public struct MultiplexLogHandler : LogHandler

A pseudo-LogHandler that can be used to send messages to multiple other LogHandlers.

Effective Logger.Level

When first initialized the multiplex log handlers’ log level is automatically set to the minimum of all the passed in log handlers. This ensures that each of the handlers will be able to log at their appropriate level any log events they might be interested in.

Example: If log handler A is logging at .debug level, and log handler B is logging at .info level, the constructed MultiplexLogHandler([A, B])‘s effective log level will be set to .debug, meaning that debug messages will be handled by this handler, while only logged by the underlying A log handler (since B’s log level is .info and thus it would not actually log that log message).

If the log level is set on a Logger backed by an MultiplexLogHandler the log level will apply to all underlying log handlers, allowing a logger to still select at what level it wants to log regardless of if the underlying handler is a multiplex or a normal one. If for some reason one might want to not allow changing a log level of a specific handler passed into the multiplex log handler, this is possible by wrapping it in a handler which ignores any log level changes.

Effective Logger.Metadata

Since a MultiplexLogHandler is a combination of multiple log handlers, the handling of metadata can be non-obvious. For example, the underlying log handlers may have metadata of their own set before they are used to initialize the multiplex log handler.

The multiplex log handler acts purely as proxy and does not make any changes to underlying handler metadata other than proxying writes that users made on a Logger instance backed by this handler.

Setting metadata is always proxied through to all underlying handlers, meaning that if a modification like logger[metadataKey: "x"] = "y" is made, all underlying log handlers that this multiplex handler was initiated with will observe this change.

Reading metadata from the multiplex log handler MAY need to pick one of conflicting values if the underlying log handlers were already initiated with some metadata before passing them into the multiplex handler. The multiplex handler uses the order in which the handlers were passed in during its initialization as a priority indicator - the first handler’s values are more important than the next handlers values, etc.

Example: If the multiplex log handler was initiated with two handlers like this: MultiplexLogHandler([handler1, handler2]). The handlers each have some already set metadata: handler1 has metadata values for keys one and all, and handler2 has values for keys two and all.

A query through the multiplex log handler the key one naturally returns handler1’s value, and a query for two naturally returns handler2’s value. Querying for the key all will return handler1’s value, as that handler was indicated “more important” than the second handler. The same rule applies when querying for the metadata property of the multiplex log handler - it constructs Metadata uniquing values.