Function fetched 

pub fn fetched(
    url: &str,
    decode: fn(&[u8]) -> Result<ImageData, String>,
) -> Fetched

What is known about the image at url, starting the fetch if this is the first ask.

Called from a view, which runs every frame: the first call starts the work and says Loading, and every later one is a lookup. That is what makes it safe to write in a view at all — a fetch per frame would be sixty requests a second for one picture.

A failure is remembered like any other, and for the same reason as milestone 372’s: a URL that 404s will 404 again, and retrying every frame turns one bad link into a permanent load on somebody’s server.