Add context (right click) menus about druid HOT 2 CLOSED

Okay, here's an actual sketch for this; threading the needle between mac and windows is a bit subtle.

Windows

In win32, context menus are intended to be displayed during handling of the WM_CONTEXTMENU message. This message is called after WM_RBUTTONUP; that is, if the application has not handled the right mouse button, then it is called again with a message telling it to display a context menu.

macOS

In Cocoa, the default implementation of rightMouseDown: calls menuForEvent:, and presents a menu if it is returned. If it is not returned, the mouse event continues up the responder chain.

A preliminary design

We would like some API that works well in both cases, without interfering with other event flow.

What seems easiest: on right mouse down, we check if the running application would like to display a context menu. We add two new methods to druid-shell::WinHandler, that look something like:

fn context_menu(&mut self, pos: Point, ctx: &mut dyn WinCtx) -> Option<Menu>;
fn context_menu_end(&mut self, cmd: Option<Action>);

context_menu is called on right mouse down. If this method returns Some, then the right mouse up/down/moved events are suppressed while the menu is displayed. If it returns None, then the right mouse events are sent as normal. context_menu_end is called anytime the menu is closed, and includes a cmd if one was selected.

What is a menu?

The type of Menu is not totally clear. There are currently Menu types declared in each druid-shell backend, and these may be reused; I'm not sure exactly the state of that code. I could also imagine Menu being a platform-independent 'menu description', that is then turned into some concrete platform-specific object in druid-shell. (This pattern echoes the changes to Color and Gradient in piet that have been recently discussed.

The type of the cmd argument is up for discussion.

from druid.

Sounds like a good start. A few other thoughts.

First, to what extent is WM_CONTEXTMENU and menuForEvent: actually necessary for correct context menu operation, and to what extent is it the platform helpfully offering assistance for mouse events? In the latter case it's not actually helpful, because we can do dispatching in a platform-independent way.

Second, it's certainly a choice to use platform menus. I'm in favor, but I'll point out that they don't necessarily match the appearance of the rest of the app. I strongly suspect this will be smoother on macOS than Windows.

The current Menu types need some more work. I suspect it's possible to make them support contextual menus as well as the traditional drop-down menu bar; on Windows it's the same objects underneath.

from druid.