[Webkit-unassigned] [Bug 85817] New: [Qt] Implement WebProcess based input method event handling

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=85817

           Summary: [Qt] Implement WebProcess based input method event
                    handling
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebKit Qt
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: hausmann at webkit.org
                CC: kenneth at webkit.org, michael.bruning at nokia.com


The current handling of Qt Input Method Events (IMEs) tries to map the event parameters to either a SetComposition or ConfirmComposition message to the web process. Unfortunately this mapping is not entirely correct and breaks with certain virtual keyboards that use the IME interface.

A very concrete example: Imagine a virtual keyboard that makes suggestions based on what you write. So far you've written "Pla" and the keyboard suggests "Planet". The incomplete "Pla" sequence however is not a composition. Upon accepting the suggestion we receive an IME with "Planet" as commit string and (-3, 3) as replacementStart/Length.

In Qt the controls that respond the IMEs implement roughly the following "algorithm":

    (1) Remove the text referred to by replacementStart/Length
    (2) Insert the commit string
    (3) Set text selection as specified via QInputMethodEvent::Selection entry from attributes()
    (4) Set preeditText() as composition along with specified attributes

Any of the above steps are optional if their argument is not present - for example step (2) is a no-op if the commit string is empty. All of the above steps should appear as one operation in terms of undo/redo to the user. Step (1) and (2) can easily be combined into one edit operation.

I suggest we implement this algorithm in one shot on the web process side, to reduce the number of messages to be sent. I suggest to map the above steps to the WebCore::Editor interface as follows:

    * Step (1) and (2) become: Set replacementStart/Length as selection, convert it to composition with setComposition(selectedText()), confirmComposition(commitString)
    * Step (4) becomes a call to setComposition(preeditText)


On the UIProcess side we need to act on the IME synchronously, which means we need to apply the provided change simultaneously and immediately on the cached selection text (modify surroundingText, cursorPosition, etc.). We should also generate an integer ID, pass it as parameter with the IME message to the WebProcess and let the WebProcess send it back to us with one final EditorStateChanged message after applying steps (1)-(4). Until that EditorStateChanged message with the expected ID has arrived, we should ignore all incoming EditorStateChanged messages or at least not inform the input method about any updates, to avoid providing an intermediate (and thus incorrect) state to the input method.

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