feat(form-file): reset file input when value set to null or empty str…

…ing (#2170)

* feat(form-file): reset file input when value  set to null or empty string

As an alternative to calling the `reset()` method, users can now clear the form file input by setting the v-model to null or an empty string.

The watcher checks for null and empty string and internally calls reset()

* Update form-file.js

* Update README.md

src/components/form-file/README.md

@@ -145,24 +145,27 @@ assitive technologies.
 
 
 ## Clearing the file selection
-Because of limitations in the value binding with `<input type="file">` elements, `v-model`
-for `<b-form-file>` is unidirectional, and cannot be used to set or clear the file(s) selection.
-To get around this limitation, `<b-form-file>` provides a `reset()` method that can be
-called to clear the file input.
+With inputs of type file, normally the `v-model` is uni-directional (meaning
+you cannot pre-set the selected files). However, you can clear the file input's 
+selected files by setting the `v-model` to either `null`, an empty string, or an
+empty array).
 
-To take advantage of the `reset()` method, you will need to obtain a reference
-to the `<b-form-file>` component:
+Alternatively,  `<b-form-file>` provides a `reset()` method that can be called to
+clear the file input. To take advantage of the `reset()` method, you will need
+to obtain a reference to the `<b-form-file>` component.
 
 ```html
-<div id="#app">
-    <b-form-file v-model="file" ref="fileinput"></b-form-file>
-    <b-button @click="clearFiles">Reset</b-button>
-</div>
-```
+<template>
+  <div>
+    <b-form-file v-model="file" ref="fileinput" class="mb-2"></b-form-file>
+    <b-button @click="clearFiles" class="mr-2">Reset via method</b-button>
+    <b-button @click="file = null">Reset via v-model</b-button>
+    <p class="mt-2">Selected file: <b>{{ file ? file.name : ''}}</b><p>
+  </div>
+</template>
 
-```js
-window.app = new Vue({
-  el: '#app',
+<script>
+export default {
   data () {
     return {
       file: null
@@ -173,8 +176,16 @@ window.app = new Vue({
       this.$refs.fileinput.reset();
     }
   }
-})
+}
+</script>
+
+<!-- form-file-reset.vue -->
 ```
 
+**Implementation note:** As not all browsers allow setting a value of a file
+input (even to null or an empty string), `b-form-input` employs a technique that
+works cross-browser that involves changing the input type to null and then back
+to type file.
+
 
 <!-- Component reference added automatically from component package.json -->

src/components/form-file/form-file.js

@@ -2,7 +2,7 @@ import idMixin from '../../mixins/id'
 import formMixin from '../../mixins/form'
 import formStateMixin from '../../mixins/form-state'
 import formCustomMixin from '../../mixins/form-custom'
-import { from as arrayFrom } from '../../utils/array'
+import { from as arrayFrom, isArray } from '../../utils/array'
 
 // temporary css until Bootstrap V4.2 is released
 import './form-file.css'
@@ -36,7 +36,8 @@ export default {
       on: {
         change: this.onFileChange,
         focusin: this.focusHandler,
-        focusout: this.focusHandler
+        focusout: this.focusHandler,
+        reset: this.onReset
       }
     })
 
@@ -80,6 +81,9 @@ export default {
     }
   },
   props: {
+    value: {
+      default: null
+    },
     accept: {
       type: String,
       default: ''
@@ -152,6 +156,11 @@ export default {
       } else {
         this.$emit('input', newVal)
       }
+    },
+    value (newVal) {
+      if (!newVal || (isArray(newVal) && newVal.length === 0)) {
+        this.reset()
+      }
     }
   },
   methods: {
@@ -216,6 +225,10 @@ export default {
         this.selectedFile = files[0]
       }
     },
+    onReset () {
+      // Triggered when the parent form (if any) is reset
+      this.selectedFile = this.multiple ? [] : null
+    },
     onDragover (evt) {
       evt.preventDefault()
       evt.stopPropagation()