Infinite lists

In mobile apps it is often necessary to display very large lists of items. One problem with this is that a large number of DOM elements must be created which can affect performance.

Onsen UI provides an element called VOnsLazyRepeat which helps rendering large numbers of items. It will automatically calculate what elements are visible and only render those. When the user scrolls, it will remove items that are outside the screen and add elements that become visible dynamically.

Notice that for a simple “load more items” functionality, it is recommended to use VOnsPage‘s infiniteScroll prop instead.

Using the component

VOnsLazyRepeat must we used inside the container component that will held all the items. Frequently it is used together with VOnsList:

<v-ons-list>
  <v-ons-lazy-repeat ...props></v-ons-lazy-repeat>
</v-ons-list>

The items will be rendered as siblings of VOnsLazyRepeat.

Defining an infinite list

VOnsLazyRepeat needs at least two props: length and renderItem. The first one is just a number that indicates the maximum amount of items that could be virtually displayed. The second one must be a generator function like the following:

renderItem(index) {
  return new Vue({
    template: `<v-ons-list-item>...</v-ons-list-item>`;
    // Set item data using 'index'
  });
}

The requirement is that it must return a new Vue component to be attached in the parent list. It must not be mounted already since VOnsLazyRepeat will mount it in the proper place. This generator function gets the index of the item in the list (absolute index, not the index of the visible list) that can be used to initialize the item’s data.

It is possible to import an existing component and extend it as well.

Calculating the height

VOnsLazyRepeat can dynamically calculate the height of every item once they are rendered, but in order to know how many items it should render beforehand we must provide a hint. By default, VOnsLazyRepeat pre-renders the very first item and takes its height to do the calculations.

However, it is also possible to pass an optional calculateItemHeight function prop that gets the element index and returns its approximate height. This can enhance the calculations and allow better scrolling.

calculateItemHeight(index) {
  return myItems[index].image.height;
}