(types): properly handle null case for init / refs

- modify _setRef to actually set internal refs to null if unmounted
- modify getCanvas() and getSignaturePad() to throw errors if the ref
  is currently null
  - use both methods internally instead of directly accessing their
    respective variables
    - make some alias variables for brevity where possible
  - add a test for the error case as well

Author: agilgur5

src/index.tsx

@@ -29,15 +29,20 @@ export default class ReactSignatureCanvas extends Component<ReactSignatureCanvas
     clearOnResize: true
   }
 
-  // this is some hack-ish init and casting to avoid `| null` everywhere :/
-  /* eslint-disable @typescript-eslint/consistent-type-assertions */
-  _sigPad: SignaturePad = {} as SignaturePad
-  _canvas: HTMLCanvasElement = {} as HTMLCanvasElement
-  /* eslint-enable @typescript-eslint/consistent-type-assertions */
+  static refNullError = new Error('react-signature-canvas is currently ' +
+    'mounting or unmounting: React refs are null during this phase.')
+
+  // shortcut reference (https://stackoverflow.com/a/29244254/3431180)
+  private readonly staticThis = this.constructor as typeof ReactSignatureCanvas
+
+  _sigPad: SignaturePad | null = null
+  _canvas: HTMLCanvasElement | null = null
 
   _setRef = (ref: HTMLCanvasElement | null): void => {
-    if (ref !== null) {
-      this._canvas = ref
+    this._canvas = ref
+    // if component is unmounted, set internal references to null
+    if (this._canvas === null) {
+      this._sigPad = null
     }
   }
 
@@ -47,7 +52,8 @@ export default class ReactSignatureCanvas extends Component<ReactSignatureCanvas
   }
 
   componentDidMount: Component['componentDidMount'] = () => {
-    this._sigPad = new SignaturePad(this._canvas, this._excludeOurProps())
+    const canvas = this.getCanvas()
+    this._sigPad = new SignaturePad(canvas, this._excludeOurProps())
     this._resizeCanvas()
     this.on()
   }
@@ -63,23 +69,30 @@ export default class ReactSignatureCanvas extends Component<ReactSignatureCanvas
 
   // return the canvas ref for operations like toDataURL
   getCanvas = (): HTMLCanvasElement => {
+    if (this._canvas === null) {
+      throw this.staticThis.refNullError
+    }
     return this._canvas
   }
 
   // return a trimmed copy of the canvas
   getTrimmedCanvas = (): HTMLCanvasElement => {
     // copy the canvas
+    const canvas = this.getCanvas()
     const copy = document.createElement('canvas')
-    copy.width = this._canvas.width
-    copy.height = this._canvas.height
+    copy.width = canvas.width
+    copy.height = canvas.height
     // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
-    copy.getContext('2d')!.drawImage(this._canvas, 0, 0)
+    copy.getContext('2d')!.drawImage(canvas, 0, 0)
     // then trim it
     return trimCanvas(copy)
   }
 
   // return the internal SignaturePad reference
   getSignaturePad = (): SignaturePad => {
+    if (this._sigPad === null) {
+      throw this.staticThis.refNullError
+    }
     return this._sigPad
   }
 
@@ -98,7 +111,7 @@ export default class ReactSignatureCanvas extends Component<ReactSignatureCanvas
       return
     }
 
-    const canvas = this._canvas
+    const canvas = this.getCanvas()
     /* When zoomed out to less than 100%, for some very strange reason,
       some browsers report devicePixelRatio as less than 1
       and only part of the canvas is cleared then. */
@@ -124,35 +137,35 @@ export default class ReactSignatureCanvas extends Component<ReactSignatureCanvas
   //
   on: SignaturePad['on'] = () => {
     window.addEventListener('resize', this._checkClearOnResize)
-    return this._sigPad.on()
+    return this.getSignaturePad().on()
   }
 
   off: SignaturePad['off'] = () => {
     window.removeEventListener('resize', this._checkClearOnResize)
-    return this._sigPad.off()
+    return this.getSignaturePad().off()
   }
 
   clear: SignaturePad['clear'] = () => {
-    return this._sigPad.clear()
+    return this.getSignaturePad().clear()
   }
 
   isEmpty: SignaturePad['isEmpty'] = () => {
-    return this._sigPad.isEmpty()
+    return this.getSignaturePad().isEmpty()
   }
 
   fromDataURL: SignaturePad['fromDataURL'] = (dataURL, options) => {
-    return this._sigPad.fromDataURL(dataURL, options)
+    return this.getSignaturePad().fromDataURL(dataURL, options)
   }
 
   toDataURL: SignaturePad['toDataURL'] = (type, encoderOptions) => {
-    return this._sigPad.toDataURL(type, encoderOptions)
+    return this.getSignaturePad().toDataURL(type, encoderOptions)
   }
 
   fromData: SignaturePad['fromData'] = (pointGroups) => {
-    return this._sigPad.fromData(pointGroups)
+    return this.getSignaturePad().fromData(pointGroups)
   }
 
   toData: SignaturePad['toData'] = () => {
-    return this._sigPad.toData()
+    return this.getSignaturePad().toData()
   }
 }

test/index.spec.tsx

@@ -226,3 +226,19 @@ describe('on & off methods', () => {
     expect(instance.on).not.toBeCalled()
   })
 })
+
+// unmounting comes last
+describe('unmounting', () => {
+  const wrapper = mount(<SignatureCanvas />)
+  const instance = rSCInstance(wrapper)
+
+  it('should error when retrieving instance variables', () => {
+    wrapper.unmount()
+    expect(() => {
+      instance.getCanvas()
+    }).toThrowError(SignatureCanvas.refNullError)
+    expect(() => {
+      instance.getSignaturePad()
+    }).toThrowError(SignatureCanvas.refNullError)
+  })
+})