feat: expose eval method

[geieredgar]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Documentation
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes. Issue #___
• No

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:
This allows executing javascript code without having to use the mpsc::channel and helps to simplify handle_event_loop in tauri-runtime-wry (see tauri-apps/tauri#1962).

[wusyong]

The problem is this method is not thread-safe. It should be called on the main thread only.
Mpsc channel could help you send the script on any thread.
I think one workaround is we could call evaluate_script immediately after dispatch_script if it's on main thread (ie. in the event loop)
@lucasfernog wdyt?

[geieredgar]

Yes, calling evaluate_script immediately after dispatch_script is also what I did in my PR tauri-apps/tauri#1963 to fix tauri-apps/tauri#1962. I just think that solution is a bit ugly, which is why I opened this PR. In my opinion it would be a lot cleaner if wry did only provide the eval method and no evaluate_script, dispatch_script and Dispatcher, with a doc comment stating that eval should only be called on the main thread (I think this information should also be added to evaluate_script) and maybe a hint that users could use channels if they want to send scripts from any thread. I actually think that using the EventLoopProxy to send scripts from any thread is a better solution than using an mpsc channel because it always wakes up the event loop, which would not happen if a script is send via Dispatcher, where we cannot call evaluate_script afterwards and would have to send a message to the event loop anyway.

[wusyong]

Yeah, I think we could do that. If @lucasfernog also agrees (since tauri will need some custom event on EventLoopProxy), let's remove the dispatcher and all its related methods and expose this instead.

[geieredgar]

FYI, tauri already uses a WebviewMessage::EvaluateScript custom event and AFAICT doesn't use the Dispatcher.

[geieredgar]

Closed in favor of #291.