Example using "Use cairo for drawing"

[joe9]

Hello,

Thanks for sharing haskell-gi.

Just want to check if there is an example or sample code using haskell-gi to render to a gtk3 window using Cairo.

Thanks

[garetxe]

Hi,

There was no example, but I had been meaning to get around to this for a while, so I decided to finally write an example. Please see https://github.com/haskell-gi/haskell-gi/blob/master/examples/Cairo.hs , which is a straight port of the cairo-clock example in gtk2hs to haskell-gi (you will need GHC 8.0 to compile this example).

The issue is that there is no useful introspection data in cairo itself, so the haskell-gi bindings are not useful at all in themselves. Luckily one can use the hand-written bindings in the cairo library without much effort. Everything is fairly straightforward, the only tricky part is the following function:

-- | This function bridges gi-cairo with the hand-written cairo
-- package. It takes a `GI.Cairo.Context` (as it appears in gi-cairo),
-- and a `Render` action (as in the cairo lib), and renders the
-- `Render` action into the given context.
renderWithContext :: GI.Cairo.Context -> Render () -> IO ()
renderWithContext ct r = withManagedPtr ct $ \p ->
runReaderT (runRender r) (Cairo (castPtr p))

I hope this helps. If anything is confusing please do not hesitate to ask.