Testing Applications with Xnee

Xnee can record user actions during a session and then replay those actions. By recording sessions when testing a program, Xnee automatically can test the program for you later. These test sessions can be replayed before every release, or every night, to ensure the quality of your program. Is it as easy as it sounds? Almost.

Xnee doesn't test only GUIs. You also can use Xnee to test command-line programs by making a few test scripts that test all the options of a command-line program and analyze the results. Xnee also has been used to test how much traffic is being sent over a large network with numerous thin clients. Support for distributing events to multiple displays has been added if you want to test the same cases on multiple machines at the same time. Besides testing programs, Xnee also is used to demonstrate programs. In this case, Xnee acts as a patient demonstrator, doing the same job over and over again without complaining.

In 1997, Henric Johansson and I wrote our master's thesis on recording and replaying X events. We implemented a nonfree recorder and replayer for a Swedish company for its internal purposes. After finding a job, I often lacked a free test program for X11, so I decided to implement one on my own, using the experience gained from the thesis. The Xnee Project started in the summer of 1999 and was licensed under GPL from the start. In November 2002, version 1.0 was released, and by the end of February 2003, Xnee was dubbed a GNU package.

Before we go on with Xnee, this short introduction to X explains a lot of the terms used in this article. X is a window-based user interface system for various platforms. The X server is a program that handles all the hardware and actually does the drawing on the screen. On GNU/Linux systems, XFree86 is the most-used X server. X programs are known as clients; examples are xterm and Galeon. The clients communicate with an X server using the X protocol.

In this article we concentrate on the packets used to send information between the X server and its clients. These packets are called Event, Request, Reply and Error and are referred to here as protocol data. The following list shows the X11 protocol data description:

The most interesting thing here is every time the user interacts with the computer using the mouse or the keyboard the X server sends the appropriate client one or more events. Some of these events result directly from user input. These events are referred to as device events. The device events are ButtonPress, ButtonRelease, MotionNotify, KeyPress and KeyRelease. If we could record all of these events during a session, we would have a complete description of all the actions the user performed. If we had a robot that could read these events if they were printed to a file or on paper, the robot then could interact with the system as the user did when recording, and we would have ourselves a test robot. Or, even better, if we had support for faking those events, we would have a test robot made of software. Fortunately, support exists for both recording and replaying in X.

To record X protocol data we can use the extensions RECORD or XTrap. There are other ways to accomplish recording, such as sniffing the X socket, but we'll focus on RECORD as it's what Xnee uses. To replay events, we can use both the XTest extension and the RECORD extension. During replay, the RECORD extension is used to synchronize what's happening when replaying with what happened when the session was recorded.

The RECORD extension sends copies of the data sent between the clients and the server to the client requesting it. Using the RECORD extension, Xnee can record all protocol data the user wants and save it to a file to replay later.

The XTest extension can reproduce or fake all device events. This extension lets Xnee fake user actions, such as moving the pointer, pressing and releasing a key or pressing and releasing a button. No other data can be replayed.