xblock html problem

[xyaw Li]

https://drive.google.com/file/d/1SPTLspHjBxmqGiE39slGQZxA5inRZpe7/view?usp=sharing

Attached file is the code.

==========================

I would like to show different content via if-else ,

<div class="box_block" >
    {%  if self.display_name == "Visualize Coding" %}
        {{ self.boxurl }}
    {% endif %}
    {% if self.display_name == "External Html" %}
        <iframe src="{self.boxurl}" frameborder="0" name="test" scrolling="yes" width="{self.boxwidth}" height="{self.boxheight}" allowfullscreen></iframe>
    {% endif %}
</div>

I try many times and still fail . 

Is there any document about the xblock-html develope? 

(Maybe somebody could tell me the problem in my code.)