servo/src/components/net/local_image_cache.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/*!
An adapter for ImageCacheTask that does local caching to avoid
extra message traffic, it also avoids waiting on the same image
multiple times and thus triggering reflows multiple times.
*/

use image_cache_task::{Decode, GetImage, ImageCacheTask, ImageFailed, ImageNotReady, ImageReady};
use image_cache_task::{ImageResponseMsg, Prefetch, WaitForImage};

use std::comm;
use std::comm::Port;
use std::task;
use servo_util::url::{UrlMap, url_map};
use extra::net::url::Url;

pub fn LocalImageCache(image_cache_task: ImageCacheTask) -> LocalImageCache {
    LocalImageCache {
        image_cache_task: image_cache_task,
        round_number: 1,
        on_image_available: None,
        state_map: url_map()
    }
}

pub struct LocalImageCache {
    priv image_cache_task: ImageCacheTask,
    priv round_number: uint,
    priv on_image_available: Option<@fn() -> ~fn(ImageResponseMsg)>,
    priv state_map: UrlMap<@mut ImageState>
}

priv struct ImageState {
    prefetched: bool,
    decoded: bool,
    last_request_round: uint,
    last_response: ImageResponseMsg
}

#[allow(non_implicitly_copyable_typarams)] // Using maps of Urls
impl LocalImageCache {
    /// The local cache will only do a single remote request for a given
    /// URL in each 'round'. Layout should call this each time it begins
    pub fn next_round(&mut self, on_image_available: @fn() -> ~fn(ImageResponseMsg)) {
        self.round_number += 1;
        self.on_image_available = Some(on_image_available);
    }

    pub fn prefetch(&self, url: &Url) {
        let state = self.get_state(url);
        if !state.prefetched {
            self.image_cache_task.send(Prefetch(copy *url));
            state.prefetched = true;
        }
    }

    pub fn decode(&self, url: &Url) {
        let state = self.get_state(url);
        if !state.decoded {
            self.image_cache_task.send(Decode(copy *url));
            state.decoded = true;
        }
    }

    // FIXME: Should return a Future
    pub fn get_image(&self, url: &Url) -> Port<ImageResponseMsg> {
        let state = self.get_state(url);

        // Save the previous round number for comparison
        let last_round = state.last_request_round;
        // Set the current round number for this image
        state.last_request_round = self.round_number;

        match state.last_response {
            ImageReady(ref image) => {
                let (port, chan) = comm::stream();
                chan.send(ImageReady(image.clone()));
                return port;
            }
            ImageNotReady => {
                if last_round == self.round_number {
                    let (port, chan) = comm::stream();
                    chan.send(ImageNotReady);
                    return port;
                } else {
                    // We haven't requested the image from the
                    // remote cache this round
                }
            }
            ImageFailed => {
                let (port, chan) = comm::stream();
                chan.send(ImageFailed);
                return port;
            }
        }

        let (response_port, response_chan) = comm::stream();
        self.image_cache_task.send(GetImage(copy *url, response_chan));

        let response = response_port.recv();
        match response {
            ImageNotReady => {
                // Need to reflow when the image is available
                // FIXME: Instead we should be just passing a Future
                // to the caller, then to the display list. Finally,
                // the compositor should be resonsible for waiting
                // on the image to load and triggering layout
                let image_cache_task = self.image_cache_task.clone();
                assert!(self.on_image_available.is_some());
                let on_image_available = self.on_image_available.get()();
                let url = copy *url;
                do task::spawn {
                    let (response_port, response_chan) = comm::stream();
                    image_cache_task.send(WaitForImage(copy url, response_chan));
                    on_image_available(response_port.recv());
                }
            }
            _ => ()
        }

        // Put a copy of the response in the cache
        let response_copy = match response {
            ImageReady(ref image) => ImageReady(image.clone()),
            ImageNotReady => ImageNotReady,
            ImageFailed => ImageFailed
        };
        state.last_response = response_copy;

        let (port, chan) = comm::stream();
        chan.send(response);
        return port;
    }

    priv fn get_state(&self, url: &Url) -> @mut ImageState {
        let state = do self.state_map.find_or_insert_with(url.clone()) |_| {
            let new_state = @mut ImageState {
                prefetched: false,
                decoded: false,
                last_request_round: 0,
                last_response: ImageNotReady
            };
            new_state
        };
        *state  // Unborrowing the state
    }
}