book.locations.generate() asynchronous replacement

[darylkell]

book.locations.generate() is slow as it runs sequentially. On one book that takes 19.6 seconds to load, this was reduced on my machine down to 0.5 seconds.

Assuming ePub is the class imported by script tag or similar, instead of using book.locations.generate() you can use the following book.ready.then() code to load the sections asynchronously.

This would be a nice addition in lieu of or in addition to book.locations.generate() as virtually every browser unloads tabs on mobile devices, causing books to be reloaded again and again when the user shifts focus.

const book = ePub(file);  // instantiate by normal means
const rendition = book.renderTo("reader", {
    width: "100%",
    height: "100%"
});

book.ready.then(() => {
    book.locations.break = 1800;
    const sections = book.spine.spineItems.filter(section => section.linear);

    // process the sections asynchronously rather than sequentially
    await Promise.all(sections.map(section => book.locations.process(section)));

    // update the book with some necessary information that .generate() does
    book.locations.total = book.locations._locations.length - 1;
    if (book.locations._currentCfi) {
        book.locations.currentLocation = book.locations._currentCfi;
    };
    
     // since we performed the process virtually in parallel, we must get the locations back in order
    const cfi = new ePub.CFI();
    book.locations._locations.sort((a, b) => cfi.compare(a, b));
}