VueJs object key/value is not reactive in v-for loop

Question

I have following problem, which I don't know how to properly tackle.

I have "list" of all purchased images on my view. I display them with v-for loop. Each image also has progress-bar element, so when user clicks on download button, downloadContent function gets executed and progress bar should be displayed.

So my html looks like this.

<section class="stripe">
    <div class="stripe__item card" v-for="(i, index) in purchasedImages">
        <progress-bar :val="i.download_progress"
                      v-if="i.download_progress > 0 && i.download_progress < 100"></progress-bar>
        <div class="card__wrapper">
            <img :src="'/'+i.thumb_path" class="card__img">
        </div>
        <div class="btn-img card__btn card__btn--left" @click="downloadContent(i.id_thumb, 'IMAGE', index)">
        </div>
    </div>
</section>

And this is my JS Code

import Vue from 'vue'
import orderService from '../api-services/order.service';
import downloadJs from 'downloadjs';
import ProgressBar from 'vue-simple-progress';

export default {
    name: "MyLocations",
    components: {
        ProgressBar: ProgressBar
    },
    data() {
        return {
            purchasedImages: {},
            purchasedImagesVisible: false,
        }
    },
    methods: {
        getUserPurchasedContent() {
            orderService.getPurchasedContent()
                .then((response) => {
                    if (response.status === 200) {

                        let data = response.data;
                        this.purchasedImages = data.images;

                        if (this.purchasedImages.length > 0) {
                            this.purchasedImagesVisible = true;
                            // Set download progress property
                            let self = this;
                            this.purchasedImages.forEach(function (value, key) {
                                self.purchasedImages[key].download_progress = 0;
                            });
                        }
                    }
                })
        },
        downloadContent(id, type, index) {
            let self = this;
            orderService.downloadContent(id, type)
                .then((response) => {
                    let download = downloadJs(response.data.link);
                    download.onprogress = function (e) {
                        if (e.lengthComputable) {
                            let percent =  e.loaded / e.total * 100;
                            let percentage = Math.round(percent);
                            if (type === 'IMAGE') {
                            // Is this proper way to set one field reactive?
                         self.purchasedImages[index].download_progress = percentage;
                                if (percentage === 100) {
                                    self.purchasedImages[index].download_progress = 0;
                                }
                            }
                        }
                    }
                })
        },
    },
    mounted: function () {
        this.getUserPurchasedContent();
    }
};

So the problem is. When user clicks on download button, download starts to execute and I get downloaded content, but I don't see progress bar. So I wonder, is this a proper way to set element reactive? How should my implementation look like? How to properly set self.purchasedImages[index].download_progress object key value, so progress bar will be visible?

If you need any additional informations, please let me know and I will provide. Thank you!

Answer 1

The snippet:

this.purchasedImages = data.images;

Leads us to believe data.images is an array of objects that do not have the download_progress property. So Vue can't detect/react when it changes.

To fix that, you can use Vue.set:

Vue.set(self.purchasedImages[key], 'download_progress', 0);

This is very well explained in Vue.js docs.

Another option: add the property before assigning to data

Just for completeness, you could also add the download_progress before assigning the array to the data property. This would allow Vue to notice it and be able to react to it.

Example:

let data = response.data;
this.purchasedImages = data.images.map(i => ({...i, download_progress: 0}));

if (this.purchasedImages.length > 0) {
    this.purchasedImagesVisible = true;
    // no need to set download_progress here as it was already set above
}

// if above could also be simplified to just:
this.purchasedImagesVisible = this.purchasedImages.length;

---

On a side note, since it is gonna be an array and not an object, I suggest you declare it as such:

data() {
    return {
        purchasedImages: [], // was: {},

This will have no effect, since you overwrite purchasedImages completely in (this.purchasedImages = data.images;), but it is a good practice as it documents that property's type.