Where to store pipeline specific options for scrolling behavior

[paulrouget]

We want to implement something like this: iframe.setOverscrollOptions(options);.

I managed to send this information to WebRender.

In render_backend.rs, I get this message: ApiMsg::SetOverscrollOptions(pipeline_id, options).

Later, in frame.rs, in scroll(), I want to access these options.

I don't know in which struct I should store the options in render_backend.rs to make it accessible in frame.rs.

I only want these options to affect how the document of the pipeline scroll within the iframe. Not pipeline's inner scrollable layers.

[paulrouget]

/cc @glennw @pcwalton

[glennw]

Instead of a new message, could you pass it through as an extra parameter to SetRootStackingContext message? (This is called once per pipeline here - https://github.com/servo/webrender/blob/master/webrender/src/render_backend.rs#L152)

Then, you can easily store it in the scene pipeline struct here:
https://github.com/servo/webrender/blob/master/webrender/src/scene.rs#L201

Would that work for your requirements?

[paulrouget]

could you pass it through as an extra parameter to SetRootStackingContext message

I could, but I want to be able to change the overscroll options anytime. When is SetRootStackingContext called?

[paulrouget]

I stored the overscroll options in ScenePipeline. Now, in frame.scroll(), I need to access these options. It doesn't sound possible within Frame to reach the scene.

RenderBacker, where scene is accessible, calls iframe.scroll(), but at this point, we don't know which pipeline is about to get scrolled.

I'm stuck.

Should I pass scene as a paramater to frame.scroll()?

[glennw]

In general the idea is that there is a step which takes the relevant information from the Scene structure and builds the Frame from it. This is mostly handled in the flatten() method in frame.rs. There is a step in this recursive function where it encounters a new iframe / pipeline and extracts the information from the scene. This is probably the right place to put it.

Is your code in a branch pushed to GH? It might be useful to take a look and see how it's being used - how often will these overscroll options be changed? What is it that controls / defines the overscroll options?

[paulrouget]

Is your code in a branch pushed to GH?

See:

• servo/servo@15c0c88
• 424f44b

how often will these overscroll options be changed?

No often. Usually, after the user navigated (clicked on link or went back/fwd).

What is it that controls / defines the overscroll options?

For now, the option tells where the user can overscroll (top, right, bottom, left). Later, we want to introduce some more complex options:

dictionary OverscrollOptions {
  boolean enabled;
  float minPanDistanceRatio;
  float springFriction;
  float springStiffness;
  float stopDistanceThreshold;
  float stopVelocityThreshold;
}

[glennw]

@paulrouget I think this can be closed now?

[paulrouget]

Yes. I might get back to this later. But as of now, it's not relevant anymore.