Spec for global action IDs (#7175)

## Summary of the Pull Request

⚠️ This spec has been moved from #6902. That version was branched off the new tab menu customization, and had a terribly convoluted git history. After discussion with the team, we've decided that it's best that this spec is merged atomically _first_, and used as the basis for #5888, as opposed to the other way around.

> This document is intended to serve as an addition to the [Command Palette Spec],
> as well as the [New Tab Menu Customization Spec].
> 
> As we come to rely more on actions being a mechanism by which the user defines
> "do something in the Terminal", we'll want to make it even easier for users to
> re-use the actions that they've already defined, as to reduce duplicated json as
> much as possible. This spec proposes a mechanism by which actions could be
> uniquely identifiable, so that the user could refer to bindings in other
> contexts without needing to replicate an entire json blob.
> 

## PR Checklist
* [x] Specs: #6899
* [x] References: #1571, #1912, #3337, #5025, #5524, #5633
* [x] I work here

## Detailed Description of the Pull Request / Additional comments
_\*<sup>\*</sup><sub>\*</sub> read the spec  <sub>\*</sub><sup>\*</sup>\*_



[Command Palette Spec]: https://github.com/microsoft/terminal/blob/master/doc/specs/%232046%20-%20Command%20Palette.md
[New Tab Menu Customization Spec]: https://github.com/microsoft/terminal/blob/master/doc/specs/%231571%20-%20New%20Tab%20Menu%20Customization.md

Author: zadjii-msft

doc/specs/#6899 - Action IDs/#6899 - Action IDs.md

@@ -0,0 +1,228 @@
+---
+author: Mike Griese @zadjii-msft
+created on: 2020-07-13
+last updated: 2020-07-22
+issue id: 6899
+---
+
+# Action IDs
+
+## Abstract
+
+This document is intended to serve as an addition to the [Command Palette Spec],
+as well as the [New Tab Menu Customization Spec].
+
+As we come to rely more on actions being a mechanism by which the user defines
+"do something in the Terminal", we'll want to make it even easier for users to
+re-use the actions that they've already defined, as to reduce duplicated json as
+much as possible. This spec proposes a mechanism by which actions could be
+uniquely identifiable, so that the user could refer to bindings in other
+contexts without needing to replicate an entire json blob.
+
+## Solution Design
+
+This spec was largely inspired by the following diagram from @DHowett:
+
+![figure 1](data-mockup.png)
+
+The goal is to introduce an `id` parameter by which actions could be uniquely
+refered to. If we'd ever like to use an action outside the list of `actions`, we
+can simply refer to the action's ID, allowing the user to only define the action
+_once_.
+
+We'll start by renaming `bindings` to `actions`. `bindings` was suggested as a
+rename for `keybindings` in [#6532], as a way to make the name more generic.
+Discussion with the team lead to the understanding that the name `actions` would
+be even better, as a way of making the meaning of the "list of actions" more
+obvious.
+
+When we're parsing `actions`, we'll make three passes:
+* The first pass will scan the list for objects with an `id` property. We'll
+  attempt to parse those entries into `ActionAndArgs` which we'll store in the
+  global `id->ActionAndArgs` map. If any entry doesn't have an `id` set, we'll
+  skip it in this phase. If an entry doesn't have a `command` set, we'll ignore
+  it in this pass.
+* The second pass will scan for _keybindings_. Any entries with `keys` set will
+  create a `KeyChord->ActionAndArgs` entry in the keybindings map. If the entry
+  has an `id` set, then we'll simply re-use the action we've already parsed for
+  the `id`, from the action map. If there isn't an `id`, then we'll parse the
+  action manually at this time. Entries without a `keys` set will be ignored in
+  this pass.
+* The final pass will be to generate _commands_. Similar to the keybindings
+  pass, we'll attempt to lookup actions for entries with an `id` set. If there
+  isn't an `id`, then we'll parse the action manually at this time. We'll then
+  get the name for the entry, either from the `name` property if it's set, or
+  the action's `GenerateName` method.
+
+For a visual representation, let's assume the user has the following in their
+`actions`:
+
+![figure 2](data-mockup-actions.png)
+
+We'll first parse the `actions` to generate the mapping of `id`->`Actions`:
+
+![figure 3](data-mockup-actions-and-ids.png)
+
+Then, we'll parse the `actions` to generate the mapping of keys to actions, with
+some actions already being defined in the map of `id`->`Actions`:
+
+![figure 4](data-mockup-actions-and-ids-and-keys.png)
+
+
+When layering `actions`, if a later settings file contains an action with the
+same `id`, it will replace the current value. In this way, users can redefine
+actions, or remove default ones (with something like `{ "id":
+"Terminal.OpenTab", "command":null }`
+
+We'd maintain a large list of default actions, each with unique `id`s set. These
+are all given `id`'s with a `Terminal.` prefix, to easily identify them as
+built-in, default actions. Not all of these actions will be given keys, but they
+will all be given `id`s.
+
+> 👉 NOTE: The IDs for the default actions will need to be manually created, not
+> autogenerated. These `id`s are not strings displayed in the user interface, so
+> localization is not a concern.
+
+As we add additional menus to the Terminal, like the customization for the new
+tab dropdown, or the tab context menu, or the `TermControl` context menu, they
+could all refer to these actions by `id`, rather than duplicating the same json.
+
+
+### Existing Scenarios
+
+Keybindings will still be stored as a `keys->Action` mapping, so the user will
+still be able to override default keybindings exactly the same as before.
+
+Similarly, commands in the Command Palette will continue using their existing
+`name->Action` mapping they're currently using. For a binding like
+
+```json
+{ "keys": "ctrl+alt+x", "id": "Terminal.OpenDefaultSettings" },
+```
+* We'll bind whatever action is defined as `Terminal.OpenDefaultSettings` to
+  <kbd>ctrl+alt+x</kbd>.
+* We'll use whatever action is defined as `Terminal.OpenDefaultSettings` to
+  generate a name for the command palette.
+
+### Future Context Menus
+
+In [New Tab Menu Customization Spec], we discuss allowing the user to bind
+actions to the new tab menu. In that spec, they can do so with something like
+the following:
+
+```json
+{
+    "newTabMenu": [
+        { "type":"action", "command": { "action": "adjustFontSize", "delta": 1 }, }
+        { "type":"action", "command": { "action": "adjustFontSize", "delta": -1 }, }
+        { "type":"action", "command": "resetFontSize", }
+        { "type":"profile", "profile": "cmd" },
+        { "type":"profile", "profile": "Windows PowerShell" },
+        { "type":"separator" },
+        {
+            "type":"folder",
+            "name": "Settings...",
+            "icon": "C:\\path\\to\\icon.png",
+            "entries":[
+                { "type":"action", "command": "openSettings" },
+                { "type":"action", "command": { "action": "openSettings", "target": "defaultsFile" } },
+            ]
+        }
+    ]
+}
+```
+
+In this example, the user has also exposed the "Increase font size", "Decrease
+font size", and "Reset font size" actions, as well as the settings files in a
+submenu. With this proposal, the above could instead be re-written as:
+
+```json
+{
+    "newTabMenu": [
+        { "type":"action", "id": "Terminal.IncreaseFontSize" },
+        { "type":"action", "id": "Terminal.DecreaseFontSize" },
+        { "type":"action", "id": "Terminal.ResetFontSize" },
+        { "type":"profile", "profile": "cmd" },
+        { "type":"profile", "profile": "Windows PowerShell" },
+        { "type":"separator" },
+        {
+            "type":"folder",
+            "name": "Settings...",
+            "icon": "C:\\path\\to\\icon.png",
+            "entries":[
+                { "type":"action", "id": "Terminal.OpenDefaultSettings" },
+                { "type":"action", "id": "Terminal.OpenSettings" },
+            ]
+        }
+    ]
+}
+```
+
+In this example, the actions are looked up from the global map using the `id`
+provided, enabling the user to re-use their existing definitions. If the user
+re-defined the `Terminal.IncreaseFontSize` action to mean something else, then
+the action in the new tab menu will also be automatically updated.
+
+Furthermore, when additional menus are added (such as the tab context menu, or
+the `TermControl` context menu), these could also leverage a similar syntax to
+the above to allow re-use of the `id` parameter.
+
+Discussion with the team also suggested that users shouldn't be able to define
+actions in these menus _at all_. The actions should exclusively be defined in
+`actions`, and other menus should only be able to refer to these actions by
+`id`.
+
+## UI/UX Design
+
+There's not a whole lot of UI for this feature specifically. This is largely
+behind-the-scenes refactoring of how actions can be defined.
+
+## Capabilities
+
+### Accessibility
+
+_(not applicable)_
+
+### Security
+
+_(no change expected)_
+
+### Reliability
+
+_(no change expected)_
+
+### Compatibility
+
+_(no change expected)_
+
+### Performance, Power, and Efficiency
+
+_(no change expected)_
+
+## Potential Issues
+
+This won't necessarily play well with iterable commands in the Command Palette,
+but that's okay. For iterable commands, users will still need to define the
+actions manually.
+
+## Future considerations
+
+* See the following issues for other places where this might be useful:
+  - [#1912] - Context Menu for Tabs
+    * See also [#5524], [#5025], [#5633]
+  - [#3337] - Right-click menu inside TerminalControl (w/ Copy & Paste?)
+    * See also [#5633] and [#5025], both those actions seem reasonable in either
+      the tab context menu or the control context menu.
+
+<!-- Footnotes -->
+[Command Palette Spec]: https://github.com/microsoft/terminal/blob/master/doc/specs/%232046%20-%20Command%20Palette.md
+[New Tab Menu Customization Spec]: https://github.com/microsoft/terminal/blob/master/doc/specs/%231571%20-%20New%20Tab%20Menu%20Customization.md
+
+[#1571]: https://github.com/microsoft/terminal/issues/1571
+[#1912]: https://github.com/microsoft/terminal/issues/1912
+[#3337]: https://github.com/microsoft/terminal/issues/3337
+[#5025]: https://github.com/microsoft/terminal/issues/5025
+[#5524]: https://github.com/microsoft/terminal/issues/5524
+[#5633]: https://github.com/microsoft/terminal/issues/5633
+[#6532]: https://github.com/microsoft/terminal/issues/6532
+[#6899]: https://github.com/microsoft/terminal/issues/6899