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Intro to Appium Drivers

As the main Overview makes clear, "drivers" are basically Appium's answer to the question, "how do we support automation of multiple, unrelated platforms?" In this doc we'll get into a little more detail about how drivers work. The specific details of how drivers work probably don't matter too much for you, unless you're planning on writing your own driver or contributing to an existing driver (things we hope you do!).

The main benefit in understanding a bit more of how drivers work is that being aware of the typical complexity or the typical driver architecture will inform your debugging process when you inevitably run into an issue in one of your tests.

Interface Implementations

At the most basic level, drivers are simply Node.js classes that extend a special class included in Appium, called BaseDriver. You could have something very close to a "working" driver, with these very simple lines of code:

import BaseDriver from '@appium/base-driver'

class MyNewDriver extends BaseDriver {

This empty driver doesn't do anything, but you could wrap it up in a Node.js module, add a few Appium-related fields to the module's manifest (package.json), and then install it using appium driver install.

So, from a technical perspective, an Appium driver is just a bit of code that inherits from some other Appium code. That's it! Now, inheriting from BaseDriver actually gives us a lot, because BaseDriver is essentially an encapsulation of the entire WebDriver protocol. So all a driver needs to do something useful is to implement Node.js methods with names corresponding to their WebDriver protocol equivalents.

So let's say I wanted to do something with this empty driver; first I have to decide which WebDriver command I want to implement. For our example, let's take the Navigate To WebDriver command. Leave aside for the moment what I want to have the driver do when this command is executed. To tell Appium the driver can handle the command, all we have to do is define a method like this in our driver class:1

async setUrl(url) {
    // do whatever we want here

That's it! How we actually implement the command is totally up to us, and depends on the platform(s) we want to support. Here are some different example implementations of this command for different platforms:

  • Browsers: execute some JavaScript to set window.location.href
  • iOS apps: launch an app using a deep link
  • Android apps: launch an app using a deep link
  • React apps: load a specific route
  • Unity: go to a named scene

So you can see there can be a lot of differences between how drivers implement the same WebDriver command across platforms.2 What is the same, though, is how they express that they can handle a protocol command.

We're going into this great amount of detail (which you don't need to remember, by the way), because it's important to stress the point that an Appium driver is not inherently anything in particular, other than a bit of JS code that can handle WebDriver protocol commands. Where you go from there is up to you, the driver author!

Automation mapping

But typically what driver authors want to do is to provide automation behaviours for a given platform(s) that are semantically very similar to the the WebDriver spec implementations for browsers. When you want to find an element, you should get a reference to a UI element. When you want to click or tap that element, the resulting behaviour should be the same as if a person were to click or tap on the element. And so on.

So the real challenge for driver authors is not how to work with the WebDriver protocol (because BaseDriver encapsulates all that for you), but how to make the actual automation happen on the target platform. Every driver relies on its own set of underlying technologies here. As mentioned in the Overview, the iOS driver uses an Apple technology called XCUITest. These underlying automation technologies usually have proprietary or idiosyncratic APIs of their own. Writing a driver becomes the task of mapping the WebDriver protocol to this underlying API (or sometimes a set of different underlying APIs--for example, the UiAutomator2 driver relies not only on the UiAutomator2 technology from Google, but also functions only available through ADB, as well as functions only available via the Android SDK inside a helper app). Tying it all together into a single, usable, WebDriver interface is the incredibly useful (but incredibly challenging) art of driver development!

Multi-level architecture

In practice, this often results in a pretty complex architecture. Let's take iOS for example again. The XCUITest framework (the one used by the Appium driver) expects code that calls it to be written in Objective-C or Swift. Furthermore, XCUITest code can only be run in a special mode triggered by Xcode (and directly or indirectly, the Xcode command line tools). In other words, there's no straightforward way to go from a Node.js function implementation (like setUrl() above) to XCUITest API calls.

What the XCUITest driver authors have done is instead to split the driver into two parts: one part written in Node.js (the part which is incorporated into Appium and which initially handles the WebDriver commands), and the other part written in Objective-C (the part which actually gets run on an iOS device and makes XCUITest API calls). This makes interfacing with XCUITest possible, but introduces the new problem of coordination between the two parts.

The driver authors could have chosen any of a number of very different strategies to model the communication between the Node.js side and the Objective-C side, but at the end of the day decided to use ... the WebDriver protocol! That's right, the Objective-C side of the XCUITest driver is itself a WebDriver implementation, called WebDriverAgent.3

  • The Appium XCUITest driver builds and manages WebDriverAgent for you, which can be a pain and involves the use of Xcode.
  • The XCUITest driver does lots more than what can be done by WebDriverAgent, for example working with simulators or devices, installing apps, and the like.

The moral of the story is that driver architectures can become quite complicated and multilayered, due to the nature of the problem we're trying to solve. It also means it can be difficult sometimes to tell where in this chain of technologies something has gone wrong, if you run into a problem with a particular test. With the XCUITest world again, we have something like the following set of technologies all in play at the same time:

  • Your test code (in its programming language) - owned by you
  • The Appium client library - owned by Appium
  • The Selenium client library - owned by Selenium
  • The network (local or Internet)
  • The Appium server - owned by Appium
  • The Appium XCUITest driver - owned by Appium
  • WebDriverAgent - owned by Appium
  • Xcode - owned by Apple
  • XCUITest - owned by Apple
  • iOS itself - owned by Apple
  • macOS (where Xcode and iOS simulators run) - owned by Apple

It's a pretty deep stack!

Proxy mode

There's one other important architectural aspect of drivers to understand. It can be exemplified again by the XCUITest driver. Recall that we just discussed how the two "halves" of the XCUITest driver both speak the WebDriver protocol---the Node.js half clicks right into Appium's WebDriver server, and the Objective-c half (WebDriverAgent) is its own WebDriver implementation.

This opens up the possibility of Appium taking a shortcut in certain cases. Let's imagine that the XCUITest driver needs to implement the Click Element command. The internal code of this implementation would look something like taking the appropriate parameters and constructing an HTTP request to the WebDriverAgent server. In this case, we're basically just reconstructing the client's original call to the Appium server!4 So there's really no need to even write a function implementing the Click Element command. Instead, the XCUITest driver can just let Appium know that this command should be proxied directly to some other WebDriver server.

If you're not familiar with the concept of "proxying," in this case it just means that the XCUITest driver will not be involved at all in handling the command. Instead it will merely be repackaged and forwarded to WebDriverAgent at the protocol level, and WebDriverAgent's response will likewise be passed back directly to the client, without any XCUITest driver code seeing it or modifying it.

This architectural pattern provides a nice bonus for driver authors who choose to deal with the WebDriver protocol everywhere, rather than constructing bespoke protocols. It also means that Appium can create wrapper drivers for any other existing WebDriver implementation very easily. If you look at the Appium Safari driver code, for example, you'll see that it implements basically no standard commands, because all of these are proxied directly to an underlying SafariDriver process.

It's important to understand that this proxying business is sometimes happening under the hood, because if you're ever diving into some open source driver code trying to figure out where a command is implemented, you might be surprised to find no implementation at all in the Node.js driver code itself! In that case, you'll need to figure out where commands are being proxied to so you can look there for the appropriate implementation.

OK, that's enough for this very detailed introduction to drivers!

  1. You might notice that setUrl doesn't look anything like Navigate To, so how did we know to use it rather than some other random string? Well, Appium's WebDriver-protocol-to-method-name mapping is defined in a special file within the @appium/base-driver package called routes.js. So if you're writing a driver, this is where you would go to figure out what method names to use and what parameters to expect. Or you could look at the source for any of the main Appium drivers! 

  2. Of course, we want to keep the semantics as similar as possible, but in the world of iOS, for example, launching an app via a deep link (a URL with a special app-specific scheme) is about as close as we are going to get to navigating to a web URL. 

  3. You could in theory, therefore, point your WebDriver client straight to WebDriverAgent and bypass Appium entirely. This is usually not convenient, however, for a few reasons: 

  4. It's not exactly the same call, because the Appium server and the WebDriverAgent server will generate different session IDs, but these differences will be handled transparently.