feat: add custom cursor interface on windows

[Kingtous]

This PR adds an additional interface to set custom cursor on windows, which fixes issues mentioned below.

Currently, Customizing MouseCursor on windows is not working, the customed mouse will restore to arrow once it moves. See flutter/flutter#31952 (comment)
flutter/flutter#99484
for details. Variable current_cursor_ in cursor handler will not update when we set our custom cursors, and we cannot change this field to prevent the cursor from restoring due to lacking of interface.

The reason is that cursor handler on windows has no interface to change current cursor to custom cursor, only support setting windows self-contained icons.

Changes:

• add new function handled by method channel of SystemCursors.mousecursors, which can change current_cursor_, make possible to handle custom cursor on windows correctly.

We have successfully tested this interface in https://github.com/Kingtous/rustdesk_flutter_custom_cursor/blob/8f3be4ac68b4fb20d05b8d30df4c13d8fd77f9c4/lib/flutter_custom_cursor.dart#L113, which works perfectly on windows.

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[Kingtous]

Thanks for the review, all comments has resolved. Is there other jobs I need to do? :)

[loic-sharma]

@Kingtous Thanks for following up quickly! Could you add a unit test that sends a setSystemCursor message and uses MockWindowBindingHandler to verify the cursor is set as expected? (If you want to go the extra mile, ideally activateSystemCursor would be tested too, but feel free to skip that!)

Once that's ready, we should get approval from @dkwingsmt as they worked in this space.

[dkwingsmt]

Thank you. I didn't know you can create ICONINFO using a buffer.

However, I'm hesitant on merging the method channel, since we are hoping to design a universal API (method channel) for all platforms, and the current method is likely platform dependent. But I also understand that we need to develop one step at a time. So I think there are two options:

1. Make this PR only includes embedding APIs (such as SetFlutterCursor), just enough for you to develop a plugin. The plugin will listen to the channel, create ICONINFO, and call the new embedding APIs.
2. Merge this PR, but we might deprecate these methods in the future. (When we come up with a universal API, we'll probably adapt the current legacy API onto the new one for a while for compatibility.)

Also, the flow I'm imagining is, instead of setting the cursor using the buffer directly, which you have to do every time you set the same cursor,

1. Create a cursor using the buffer, and assign it with a name (string)
2. Activate the cursor using the string.

[Kingtous]

Thank you. I didn't know you can create ICONINFO using a buffer.

However, I'm hesitant on merging the method channel, since we are hoping to design a universal API (method channel) for all platforms, and the current method is likely platform dependent. But I also understand that we need to develop one step at a time. So I think there are two options:

1. Make this PR only includes embedding APIs (such as SetFlutterCursor), just enough for you to develop a plugin. The plugin will listen to the channel, create ICONINFO, and call the new embedding APIs.
2. Merge this PR, but we might deprecate these methods in the future. (When we come up with a universal API, we'll probably adapt the current legacy API onto the new one for a while for compatibility.)

Also, the flow I'm imagining is, instead of setting the cursor using the buffer directly, which you have to do every time you set the same cursor,

1. Create a cursor using the buffer, and assign it with a name (string)
2. Activate the cursor using the string.

Sounds great for a universal API to change cursor, which is a very important feature, especially for games. Although this PR is providing a new feature, it more like a bugfix. It fills the limitation(or fix bugs?) with custom cursor on windows, currently.(by extending offcial MouseCursor, MouseCursorSession abstract class, linux/macOS works great)

This problem has blocked so many developers as mentioned in flutter/flutter#31952 (comment) flutter/flutter#99484

I think the two options are all ok, and I prefer Option 2. Currently, this PR can be a temporary and fast workaround to uniform custom cursor behaviors on all platforms. Devs can change custom cursor easily by providing a raw BGRA buffer, without any win32 knowledge.

here are some questions:
Option 1: I am not very familiar with embedding API, can it be invoked without a MethodChannel?

I've thought about custom cursor cache, but it may bring a more complex custom cursor logic, which may not suitable for keeping this embedding API simple. Also, a cursor buffer is not so big, it only is invoked when the system cursor hovers on the specific item.

[Kingtous]

@Kingtous Thanks for following up quickly! Could you add a unit test that sends a setSystemCursor message and uses MockWindowBindingHandler to verify the cursor is set as expected? (If you want to go the extra mile, ideally activateSystemCursor would be tested too, but feel free to skip that!)

Once that's ready, we should get approval from @dkwingsmt as they worked in this space.

I noticed that cursor_handler.cc does not have unittest now. btw, can I manually generate a dummy cursor (buffer are all zero) for this test? I think a pure binary cursor is not suitable for the unittest.

[Kingtous]

@Kingtous Thanks for following up quickly! Could you add a unit test that sends a setSystemCursor message and uses MockWindowBindingHandler to verify the cursor is set as expected? (If you want to go the extra mile, ideally activateSystemCursor would be tested too, but feel free to skip that!)

Once that's ready, we should get approval from @dkwingsmt as they worked in this space.

I've extracted the cursor creation logic to a function named GetCursorFromBuffer. And I added a unittest which will check if the HCURSOR is not null. I think this approach may keep this unit tests simple?

[dkwingsmt]

@stuartmorgan What do you think? (Also I'm not familiar enough with the capability of plugins.)

[dkwingsmt]

No problem, let me know when it's ready :) Thank you so much for working on this!

[Kingtous]

No problem, let me know when it's ready :) Thank you so much for working on this!

I've tested this commit with the latest flutter/engine, it works perfectly with https://github.com/Kingtous/rustdesk_flutter_custom_cursor/blob/refactor/flutter/master/lib/cursor_manager.dart

The test cursor has successfully shown without a delay or an error.

All comments have been resolved now.

Later I consider to publish this custom plugin to pub.dev to make customizing cursors on flutter easier, which brings convinience for all flutter devs. Thanks. :)

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[dkwingsmt]

@stuartmorgan Would you like to give a second review?

[dkwingsmt]

@Kingtous I've approved this PR, but it is required that each PR needs the participance of 2 members of flutter/flutter-hackers, or in the case of this PR, 2 approvals. I'm looking for a second reviewer.

[SahajRana]

How long will this take?

[chinmaygarde]

@dkwingsmt This looks good to go. Please CQ when ready :)