allow to draw signature on mobile

app/javascript/draw.js

@@ -0,0 +1,113 @@
+import SignaturePad from 'signature_pad'
+import { cropCanvasAndExportToPNG } from './submission_form/crop_canvas'
+
+window.customElements.define('signature-form', class extends HTMLElement {
+  connectedCallback () {
+    const scale = 3
+
+    this.canvas.width = this.canvas.parentNode.clientWidth * scale
+    this.canvas.height = this.canvas.parentNode.clientHeight * scale
+
+    this.canvas.getContext('2d').scale(scale, scale)
+
+    this.pad = new SignaturePad(this.canvas)
+
+    this.pad.addEventListener('endStroke', () => {
+      this.updateSubmitButtonVisibility()
+    })
+
+    this.clearButton.addEventListener('click', (e) => {
+      e.preventDefault()
+
+      this.clearSignaturePad()
+    })
+
+    this.form.addEventListener('submit', (e) => {
+      e.preventDefault()
+
+      this.submitButton.disabled = true
+
+      this.submitImage().then((data) => {
+        this.valueInput.value = data.uuid
+
+        return fetch(this.form.action, {
+          method: 'PUT',
+          body: new FormData(this.form)
+        }).then((response) => {
+          this.form.classList.add('hidden')
+          this.success.classList.remove('hidden')
+
+          return response
+        })
+      }).finally(() => {
+        this.submitButton.disabled = false
+      })
+    })
+  }
+
+  clearSignaturePad () {
+    this.pad.clear()
+    this.updateSubmitButtonVisibility()
+  }
+
+  updateSubmitButtonVisibility () {
+    if (this.pad.isEmpty()) {
+      this.submitButton.style.display = 'none'
+      this.placeholderButton.style.display = 'block'
+    } else {
+      this.submitButton.style.display = 'block'
+      this.placeholderButton.style.display = 'none'
+    }
+  }
+
+  async submitImage () {
+    return new Promise((resolve, reject) => {
+      cropCanvasAndExportToPNG(this.canvas, { errorOnTooSmall: true }).then(async (blob) => {
+        const file = new File([blob], 'signature.png', { type: 'image/png' })
+
+        const formData = new FormData()
+
+        formData.append('file', file)
+        formData.append('submitter_slug', this.dataset.slug)
+        formData.append('name', 'attachments')
+
+        return fetch('/api/attachments', {
+          method: 'POST',
+          body: formData
+        }).then((resp) => resp.json()).then((attachment) => {
+          return resolve(attachment)
+        })
+      }).catch((error) => {
+        return reject(error)
+      })
+    })
+  }
+
+  get submitButton () {
+    return this.querySelector('button[type="submit"]')
+  }
+
+  get clearButton () {
+    return this.querySelector('button[aria-label="Clear"]')
+  }
+
+  get placeholderButton () {
+    return this.querySelector('button[disabled]')
+  }
+
+  get canvas () {
+    return this.querySelector('canvas')
+  }
+
+  get valueInput () {
+    return this.querySelector('input[name^="values"]')
+  }
+
+  get form () {
+    return this.querySelector('form')
+  }
+
+  get success () {
+    return this.querySelector('#success')
+  }
+})