如何在Knockout组件中访问viewModel？

<!-- 父级viewModel -->
<script>
    function ParentViewModel() {
        var self = this;
        self.message = "Hello from parent viewModel";
    }
    ko.applyBindings(new ParentViewModel());
</script>

<!-- Knockout组件 -->
<script>
    ko.components.register('child-component', {
        viewModel: function(params) {
            var self = this;
            self.parentMessage = params.$parent.message;
            self.rootMessage = params.$root.message;
        },
        template: '<div>' +
                    '<p>Parent Message: <span data-bind="text: parentMessage"></span></p>' +
                    '<p>Root Message: <span data-bind="text: rootMessage"></span></p>' +
                  '</div>'
    });
</script>

<!-- 使用Knockout组件 -->
<child-component></child-component>