FIRStorageObservableTask
@interface FIRStorageObservableTask : FIRStorageTaskAn extended StorageTask providing observable semantics that can be used for responding to changes
in task state.
Observers produce a StorageHandle, which is used to keep track of and remove specific
observers at a later date.
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                  Observes changes in the upload status: Resume, Pause, Progress, Success, and Failure. \param status The StorageTaskStatuschange to observe.\param handler A callback that fires every time the status event occurs, containing a StorageTaskSnapshotdescribing task state.returns: A task handle that can be used to remove the observer at a later date. DeclarationObjective-C - (NSString *_Nonnull) observeStatus:(enum FIRStorageTaskStatus)status handler:(void (^_Nonnull)(FIRStorageTaskSnapshot *_Nonnull))handler;
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                  Removes the single observer with the provided handle. \param handle The handle of the task to remove. DeclarationObjective-C - (void)removeObserverWithHandle:(NSString *_Nonnull)handle;
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                  Removes all observers for a single status. \param status A StorageTaskStatusto remove all listeners for.DeclarationObjective-C - (void)removeAllObserversForStatus:(enum FIRStorageTaskStatus)status;
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                  Removes all observers. DeclarationObjective-C - (void)removeAllObservers;