Generic Prompt Type #696
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Generic Prompt Type #696
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dmlary
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Apr 6, 2024
rustyline doesn't currently support changing the prompt while in the core readline loop. There are a number of open PRs and issues for this functionality, but all of them appear to be stalled for more than a year. Looking at kkawakam#696 and 4ec26e8, the traditional appoach to this is to provide a reference to a trait object (`Prompt` or `ToString`), but with that appoach there's no way to cause the prompt to be redrawn for a change without user input. This means for these appoaches the prompt could change without being displayed to the user. There's an existing mechanism to allow another async task/thread to push input into the core readline loop, the `ExternalPrinter`. In this commit, I expand `ExternalPrinter` to add `set_prompt()`. With various plumbing, this function results in `wait_for_input` to return `Cmd::SetPrompt(String)`. One of key change here is `State.prompt` changes from `&str` to `String`. There is a performance hit here from the copy, but rustyline would need to prompt and receive input hundreds of times per second for the copy to have a noticable performance inpact. Added examples/dynamic_prompt.rs to demonstrate the functionality. Closes kkawakam#417 Related kkawakam#208, kkawakam#372, kkawakam#369, kkawakam#417, kkawakam#598, kkawakam#696
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Sometimes we want to asynchronously update the prompt. For example, if we are using rustyline as a remote terminal, we want to display the connection info in the prompt. When disconnected, it is nicer to show a message and change the prompt to a ready state. With a Generic prompt type, it is possible to do that.
Performance overhead: in order to display the prompt, it should implement ToString trait. Therefore, it has a little performance overhead because it calls to_string every time it needs the prompt as a string (printing the prompt or calculating the cursor position). However, it is negligible as the whole system is a human interface that does not affect functionality.