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Tutoriel: Evaluer une expression lorsque l'expression debugger; est exécutée

Cette page décrit comment il est possible d'essayer l'API Debugger vous même en utilisant l'Ardoise JavaScript de Firefox. Nous utilisons une API pour évaluer une expression dans la page lorsque celle ci éxécute une expression JavaScript debugger;.

  1. Visitez l'URL about:config, et passez la préférence à true :

    Setting the preference

    Configurer la préférence ''

  2. Sauvegarder le fichier HTML suivant sur votre ordinateur et l'ouvrir dans votre navigateur :

    <div onclick="var x = 'snoo'; debugger;">Click me!</div>
  3. Open a developer Scratchpad (Menu button > Developer > Scratchpad), and select "Browser" from the "Environment" menu. (This menu will not be present unless you have changed the preference as explained above.)

    Selecting the 'browser' context in the Scratchpad

    Selecting the 'browser' context in the Scratchpad

  4. Enter the following code in the Scratchpad:

    // This simply defines 'Debugger' in this Scratchpad;
    // it doesn't actually start debugging anything.
    // Create a 'Debugger' instance.
    var dbg = new Debugger;
    // Get the current tab's content window, and make it a debuggee.
    var w = gBrowser.selectedBrowser.contentWindow.wrappedJSObject;
    // When the debuggee executes a 'debugger' statement, evaluate
    // the expression 'x' in that stack frame, and show its value.
    dbg.onDebuggerStatement = function (frame) {
        alert('hit debugger statement; x = ' + frame.eval('x').return);
  5. In the Scratchpad, ensure that no text is selected, and press the "Run" button.

  6. Now, click on the text that says "Click me!" in the web page. This runs the div element's onclick handler. When control reaches the debugger; statement, Debugger calls your callback function, passing a Debugger.Frame instance. Your callback function evaluates the expression x in the given stack frame, and displays the alert:

    The Debugger callback displaying an alert

    The Debugger callback displaying an alert

  7. Press "Run" in the Scratchpad again. Now, clicking on the "Click me!" text causes two alerts to show---one for each Debugger instance.

    Multiple Debugger instances can observe the same debuggee. Re-running the code in the Scratchpad created a fresh Debugger instance, added the same web page as its debuggee, and then registered a fresh debugger; statement handler with the new instance. When you clicked on the div element, both of them ran. This shows how any number of Debugger-based tools can observe a single web page simultaneously---although, since the order in which their handlers run is not specified, such tools should probably only observe, and not influence, the debuggee's behavior.

  8. Close the web page and the Scratchpad.

    Since both the Scratchpad's global object and the debuggee window are now gone, the Debugger instances will be garbage collected, since they can no longer have any visible effect on Firefox's behavior. The Debugger API tries to interact with garbage collection as transparently as possible; for example, if both a Debugger.Object instance and its referent are not reachable, they will both be collected, even while the Debugger instance to which the shadow belonged continues to exist.

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