Completely refactored and restructured.

Now use $.event.special to override jQuery function only when there
are bound mutation events.

src/mutations.attr.js

@@ -0,0 +1,61 @@
+/*
+ * jQuery.attr Mutation Events
+ *
+ * Copyright (c) 2009 Adaptavist.com Ltd
+ * Dual licensed under the MIT (MIT-LICENSE.txt)
+ * and GPL (GPL-LICENSE.txt) licenses.
+ *
+ * Author: Mark Gibson (jollytoad at gmail dot com)
+ */
+(jQuery.mutations && (function($) {
+
+$.mutations.register({
+	// The event type triggered after a mutation,
+	// "pre-" is prepended to this for the pre-mutation event.
+	type: 'attr',
+
+	// The blacklist can hold attributes that should never trigger an event
+	blacklist: {},
+
+	// Hook into jQuery when an event of this type is first bound
+	setup: function() {
+		var attr = $.attr,
+			blacklist = this.blacklist,
+			trigger = $.mutations.trigger;
+
+		// Save the original $.attr function
+		this.original = attr;
+
+
+		// Override $.attr
+		$.attr = function( elem, name, newValue, silent ) {
+			var prevValue = attr(elem, name);
+
+			if ( newValue === undefined ) {
+				return prevValue;
+			}
+
+			if ( silent || blacklist[name] ) {
+
+				return attr(elem, name, newValue);
+
+			} else if ( ""+newValue !== ""+prevValue ) {
+
+				return trigger( elem, 'attr', name, prevValue, newValue === "" ? undefined : newValue,
+					function( event ) {
+						attr( event.target, event.attrName, event.newValue );
+					}
+				);
+			}
+		};
+	},
+
+	// Remove hooks once all events of this type have been unbound
+	teardown: function() {
+		$.attr = this.original;
+		delete this.original;
+	}
+});
+
+})(jQuery));
+

src/mutations.core.js

@@ -0,0 +1,94 @@
+/*!
+ * jQuery Mutation Events
+ *
+ * Copyright (c) 2009 Adaptavist.com Ltd
+ * Dual licensed under the MIT (MIT-LICENSE.txt)
+ * and GPL (GPL-LICENSE.txt) licenses.
+ *
+ * Author: Mark Gibson (jollytoad at gmail dot com)
+ */
+(jQuery.mutations || (function($) {
+
+var m;
+m = $.mutations = {
+
+	REMOVAL:		3,
+	ADDITION:		2,
+	MODIFICATION:	1,
+
+	// Construct a new mutation event
+	event: function( eventType, attrName, prevValue, newValue ) {
+		var event = new $.Event( eventType );
+
+		// Add MutationEvent fields
+		event.attrName = attrName;
+		event.newValue = newValue;
+		event.prevValue = prevValue;
+
+		event.attrChange =
+			newValue === null || newValue === undefined   ? m.REMOVAL :
+			prevValue === null || prevValue === undefined ? m.ADDITION :
+															m.MODIFICATION;
+
+		return event;
+	},
+
+	// Trigger a pre and post mutation event, the pre mutation handlers may
+	// cancel the mutation using event.preventDefault(), it may also modify
+	// the mutation by setting the event fields.
+	trigger: function( elem, eventType, attrName, prevValue, newValue, commit ) {
+		var event = m.event('pre-' + eventType, attrName, prevValue, newValue),
+			ret;
+
+//		console.log('trigger %s %s: %o -> %o %o', event.type, event.attrName, event.prevValue, event.newValue, elem);
+
+		$.event.trigger( event, undefined, elem );
+
+		if ( !event.isDefaultPrevented() ) {
+			event.type = eventType;
+			ret = commit(event);
+			$.event.trigger( event, undefined, event.target );
+		}
+
+		return ret;
+	},
+
+	type: {},
+
+	// Register a new mutation event
+	register: function( opts ) {
+		m.type[opts.type] = opts;
+
+		// Track how many bindings we have for this event type
+		opts.usage = 0;
+
+		// Register the pre/post mutation event types as special event type
+		// so we can hook into jQuery on the first binding of this type
+		$.event.special['pre-'+opts.type] =
+		$.event.special[opts.type] = {
+
+			add: function() {
+				// Call the setup on the first binding
+				if ( !opts.usage++ ) {
+					opts.setup();
+				}
+			},
+
+			remove: function() {
+				// Call teardown when last binding is removed
+				if ( !--opts.usage ) {
+					opts.teardown();
+				}
+			}
+		};
+	},
+
+	unregister: function( type ) {
+		delete $.event.special['pre-'+type];
+		delete $.event.special[type];
+		delete this[type];
+	}
+};
+
+})(jQuery));
+

test/index.html

@@ -7,7 +7,8 @@
 	<!-- Includes -->
 	<script type="text/javascript" src="include/jquery.js"></script>
 	<script type="text/javascript" src="qunit/testrunner.js"></script>
-	<script type="text/javascript" src="../src/jquery.mutations.js"></script>
+	<script type="text/javascript" src="../src/mutations.core.js"></script>
+	<script type="text/javascript" src="../src/mutations.attr.js"></script>
 	<script type="text/javascript" src="unit/attr.js"></script>
 	<script type="text/javascript" src="unit/data.js"></script>
 </head>