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• To be sure there is a recent loop closure constraint, you can query PoseGraphInterface::constraints(), loop over all constraints and check if there is a recent one of type INTER that spans some time.

• A simpler solution is to subscribe to the global slam callback GlobalSlamOptimizationCallback by setting PoseGraphInterface::SetGlobalSlamOptimizationCallback. This will call you back after Cartographer has searched for loop closures. It does not tell you if loop closures were found, it mostly tells you if the background loop closure search is keeping up or falls behind.