plus监听backbutton

<script>
	var plusReady = function(callback) {
		if (window.plus) {
			callback();
		} else {
			document.addEventListener('plusready', callback);
		}
	};
	plusReady(function() {
		var firstBack = 0;
		var handleBack = function() {
			var now = Date.now || function() {
				return new Date().getTime();
			};
			if (location.hash == '#/app/xindex') {
				if (!firstBack) {
					firstBack = now();
					plus.nativeUI.toast('再按一次退出应用');
					setTimeout(function() {
						firstBack = 0;
					}, 2000);
				} else if (now() - firstBack < 2000) {
					plus.runtime.quit();
				}
			} else {
				// console.log(location.hash);
				history.back();
			}
		};
		plus.key.addEventListener('backbutton', handleBack);
	});
</script>