Simulate mouse / touch events with jQuery in PhantomJS and the browser
For my little project jQuery Finger, I wanted to be able to continuously test it while coding. This can be easily achieved using the magic combo of Grunt, Mocha (with Chai) and PhantomJS.
I just had to launch grunt watch and listen to the evil bip telling me "test have failed, you suck nooby noob!".
This is what I wanted, but things became more complicated when I wanted to run tests with more advanced user inputs such as tap, double tap, long tap, drag and swipe, across multiple elements.
I have tried to use MouseEvents and TouchEvent to simulate it, but I couldn't make it work properly with PhantomJS. If somebody has successfully achieved this, I'm interested in!
The code of this article is hosted here. Use it, fork it!
jQuery is your best friend
For jQuery Finger, I must use jQuery as a direct dependency. That's convenient I guess because jQuery has the ability to trigger events, and particulary mouse/touch events. It adds an abstraction above native events which was very handy for me.
The caveat of using jQuery to trigger events is that you must register jQuery events specifically. You can't trigger a jQuery click and hope catching it if you registered your listener with addEventListener. jQuery simply triggers an event using its own internal sugar. If this is a blocker for you, this article might still be interesting because the global idea remains the same.
Manage your virtual pointer
So how can we try to simulate pointer events ?
We can use the magic of the document.elementFromPoint function. It tells which DOM element lives at the given position. AFAIK it is supported since quite a long time.
All we have to do, is to keep track of coordinates. When we want to trigger an event, we retreive the DOM element, and call jQuery.fn.trigger on it. Simple as pie.
Here is the first draft of our virtual pointer:
With this first version, we can easily:
change the position of the pointer with x and y.
simulate a mousedown mouseup / touchstart touchend by calling tap, assuming we register events with VirtualPointer.START_EVENT or VirtualPointer.STOP_EVENT.
A classic jQuery event is built on top of a native event that you can access with the originalEvent property. Some handy properties are directly copied to the jQuery event object.
pageX and pageY are one of these. They give the pointer absolute coordinates when the event is fired. jQuery tries to unify these properties accross every browser. But, on some mobile browsers, such as Mobile Safari, pageX and pageY stay to 0. The only trustable source for coordinates resides in the event.originalEvent.touches[0] properties.
The problem is when you manually trigger a jQuery event, it is created from scratch. So pageX, pageY and originalEvent are missing. We have to create them manually and forward our virtual pointer's coordinates.
Let's add press and doubletap event
press event is an alias for a long tap event.
Here is the updated version of our initial virtual pointer:
The code is pretty self explanatory.
I will just explain the security multiplier. setTimeout is a cool bro, but he is not very punctual. So, if your code really depends on a time threshold, you should never put the setTimeout value to that threshold, or near it. By applying a +0.5/-0.5 security multiplier, you are always sure that you will be either on one or the other side.
Now let's add some motion
To handle motion, we will have to pretend that the cursor is ... moving, woot!
We will need destination coordinates and a duration. Our job will be to interpolate coordinates between current and destination, and call the mousemove/touchmove event at a given interval.
In real life, motion events may be triggered at a very, perhaps sometimes too high rate (i.e. *Chrome's rate is around every 2ms).
We won't be able to be so precise, but who cares? We will just use our good old friend setTimeout with a 0 timeout to simulate AMAP fake moves.
Now, we can call move to simulate motion, and tapStart, move, tapEnd sequence to simulate some kind of drag. Great!
With those three utility functions, we are now able to simulate gestures.
Right of the Batman, here is our final code:
That's it! This is a basic implementation of our virtual pointer. It should allow us to simulate mouse/touch events in the browser and in PhantomJS.
As a side note, PhantomJS is detected as a touch device because window.ontouchstart is defined. This appears to be something common with some WebKit forks.
Bonus: prevent user interaction
During the simulation we don't want any real event messing around with our tests.
Well, keep your hand away from your mouse, that's it!
When you trigger an event with jQuery, it comes with an additional property isTrigger. So, basically, in your event handlers you can tell if this is a fake or a real event.
In our case, we are only interested in fake events, not real ones.
The only satisfying solution I found is proxying each jQuery event handlers and skipping real ones.
We could just prepend our event listeners with something like this:
But do we? Not really... Let's use jQuery.event.special in our test environment to proxy every jQuery event handler dynamically:
Just be sure you execute this code before running your tests and registering your events. You will then be able to harlem shake your mouse / fingers without any side effect.
Everything in this article is included in the Github version, plus some additional bonuses:
an example of Mocha integration in the very simple test suite.
the possibility to specify a custom scope for callbacks.
the possibility to auto reset coordinates on each tapStart.
Also, you can take a look at jQuery Finger :-)
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