How do I get and set the cursor position? Where is the buffer?

[Thraka]

Trying to learn how this works and abandon System.Console. I'm unsure though how I get/set the cursor to a specific position. Also, how do I determine the screen/buffer position?

[alexrp]

I'm unsure though how I get/set the cursor to a specific position.

Setting the cursor position is done with the MoveCursor* family of methods on Terminal.

The only cursor tracking currently supported is via the SaveCursorPosition/RestoreCursorPosition methods on Terminal, which use DECSC and DECSR.

The reason that there isn't a more general CursorPosition property is that it's actually really complicated to do in VT100. You have to write a DECXCPR and then read a response. That sounds simple enough on the surface until you realize that there can be arbitrary data coming in on standard input between sending the query and getting the response. That data then has to be put into a buffer of some kind that sits in front of standard input. You also need to contend with the fact that another thread might be reading from standard input at the same time. There is no solution to this problem that doesn't have drawbacks (e.g. the ability to cause deadlocks under certain circumstances). There's also the whole issue about character widths in terminal emulators not being standardized in any meaningful way.

If you're curious about how System.Console does it, see here.

Given the drawbacks, complexity, and unknowns of implementing this, I've opted not to do it for now. I'm not necessarily opposed to doing it at some point, but I reasoned that most applications would probably be fine with SaveCursorPosition + RestoreCursorPosition and/or would have to do manual tracking anyway for more complex TUIs and the like.

Also, how do I determine the screen/buffer position?

Do you mean screen position as in where the console window is on the monitor? If so, there's no way to get that information on Unix platforms, so the API doesn't currently expose it.

It's certainly possible to expose this information regardless and just throw PlatformNotSupportedException on Unix platforms, but a big motivator for creating System.Terminal in the first place was to expose a unified, cross-platform terminal API, as opposed to .NET Core's System.Console which has many methods/properties that throw on Unix platforms. I worry that exposing platform-specific stuff would encourage developers using those platforms to write non-portable code.

[Thraka]

Hi @alexrp thank you for the answer. I'm trying to design an interactive console app where you can use the arrow keys to select output and being able to at least know where the cursor is lets me understand what parts i need to clear and rewrite. I guess I can try and figure out some logic around it, but it still will be a bit tricky.

What about when that command is sent, turning on a peek within the code that is watching the incoming stream looking for the appropriate codes. When the codes are found, get the result, turn off the peek. Each peek on the stream is passing back to the normal terminal output when it's not matching what is being looked for. You can also have a timeout that gives up on looking and either raises an event or just does nothing.

When I was talking about the buffer, I was meaning where top line of the screen within the buffer area. Like the buffer is currently 1000 lines long and the screen top is looking at line 24 of the buffer and the screen is 25 lines tall, meaning the bottom of the screen is at line 49 or something along those lines.

Thanks!

[alexrp]

I'm trying to design an interactive console app where you can use the arrow keys to select output and being able to at least know where the cursor is lets me understand what parts i need to clear and rewrite. I guess I can try and figure out some logic around it, but it still will be a bit tricky.

My recommendation would be to keep track of the cursor position as you write output, if that's possible in your case.

When I was talking about the buffer, I was meaning where top line of the screen within the buffer area. Like the buffer is currently 1000 lines long and the screen top is looking at line 24 of the buffer and the screen is 25 lines tall, meaning the bottom of the screen is at line 49 or something along those lines.

Similarly, I think you would have to keep track of this yourself as you write output. I don't know that there's any way to query this from the terminal.

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What about when that command is sent, turning on a peek within the code that is watching the incoming stream looking for the appropriate codes. When the codes are found, get the result, turn off the peek. Each peek on the stream is passing back to the normal terminal output when it's not matching what is being looked for. You can also have a timeout that gives up on looking and either raises an event or just does nothing.

Sure, there are many ways it could be done. It's just that all of them are fairly tricky and seem to involve user-visible trade-offs - e.g. in System.Console's case, the inability to read from standard input at the same time as accessing Cursor* properties. That's why I've held off on implementing this because I want to try to come up with a better design, if at all possible.

[Thraka]

Thanks!