Asynchronous Command Streaming

### Synopsis

`[jqxhr=] loadcmd(command [,target] [,filter] [,timeout])`

The `loadcmd()` function executes a shell command or evaluates javascript code with asynchronous streaming of the output into a target element or onto a function.

### Basic Usage

As this is the underlying function for command buttons, basic usage is very similar, but fully scriptable:

As with command buttons, you can append the output by prefixing the target selector with "+". You can omit the output by passing null as the target. If target is a DOM element, loadcmd automatically sets the `loading` class on the target while the command is processed.

The target element's min-height is automatically fixed to the current height of the target before the output is set. This avoids page layout jumps when reusing a target for commands.

If the target is scrollable and the content exceeds the visible area, the element is automatically scrolled to show the new content, unless the user has done a manual scrolling on the element.

If the command output stops for `timeout` seconds, the request is aborted and a timeout error is shown. Default timeout is 20 seconds for standard commands, 300 seconds for known long running commands.

### Evaluate Javascript Code

To execute Javascript, either pass an object instead of the `command` string, containing the command as property "command" and a second property "type" with value "js", or simply use the wrapper call `loadjs()`. Example:

Javascript evaluation is not limited to a single command or line. Hint: to avoid encoding complex JS code in the onclick attribute, store the code in some hidden DOM element and read it via `$(…).text()`.

### Output Encoding & Binary Streams

To allow binary data to be sent by a command and to enable processing of the result by Javascript on the browser without character encoding issues, the command API supports output "binary" mode.

Primary application for this is exchanging CBOR encoded objects between the module and the web frontend. To fully support this, the OVMS web frontent includes the CBOR library by Patrick Gansterer. Example use:

        loadjs({ command: "auxbatmon.dump('CBOR')", output: "binary" }).done((data) => {
          history = CBOR.decode(data);
        });

See the AuxBatMon and PwrMon OVMS plugins for full examples.

`output` modes supported by the command API are:

- `text` → Content-Type: text/plain; charset=utf-8
- `json` → Content-Type: application/json; charset=utf-8
- `binary` → Content-Type: application/octet-stream; charset=x-user-defined
- default/other → Content-Type: application/octet-stream; charset=utf-8

### jqXHR Object

`loadcmd()` returns the jqXHR (XMLHttpRequest) object in charge for the asynchronous execution of the request. This can be used to track the results of the execution, check for errors or to abort the command.

The jQuery XHR object is also a "thenable", so actions to be performed after the command execution can simply be chained to the object. Example:

See jQuery documentation for full details and options.

### Filter / Process Output

Supply a filter function to hook into the asynchronous output stream. Use filters to filter (ahem…) / preprocess / reformat the command output, scan the stream for some info you'd like to know as soon as possible, or completely take over the output processing.

The filter function is called when a new chunk of output has arrived or when a stream error has occurred. The function gets a message object containing the `request` object and either a `text` for normal outputs or an `error`, which is a preformatted error output you can use or ignore.

If the filter function returns a string, that will be added to the output target. If it returns `null`, the target will remain untouched.

Hint: if you just want to scan the text for some info, you can pass on the message after your scan to the default `standardTextFilter()`.

Example: let's reformat a `can status` dump into a nice Bootstrap table:

Key | Value
--- | ---