Sequential thumbnail generation

core.js

  constructor(opts) {
    ...
    this.queue = []
    this.queueProcessing = false
    ...
  }
   
  actions () {
    ...
    this.on('core:file-added', (file) => {
      this.addToPreviewQueue(file)
    })
    ...
  }

  addToPreviewQueue (file) {
    this.queue.push(file)
    if (this.queueProcessing === false) {
      this.processPreviewQueue()
    }
  }

  generatePreview (file) {
    return new Promise((resolve, reject) => {
      if (Utils.isPreviewSupported(file.type) && !file.isRemote) {
        Utils.createThumbnail(file, 200).then((thumbnail) => {
          this.setPreviewURL(file.id, thumbnail)
          resolve()
        }).catch(function (err) {
          console.warn(err.stack || err.message)
          reject(err)
        })
      } else {
        reject(new Error('no preview'))
      }
    })
  }

  processPreviewQueue () {
    if (this.queue.length > 0) {
      this.queueProcessing = true
      const currentFile = this.queue.shift()
      this.generatePreview(currentFile).catch(() => {
      }).then(() => {
        if (this.queue.length > 0) {
          this.processPreviewQueue()
        } else {
          this.queueProcessing = false
        }
      })
    }
  }