FIRStorageTask
@interface FIRStorageTask : NSObjectA superclass to all Storage tasks, including StorageUploadTask
and StorageDownloadTask, to provide state transitions, event raising, and common storage
for metadata and errors.
Callbacks are always fired on the developer-specified callback queue.
If no queue is specified, it defaults to the main queue.
This class is thread-safe.
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                  An immutable view of the task and associated metadata, progress, error, etc. DeclarationObjective-C @property (nonatomic, strong, readonly) FIRStorageTaskSnapshot *_Nonnull snapshot;
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                  Undocumented DeclarationObjective-C - (nonnull instancetype)init SWIFT_UNAVAILABLE;
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                  Undocumented DeclarationObjective-C + (nonnull instancetype)new SWIFT_UNAVAILABLE_MSG("-init is unavailable");