This page is out of date and is in need of a rewrite. If you know anything about traps, please contribute!
addon.tab.traps is a public API for userscripts that can be used to obtain objects that are not otherwise available. The actual traps are coded at prototype-handler.
They share some APIs with “Once” traps, but is very different and is documented at Fake redux state separately.
editor-stepping addon shows how to manipulate
Thread object trapped.
There are two types of traps, named after how many times they get trapped. Singleton instance like VM or ScratchBlocks get trapped only once per trap, so they are called “Once”. Objects like Thread can get trapped multiple times, so they are called “Many”.
Difference between Once and Many is simple: the addon only stores reference for Once.
|vm||Scratch VM, trapped by Function.prototype.bind||Once|
|vm.propsVMBind||Scratch VM, trapped from props.vm using bind||Once|
|vm.propsVMAssign||Scratch VM, trapped from props.vm using assign||Once|
|thread||Thread object used in Scratch VM||Many|
onceValues attribute of
addon.tab.traps stores references for “Once” traps. It is an prototypeless object - e.g. you cannot call
Keys are the “trap name” listed above, and values are the trapped values.
Example: to get VM object, use
addon.tab.traps itself is not an
EventTarget. However, it’s possible to add or remove listeners, by using some public APIs.
removeOnceListener is for “Once” traps, and
removeManyListener is for “Many” traps. Note that listeners for “Once” objects can fire multiple times under some conditions, and may never fire if the object is trapped before loading the userscript.
These methods work like
removeEventListener, but third argument is not supported at all. The first argument is for trap name; for example, if it needs to be fired when
thread is trapped, do
addManyListener("thread", callback). The second argument is callback function that can take one argument for CustomEvent object, which contains the trapped value.
The CustomEvent object passed to callback always has
value attribute for trapped value, and may have
trapName if the first argument for the listener is
* (see below).
Special event: if the trap name passed to these methods is
*, it will fire when any object for the category is trapped. For example, callback in
addOnceListener("*", callback) gets fired whenever an object in Once category is trapped. To check what is trapped, the CustomEvent passed to the callback also has
trapName attribute for the trap name.