XCTFail(@"unknown query purpose value: %@", value);
+      return QueryPurpose::Listen;
+  }
+}
+
 - (DocumentViewChange)parseChange:(NSDictionary *)jsonDoc ofType:(DocumentViewChange::Type)type {
   NSNumber *version = jsonDoc[@"version"];
   NSDictionary *options = jsonDoc[@"options"];
@@ -781,7 +795,7 @@ - (void)validateExpectedState:(nullable NSDictionary *)expectedState {
 
               QueryPurpose purpose = QueryPurpose::Listen;
               if ([queryData objectForKey:@"targetPurpose"] != nil) {
-                purpose = static_cast<QueryPurpose>([queryData[@"targetPurpose"] intValue]);
+                purpose = [self parseQueryPurpose:queryData[@"targetPurpose"]];
               }
 
               TargetData target_data(query.ToTarget(), targetID, 0, purpose);