Question about canvas / shape memory management

[dtromb]

I don't think I'm quite understanding the memory management model in thorvg.

In other vector libraries I've used, the move, draw, curve, etc. primitives just function as commands, changing the state of a buffer or setting primitives to be used for a single frame / iteration of a draw operation.

Here however, they are shape objects which have a lifecycle. That's OK, but...

Suppose I have one canvas, with some shape objects that I want to be longer-lived (for example, I loaded an svg into a Picture, which is expensive, and I want to retain that object for reuse), as well as some objects that do not need to be long-lived (for example, some lines drawn on top of the svg.)

Now, when I want to clear the canvas, it seems my only choices are to completely destroy all of the shape objects (Canvas::clear with free=true), or leave them in the canvas (Canvas::clear with free=false).

What I would like to do is /remove all of the shapes from the canvas, but keep the objects alive/ so that I can dispose the ones I don't need, and keep the ones I do to reuse in the next draw.

When I call Canvas::clear with free=false and re-add the shapes, the very next draw fails with "INSUFFICIENT CONDITION" error (and no more information that =/) and the documentation is very unclear in general about how lifecycle of Shape objects is supposed to work....

When I call Canvas::clear with free=true and destroy the objects, I get double-free stack corruption.

How can I clear the canvas without destroying objects?

[hermet]

@dtromb Hello, I just noticed this answer is missing.

You have 2 options:

1. Destroy shapes with canvas->clear(true);.
   You can imagine shapes as rendering contexts. You can build a context like traditional drawing commands and push it to the canvas with every frame update.

2. Retain shapes with Canvas::clear(false);.
   In this case, the canvas will not destroy the shapes, so the ownership of the shape instances remains on your side. You can see examples/DirectUpdate.cpp for this example.

Sorry for late.