[Webkit-unassigned] [Bug 167549] New: Invoking callback should allow setting the `this` object

Sat Jan 28 02:28:42 PST 2017

https://bugs.webkit.org/show_bug.cgi?id=167549

            Bug ID: 167549
           Summary: Invoking callback should allow setting the `this`
                    object
    Classification: Unclassified
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebCore Misc.
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: joepeck at webkit.org
                CC: cdumez at apple.com, sam at webkit.org

Summary:
Invoking callback should allow setting the `this` object.

Cases like IntersectionObserver, PerformanceObserver and MutationObserver.

For example PerformanceObserver:

    <script>
    let observer = new PerformanceObserver(function f(list, obs) {
        console.log("this", this); // Expected `observer`, was `arguments.callee`
    })
    observer.observe({entryTypes: ["mark"]});
    performance.mark("mark1");
    </script>

The callbacks and their interface are autogenerated from WebIDL and have a handleEvent method. Seems we need a way to the `this` object in some cases. (Probably not all, requestAnimationFrame for example prolly doesn't have the same constraints).

Currently this works with MutationObserver, which has a custom implementation of the callback (not generated). See JSMutationCallback.

    <body>
    <div id="my-div"></div>
    <script>
    var target = document.getElementById("my-div");
    var observer = new MutationObserver(function(mutations) {
        console.log("this", this); // Expected `observer` but is `arguments.callee`
    });
    observer.observe(target, {childList: true});
    target.appendChild( document.createElement("span") );
    </script>

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