SpecialPowers

SpecialPowers is a set of APIs available to Mochitest tests. Mochitests are intended to be written like regular web pages. However, sometimes tests need to do things that regular web pages are not permitted to do for security reasons. In these cases, the SpecialPowers APIs can be used to perform specific actions outside of the reach of normal web pages.

Method overview

Preference APIs

Permission APIs

Event Listener / Observer APIs

TBD

Garbage Collection APIs

TBD

Privilege Wrapper APIs

XPCOM Components APIs

Log APIs

TBD

Environment APIs

Focus Management APIs

TBD

Mock APIs

TBD

Form History APIs

TBD

Snapshot APIs

TBD

Clipboard APIs

TBD

Console APIs

TBD

Layout APIs

TBD

Frame Message APIs

TBD

Apps APIs

TBD

Other APIs

Attributes

Methods

Preference APIs

See nsIPrefBranch for documentation on the APIs that these methods use internally.

getBoolPref(aPrefName)

Gets the value of the preference aPrefName as a boolean.

getIntPref(aPrefName)

Gets the value of the preference aPrefName as an integer.

getCharPref(aPrefName)

Gets the value of the preference aPrefName as a string.

getComplexValue(aPrefName)

Gets the value of the preference aPrefName as an XPCOM object.

setBoolPref(aPrefName, aValue)

Sets the value of the preference aPrefName to the boolean value aValue.

setIntPref(aPrefName, aValue)

Sets the value of the preference aPrefName to the integer value aValue.

setCharPref(aPrefName, aValue)

Sets the value of the preference aPrefName to the string value aValue.

setComplexValue(aPrefName, aValue)

Sets the value of the preference aPrefName to the XPCOM object aValue.

clearUserPref(aPrefName)

Resets the preference aPrefName to its default value.

Permission APIs

TBD

Event Listener / Observer APIs

addChromeEventListener(type, listener, capture, allowUntrusted)

Adds an event listener to the TabChildGlobal object.

removeChromeEventListener(type, listener, capture)

Removes an event listener from the TabChildGlobal object.

Garbage Collection APIs

gc()

Forces a round of garbage collection to be performed.

Privilege Wrapper APIs

Object wrap(Object)

Wraps a chrome object so that it can be accessed.  Needed for accessing the return values of methods returned by XPCOM components (for example).

XPCOM Components APIs

Cc

The value of Components.classes, as available to chrome code.

Ci

The value of Components.interfaces, as available to chrome code.

Cr

The value of Components.results, as available to chrome code.

Cu

The value of Components.utils, as available to chrome code.

Log APIs

TBD

Environment APIs

TBD

Focus Management APIs

TBD

Mock APIs

MockFilePicker

This replaces the standard File Picker with one that can be controlled via script, for testing load and save code. To use it, add the following lines of code to your test:

var MockFilePicker = SpecialPowers.MockFilePicker;
MockFilePicker.reset(); // You must call reset before each test

This patch has examples of how to use the MockFilePicker, and also how to use it for XPCShell tests. The code in testing/mochitest/MockFilePicker.jsm might also be helpful.

Form History APIs

TBD

Snapshot APIs

TBD

Clipboard APIs

TBD

Console APIs

TBD

Layout APIs

TBD

Frame Message APIs

loadChromeScript()

TBD

Apps APIs

TBD

Other APIs

createSystemXHR()

Creates and returns an XMLHttpRequest instance which has full "system privileges". In other words, it can:

• Make cross-site requests without any restrictions, i.e. the target server does not need to support CORS.
• Set any header using xhr.setRequestHeader.
• Read any response using xhr.getResponseHeader and xhr.getAllResponseHeaders.
• Load and parse XUL contents using the xhr.responseXML property.
• Make requests without a referer (sic) header. If you want a referer header set, you have to do so manually using xhr.setRequestHeader.

However, any document parsed by the xhr object and accessed through xhr.responseXML is created using a null principal, which limits what the document can do.

sanityCheck()

Returns the string "foo".

Adding new APIs

If your test requires privileged functionality that's not currently present, you can add new APIs to the SpecialPowers object.

Because of the support for out-of-process content, the SpecialPowers implementation is split into two separate files:

• SpecialPowersObserver.jsm, which is always run in the parent process.
• specialpowers.js, which is a content script, and may be run in the content process.

Both files execute with chrome privileges, but certain XPCOM APIs are not available in content processes. Consult with an Electrolysis or Mobile peer if you are unsure if you can use a specific API there. You may also want to read the Message Manager documentation for more detailed information on how cross-process messaging works.

A simple example

Let's say you wanted to expose the numberOfScreens attribute from the nsIScreenManager interface. Let's also suppose that this interface is not available in the content process, just to make our example more complete. To start, first you would need to define the new API on the SpecialPowers object that is exposed to content. This object is defined in the content script.

Let's first add a numberOfScreens getter to SpecialPowers. It will simply send a blocking message to the chrome process, asking it to return the value in its response:

var SpecialPowers = {
  // existing APIs
  //...

  // Provide nsIScreenManager.numberOfScreens
  get numberOfScreens() {
    // You could pass additional parameters in the second parameter, consult the message manager documentation for more details.
    // Ideally this would be a memoizing getter, that's somewhat out of scope for this document.
    return sendSyncMessage("SPNumberOfScreens", {})[0];
  }
};

Now you need to provide a handler for this message in the chrome observer script. In the SpecialPowersObserver.observe function, register your message underneath the existing messages:

// Register for any messages our API needs us to handle
messageManager.addMessageListener("SPPrefService", this);
messageManager.addMessageListener("SPNumberOfScreens", this);

Then, in the SpecialPowersObserver.receiveMessage function, add a branch to handle your new message and return the result:

receiveMessage: function(aMessage) {
  switch(aMessage.name) {
    case "SPPrefService":
    // existing code...

    case "SPNumberOfScreens":
      var screenManager = Components.classes["@mozilla.org/gfx/screenmanager;1"]
                    .getService(Components.interfaces.nsIScreenManager);}
      return screenManager.numberOfScreens;

    default:

That's it! You will need to rebuild in the testing/mochitest directory in your object directory for your changes to show up; then you should be able to use your new SpecialPowers.numberOfScreens property in Mochitests.