The func value set of type:fabric customFabricFrameSource does not recieve any parameters. For example, you cannot pass a text to https://github.com/mifi/editly/blob/master/examples/customFabric.js.

Following patch can fix it.

diff --git a/sources/fabricFrameSource.js b/sources/fabricFrameSource.js
index 74b8299..c88238c 100644
--- a/sources/fabricFrameSource.js
+++ b/sources/fabricFrameSource.js
@@ -308,7 +308,7 @@ async function createCustomCanvasFrameSource({ width, height, params }) {
 }
 
 async function customFabricFrameSource({ canvas, width, height, params }) {
-  return params.func(({ width, height, fabric, canvas }));
+  return params.func(({ width, height, fabric, canvas, params }));
 }
 
 function registerFont(...args) {