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Conditional set: set_if atomically sets the value only when the current value satisfies a predicate, useful for state machine transitions that should only happen from a specific state.，推荐阅读91视频获取更多信息
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This callback is also given the key as an argument, which can be useful for cases where the default value is based on the key.
println("finished")。业内人士推荐快连下载作为进阶阅读