callbacksExt Extension

Python Callback Systemedit

The CallbackExt extension provides Python callback functionality to custom components. All callbacks will be passed a single argument containing an info dictionary. Callbacks can be defined in the DAT named in the Callback DAT parameter. To report all callbacks to the textport, turn on the Print Callbacks toggle parameter. The Callback DAT and Print Callbacks parameters are often found on the Callbacks parameter page, but their location can be customized.

The info dictionary always contains an "ownerComp" key. It will also have a "callbackName" key holding the callback name. It will sometimes contain an "about" key, describing the callback, and should always contain this key if a return value is expected. Generally, callbacks are called AFTER the internal method they are associated with, to allow over-riding of whatever that method does.

See also: CallbacksExt Extension

Adding CallbackExt to a Componentedit

The standard way to add CallbackExt to a component is to add a Select DAT inside that component and rename it "CallbacksExt". Change the DAT parameter of CallbacksExt to expression mode and enter the expression:


Next, set one of the component's Extension Object parameters to:


If you want to access the callback functionality directly via the component, toggle the "Promote Extension" parameter for that extension.

To implement a system allowing a user callback DAT, you will want to add a custom "Callbackdat" DAT parameter. For testing you will usually want a custom "Printcallbacks" toggle parameter. Adding these parameters is generally done with the Component Editor.


DoCallback(callbackName, callbackInfo=None, callbackOrDat=None) → callbackInfo

If it exists, call the named callback in ownerComp.par.Callbackdat and return callbackInfo with the callback's return value placed in callbackInfo['returnValue']. If no callback is found, returns None. If a callback is found, the info dictionary is returned with the addition of a retValue key containing the returned value.
If ownerComp has a parameter called Printcallbacks, and that parameter is True, callback debug info will be printed to textport.
  • callbackName - Name of callback to be called
  • callbackInfo - (Optional) Dictionary of info to be passed to callback. callbackInfo['ownerComp'] is automatically set to ownerComp. If callback needs special instructions, put them in callbackInfo['about'].
  • callbackOrDat - (Optional) Used to redirect callback to a DAT or specific function. Note: This is an expert feature, not for general use.

SetAssignedCallback(self, callbackName, callback, details=None)

Note: This is an expert feature, not for general use.
An assigned callback is a python function assigned to a particlar callback name. When doCallback is called with callbackName, callback will be invoked instead of searching CallbackDat
  • callbackName - Name of callback to be called
  • callback - The callback or None to remove the assigned callback
  • details - Optional extra info to be passed in the ['details'] key of infoDict when callback is called.

PassCallbacksTo(self, passTarget)

Note: This is an expert feature, not for general use.
Set a target DAT or function for passing on unfound callbacks to.
  • passTarget - a DAT or function

PassOnCallback(self, info)

Note: This is an expert feature, not for general use.
Pass this callback (as named in info) to the callback stored in extension's PassTarget member. Use PassCallbacksTo to set target
  • info - the callback's info dict




Get or set the dat containing user callbacks. This will set the value of the Callbackdat parameter as well, if it exists.


Get or set whether to print callback info to the Textport