How to run `go fmt` on save, in Visual Studio Code?

I'm not familiar with 'go fmt' specifically, but you can create a simple vscode extension to handle on save event and execute any arbitrary command passing the file path as an argument.

Here's a sample that just calls echo $filepath:

import * as vscode from 'vscode';
import {exec} from 'child_process';

export function activate(context: vscode.ExtensionContext) {

    vscode.window.showInformationMessage('Run command on save enabled.');

    var cmd = vscode.commands.registerCommand('extension.executeOnSave', () => {

        var onSave = vscode.workspace.onDidSaveTextDocument((e: vscode.TextDocument) => {

            // execute some child process on save
            var child = exec('echo ' + e.fileName);
            child.stdout.on('data', (data) => {
                vscode.window.showInformationMessage(data);
            });
        });
        context.subscriptions.push(onSave);
    });

    context.subscriptions.push(cmd);
}

And the package file:

{
    "name": "Custom onSave",
    "description": "Execute commands on save.",
    "version": "0.0.1",
    "publisher": "Emeraldwalk",
    "engines": {
        "vscode": "^0.10.1"
    },
    "categories": [
        "Other"
    ],
    "activationEvents": [
        "onCommand:extension.executeOnSave"
    ],
    "main": "./out/src/extension",
    "contributes": {
        "commands": [{
            "command": "extension.executeOnSave",
            "title": "Execute on Save"
        }]
    },
    "scripts": {
        "vscode:prepublish": "node ./node_modules/vscode/bin/compile",
        "compile": "node ./node_modules/vscode/bin/compile -watch -p ./"
    },
    "devDependencies": {
        "typescript": "^1.6.2",
        "vscode": "0.10.x"
    }
}

The extension is enabled via cmd+shift+p then typing "Execute on Save", but it could be reconfigured to start via another command including "*" which would cause it to load any time VSCode loads.

Once the extension is enabled, the event handler will fire whenever a file is saved (NOTE: this doesn't appear to work when file is first created or on Save as...)

This is just a minor modification of a yo code scaffolded extension as outlined here: https://code.visualstudio.com/docs/extensions/example-hello-world

Update

Here's a Visual Studio Code extension I wrote for running commands on file save. https://marketplace.visualstudio.com/items/emeraldwalk.RunOnSave.

Now, the feature has been implemented, you can enable format on save:

1. Open Settings (Ctrl + ,)
2. Search for editor.formatOnSave and set it to true

Your Go code will be formatted automatically on Ctrl + s

Its not possible at the moment but its being worked on https://github.com/Microsoft/vscode-go/issues/14