As discussed in [the Flow View section of the mitmproxy
overview](@!urlTo("mitmproxy.html")!@), mitmproxy allows you to inspect and
manipulate flows.  When inspecting a single flow, mitmproxy uses a number of
heuristics to show a friendly view of various content types; if mitmproxy
cannot show a friendly view, mitmproxy defaults to a __raw__ view.

Each content type invokes a different flow viewer to parse the data and display
the friendly view. Users can add custom content viewers by adding a view class
to contentview.py, discussed below.

## Adding a new View class to contentview.py

The content viewers used by mitmproxy to present a friendly view of various
content types are stored in contentview.py.  Reviewing this file shows a number
of classes named ViewSomeDataType, each with the properties: __name__,
__prompt__, and __content\_types__ and a function named __\_\_call\_\___.

Adding a new content viewer to parse a data type is as simple as writing a new
View class.  Your new content viewer View class should have the same properties
as the other View classes: __name__, __prompt__, and __content\_types__ and a
__\_\_call\_\___ function to parse the content of the request/response.

* The __name__ property should be a string describing the contents and new content viewer;
* The __prompt__ property should be a two item tuple:

  - __1__: A string that will be used to display the new content viewer's type; and
  - __2__: A one character string that will be the hotkey used to select the new content viewer from the Flow View screen;

* The __content\_types__ property should be a list of strings of HTTP Content\-Types that the new content viewer can parse.
  * Note that mitmproxy will use the content\_types to try and heuristically show a friendly view of content and that you can override the built-in views by populating content\_types with values for content\_types that are already parsed -- e.g. "image/png".

After defining the __name__, __prompt__, and __content\_types__ properties of
the class, you should write the __\_\_call\_\___ function, which will parse the
request/response data and provide a friendly view of the data.  The
__\_\_call\_\___ function should take the following arguments: __self__,
__hdrs__, __content__, __limit__; __hdrs__ is a MultiDict object containing
the headers of the request/response; __content__ is the content of the
request/response, and __limit__ is an integer representing the amount of data
to display in the view window.

The __\_\_call\_\___ function returns two values: (1) a string describing the
parsed data; and (2) the parsed data for friendly display.  The parsed data to
be displayed should be a list of strings formatted for display.  You can use
the __\_view\_text__ function in contentview.py to format text for display.
Alternatively, you can display content as a series of key-value pairs; to do
so, prepare a list of lists, where each list item is a two item list -- a key
that describes the data, and then the data itself; after preparing the list of
lists, use the __common.format\_keyvals__ function on it to prepare it as text
for display.

If the new content viewer fails or throws an exception, mitmproxy will default
to a __raw__ view.