Priority of async tasks

[l4rzy]

Priority of async tasks is required to make the GUI fast.
For example, when Home is loading, clicking on a Book to view to see the book's cover and preview pages must have higher priority than async tasks that are loading images in Home.

[nomi-san]

Have you tried multi-threading?
In many platforms, GUI cannot process on multiple threads.
But the image data can be loading in background on another thread.

e.g

/* thread 2 */
void imgLoader(...) {
    data = net.get(...);
}

/* callback */
void onInitialized() {
    thread.run(imgLoader, args...);
}

/* callback */
void onChangeScene() {
    if (scene.type == SC_BOOK) {    // switch
        await(imgResult);
        ...
    }
}

/* main thread */
int main() {
    prepare();
    app.run(win);
}

[l4rzy]

Thanks for replying.

From the very first day developing this application, I've used asynchronous task to load images. Interestingly, I've found that asynchronous functions blocked the UI just a little bit, especially when the internet is slow, as pointed out at this. And yes, I've successfully implemented network requests in another thread and everything worked alright.

But that was a different problem.

This issue is more like, how to pause an async task, and resume it later. The very common use case of this is when you have many async tasks, but some tasks have higher priority, e.g. your homepage has 30 images, but you want to load images that are in the perceptible field first, as soon as the user scroll to that point.

[nomi-san]

I think, all images in the page should be loaded after the user accesses the page.
Unloaded images can be placed by something alternative (alt image/text). After loaded, image loader triggers event to GUI and shows.

void imgLoader(...) {
    data[imgIndex] = net.get();
    setStateOrYield(loadingState, ...);
}

void onScroll(...) {
    img = getImage(idx);
    if (img) {
        img.show(position);
    }
    else {
        delayOrWaitOrShowAlternativeImg(...);
    }
}

[l4rzy]

I did some research on pausing and resuming async tasks in Vala, still no luck though.

I think your approach was clear and feasible to implement. By delaying loading and instead of loading and pausing, this would be more efficient.

Thanks!