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Composite Loaders

loaders.gl enables loaders to call other loaders (referred to as "sub-loaders" in this section). This enables loaders for "composite formats" to be "composed" out of loaders for the primitive parts.

An example of sub-loaders can be seen in the GLTFLoader, which delegates:

  • Draco mesh decoding to the DracoLoader
  • image decoding to the various ImageLoaders and the BasisLoader.

Naturally, composite loaders can call other composite loaders, which is for instance used by the Tiles3DLoader which uses the GLTFLoader to parse embedded glTF data in certain tiles.

Calling loaders inside loaders​

To call another loader from the parse function of your loader, use the appropriate parse*WithContext() function provided by @loaders.gl/loader-utils. Make sure to supply in the context parameter .

A conceptual example of a 3D Tiles loader calling the GLTFLoader with some additional options.

import {parseWithContext} from '@loaders.gl/loader-utils';

export async function parse3DTile(arrayBuffer, options, context) {
  const tile = {};
  // Extract embedded GLB (if present) into `tile.gltfArrayBuffer`
  ...
  if (tile.gltfArrayBuffer) {
    tile.gltf = await parseWithContext(tile.gltfArrayBuffer, GLTFLoader, {gltf: {...}}, context);
  }
}
caution

While loaders could import @loaders.gl/core and use the core parse*() functions to call sub-loaders, it is strongly discouraged for multiple reasons. Most importantly it prevents loaders.gl from properly handling certain use cases such as allowing worker-loaders to call other loaders. The parse*WithContext() functions exported by @loaders.gl/loader-utils are the right tool for the job.

LoaderContext​

When a loader is being called (i.e. one of its parse*() functions is being called), a LoaderContext object is supplied.