A problem about 'animation '

[chuyinweilai]

I use the 'animate follow the document of ‘API’, but some thing is wrong.
its my code:

        var animation = swan.createAnimation();
        animation.height(300).step();
        /* i also try :
            animation.height('auto').step();
            animation.height('300px').step();
         */
        this.setData({
            animationData: animation.export()
        });

it dose not work;
i try other props:

        var animation = swan.createAnimation();
        animation.opacity(0.5).step();
        this.setData({
            animationData: animation.export()
        });

it works！
so, can you check the API? or give us a more detail demo??