Add a spec draft for Keybindings Arguments.

  Specs #1142.

  Just read the spec :)

doc/cascadia/Keybindings-Arguments.md

@@ -0,0 +1,291 @@
+---
+author: Mike Griese @zadjii-msft
+created on: 2019-06-19
+last updated: 2019-06-19
+issue id: 1142
+---
+
+# Arbitrary Keybindings Arguments
+
+## Abstract
+
+The goal of this change is to both simplify the keybindings, and also enable far
+more flexibility when editing a user's keybindings.
+
+Currently, we have many actions that are very similar in implementation - for
+example, `newTabProfile0`, `newTabProfile1`, `newTabProfile2`, etc. All these
+actions are _fundamentally_ the same function. However, we've needed to define 9
+different actions to enable the user to provide different values to the `newTab`
+function.
+
+With this change, we'll be able to remove these _essentially_ duplicated events,
+and allow the user to specify arbitrary arguments to these functions.
+
+## Inspiration
+
+Largely inspired by the keybindings in VsCode and Sublime Text.
+
+## Solution Design
+
+We'll need to introduce args to some actions that we already have defined. These
+are the actions I'm thinking about when writing this spec:
+
+```c#
+    // These events already exist like this:
+    delegate void NewTabWithProfileEventArgs(Int32 profileIndex);
+    delegate void SwitchToTabEventArgs(Int32 profileIndex);
+    delegate void ResizePaneEventArgs(Direction direction);
+    delegate void MoveFocusEventArgs(Direction direction);
+
+    // These events either exist in another form or don't exist.
+    delegate void CopyTextEventArgs(Boolean copyWhitespace);
+    delegate void ScrollEventArgs(Int32 numLines);
+    delegate void SplitProfileEventArgs(Orientation splitOrientation, Int32 profileIndex);
+```
+
+Ideally, after this change, the bindings for these actions would look something
+like the following:
+
+```js
+{ "keys": ["ctrl+shift+1"], "command": "newTabProfile", "args": { "profileIndex":0 } },
+{ "keys": ["ctrl+shift+2"], "command": "newTabProfile", "args": { "profileIndex":1 } },
+// etc...
+
+{ "keys": ["alt+1"], "command": "switchToTab", "args": { "index":0 } },
+{ "keys": ["alt+2"], "command": "switchToTab", "args": { "index":1 } },
+// etc...
+
+{ "keys": ["alt+shift+down"], "command": "resizePane", "args": { "direction":"down" } },
+{ "keys": ["alt+shift+up"], "command": "resizePane", "args": { "direction":"up" } },
+// etc...
+
+{ "keys": ["alt+down"], "command": "moveFocus", "args": { "direction":"down" } },
+{ "keys": ["alt+up"], "command": "moveFocus", "args": { "direction":"up" } },
+// etc...
+
+{ "keys": ["ctrl+c"], "command": "copy", "args": { "copyWhitespace":true } },
+{ "keys": ["ctrl+shift+c"], "command": "copy", "args": { "copyWhitespace":false } },
+
+{ "keys": ["ctrl+shift+down"], "command": "scroll", "args": { "numLines":1 } },
+{ "keys": ["ctrl+shift+up"], "command": "scroll", "args": { "numLines":-1 } },
+
+{ "keys": ["ctrl+alt+1"], "command": "splitProfile", "args": { "orientation":"vertical", "profileIndex": 0 } },
+{ "keys": ["ctrl+alt+shift+1"], "command": "splitProfile", "args": { "orientation":"horizontal", "profileIndex": 0 } },
+{ "keys": ["ctrl+alt+2"], "command": "splitProfile", "args": { "orientation":"vertical", "profileIndex": 1 } },
+{ "keys": ["ctrl+alt+shift+2"], "command": "splitProfile", "args": { "orientation":"horizontal", "profileIndex": 1 } },
+// etc...
+```
+
+Note that instead of having 9 different `newTabProfile<N>` actions, we have a
+singular `newTabProfile` action, and that action requires a `profileIndex` in
+the `args` object.
+
+Also, pay attention to the last set of keybindings, the `splitProfile` ones.
+This is a function that requires two arguments, both a `orientation` and a
+`profileIndex`. Before this change we would have needed to create 20 separate
+actions (10 profile indicies * 2 directions) to handle these cases. Now it can
+be done with a single action that can be much more flexible in its
+implementation.
+
+### Parsing KeyBinding Arguments
+
+We'll add a new `IKeyBindingArgs` interface. All current keybinding events will
+be changed from their current types to `TypedEventHandler`s. These
+`TypedEventHandler`s second param will always be an instance of
+`IKeyBindingArgs`. So for example:
+
+```C#
+
+delegate void CopyTextEventArgs();
+delegate void NewTabEventArgs();
+delegate void NewTabWithProfileEventArgs(Int32 profileIndex);
+// ...
+
+[default_interface]
+runtimeclass AppKeyBindings : Microsoft.Terminal.Settings.IKeyBindings
+{
+    event CopyTextEventArgs CopyText;
+    event NewTabEventArgs NewTab;
+    event NewTabWithProfileEventArgs NewTabWithProfile;
+```
+
+Becomes:
+
+```C#
+interface IKeyBindingArgs {}
+runtimeclass CopyTextEventArgs : IKeyBindingArgs
+{
+    Boolean CopyWhitespace;
+}
+runtimeclass NewTabEventArgs : IKeyBindingArgs { }
+runtimeclass NewTabWithProfileEventArgs : IKeyBindingArgs
+{
+    Int32 ProfileIndex;
+}
+
+[default_interface]
+runtimeclass AppKeyBindings : Microsoft.Terminal.Settings.IKeyBindings
+{
+    event Windows.Foundation.TypedEventHandler<AppKeyBindings, CopyTextEventArgs> CopyText;
+    event Windows.Foundation.TypedEventHandler<AppKeyBindings, NewTabEventArgs> NewTab;
+    event Windows.Foundation.TypedEventHandler<AppKeyBindings, NewTabWithProfileEventArgs> NewTabWithProfile;
+```
+
+We'll also change our existing map in the `AppKeyBindings` implementation.
+Currently, it's a `std::unordered_map<KeyChord, ShortcutAction, ...>`, which
+uses the `KeyChord` to lookup the `ShortcutAction`. We'll need to introduce a
+new type `ActionAndArgs`:
+
+```C#
+runtimeclass ActionAndArgs
+{
+    ShortcutAction action;
+    IKeyBindingArgs args;
+}
+```
+
+and we'll change the map in `AppKeyBindings` to a `std::unordered_map<KeyChord,
+ActionAndArgs, ...>`.
+
+When we're parsing keybindings, we'll need to construct args for each of the
+events to go with each binding. When we find some key chord bound to a given
+Action, we'll construct the `IKeyBindingArgs` for that action. FOr many actions,
+these args will be an empty class. However, when we do find an action that needs
+additional parsing, `AppKeyBindingsSerialization` will do the extra work to
+parse the args for that action.
+
+Once the `IKeyBindingArgs` is built for the keybinding, we'll set it in
+`AppKeyBindings` with a updated `AppKeyBindings::SetKeyBinding` call.
+`SetKeyBinding`'s signature will be updated to take a `ActionAndArgs` instead.
+
+### Executing KeyBinding Actions with Arguments
+
+When we're handling a keybinding in `AppKeyBindings::_DoAction`, we'll trigger
+the event handlers with the `IKeyBindingArgs` we've stored in the map with the
+`ShortcutAction`.
+
+Then, in `App`, we'll handle each of these events. We set up lambdas as event
+handlers for each event in `App::_HookupKeyBindings`. In each of those
+functions, We'll inspect the `IKeyBindingArgs` parameter, and use args from its
+implementation to call callbacks in the `App` class.
+
+Instead of:
+```c++
+    void App::_HookupKeyBindings(TerminalApp::AppKeyBindings bindings) noexcept
+    {
+        // ...
+        bindings.NewTabWithProfile([this](const auto index) { _OpenNewTab({ index }); });
+    }
+```
+
+The code will look like:
+
+```c++
+    void App::_HookupKeyBindings(TerminalApp::AppKeyBindings bindings) noexcept
+    {
+        // ...
+        bindings.NewTabWithProfile([this](const auto& sender, const auto& args) { _OpenNewTab({ args.ProfileIndex() }); });
+    }
+```
+
+We could even go so far as to update `App` to have methods defined with the
+actual keybinding function signatures, so instead we'd just have:
+
+
+```c++
+    void App::_HookupKeyBindings(TerminalApp::AppKeyBindings bindings) noexcept
+    {
+        // ...
+        bindings.NewTabWithProfile({ this, &App::_OpenNewTab });
+    }
+
+    void App::_OpenNewTab(const TerminalApp::AppKeyBindings& sender, const NewTabEventArgs& args)
+    {
+        auto profileIndex = args.ProfileIndex();
+        // ...
+    }
+```
+
+### Serializing KeyBinding Arguments
+
+Similar to how we parse arguments from the json, we'll need to update the
+`AppKeyBindingsSerialization` code to be able to serialize the arguments from a
+particular `IKeyBindingArgs`.
+
+## UI/UX Design
+
+### Keybindings in the New Tab Dropdown
+
+Small modifications will need to be made to the code responsible for the new tab
+dropdown. The new tab dropdown currently also displays the keybindings for each
+profile in the new tab dropdown. It does this by querying for the keybinding
+associated with each action. As we'll be removing the old `ShortcutAction`s that
+this dropdown uses, we'll need a new way to find which key chord corresponds to
+opening a given profile.
+
+We'll need to be able to not only lookup a keybinding by `ShortcutAction`, but
+also by a `ShortcutAction` and `IKeyBindingArgs`. We'll need to update the
+`AppKeyBindings::GetKeyBinding` method to also accept a `IKeyBindingArgs`. We'll
+also probably want each `IKeyBindingArgs` implementation to define `operator==`,
+so that we can easily check if two different `IKeyBindingArgs` are the same in
+this method.
+
+## Capabilities
+### Accessibility
+
+N/A
+
+### Security
+
+N/A
+
+### Reliability
+
+N/A
+
+### Compatibility
+
+This change will need to carefully be crafted to enable upgrading the legacy
+keybindings seamlessly. For most actions, the upgrade should be seamless. Since
+they already don't have args, their serializations will remain exactly the same.
+
+However, for the following actions that we'll be removing in favor of actions
+with arguments, we'll need to leave legacy deserialization in place to be able
+to find these old actions, and automatically build the correct `IKeyBindingArgs`
+for them:
+
+* `newTabProfile<n>`
+    - We'll need to make sure to build args with the right `profileIndex`
+      corresponding to the old action.
+* `switchToTab<n>`
+    - We'll need to make sure to build args with the right `index` corresponding
+      to the old action.
+* `resizePane<direction>` and `moveFocus<direction>`
+    - We'll need to make sure to build args with the right `direction`
+      corresponding to the old action.
+* `scroll<direction>`
+    - We'll need to make sure to build args with the right `amount` value
+      corresponding to the old action. `Up` will be -1, and `Down` will be 1.
+
+### Performance, Power, and Efficiency
+
+N/A
+
+## Potential Issues
+
+N/A
+
+## Future considerations
+
+* Should we support some sort of conversion from num keys to an automatic arg?
+  For example, by default, <kbd>Alt</kbd>+<kbd>&gt;N&lt;</kbd> to focuses the
+  Nth tab. Currently, those are 8 separate entries in the keybindings. Should we
+  enable some way for them be combined into a single binding entry, where the
+  binding automatically recieves the number pressed as an arg? I couldn't find
+  any prior art of this, so it doesn't seem worth it to try and invent
+  currently. This might be something that we want to loop back on, but for the
+  time being, it remains out of scope of this PR.
+
+## Resources
+
+N/A