From 6fb672c452bd45ccd390778d1372dfd10bc906bc Mon Sep 17 00:00:00 2001
From: Oliver Hoff <oliver@hofff.com>
Date: Fri, 22 Mar 2019 14:28:06 +0100
Subject: [PATCH] transparent-v-model: first draft

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+- Start Date: 2019-03-22
+- Target Major Version: 2.x
+- Reference Issues: https://github.com/vuejs/vue/issues/9749 https://github.com/vuejs/vue/issues/8403
+- Implementation PR:
+
+# Summary
+
+Transparently use the `v-model` directive on data **and props**.
+
+# Basic example
+
+## Vue 2.6
+
+```js
+const child = {
+  template: `<input type="text" :value="value" @input="$emit('input', event.target.value)">`,
+  props: {
+    value: String,
+  },
+};
+const parent = {
+  components: { child },
+  template: `<child v-model="value">`,
+  data() {
+    return { value: '' };
+  },
+};
+```
+
+## Proposed behavior
+
+```js
+const child = {
+  template: `<input type="text" v-model="value">`,
+  props: {
+    value: String,
+  },
+};
+const parent = {
+  components: { child },
+  template: `<child v-model="value">`,
+  data() {
+    return { value: '' };
+  },
+};
+```
+
+
+# Motivation
+
+Wrapping model exposing components is a rather cumbersome and error prone task,
+which involves writing boilerplate code. This gets even worse when dealing with
+keyed models or dynamic components (*What do I actually need to forward*).
+
+The `v-model` improvements proposed here can also be applied to the `.sync`
+modifier, depending on https://github.com/vuejs/rfcs/pull/8.
+
+# Detailed design
+
+While the initial implementation path seems simple, there are a lot of corner
+cases to respect.
+
+The following example should contain all corner cases that need to be respected:
+```js
+const editAttribute = {
+  template: `
+    <component
+      :is="tag"
+      v-bind="tag === 'input' ? { type: "text" } : {}"
+      v-model="model.attributes[attribute]"
+    >
+  `,
+  model: {
+    prop: 'model',
+    event: 'attributeUpdated',
+  },
+  props: {
+    model: Object,
+    attribute: String,
+  },
+  data() {
+    return { tag: 'input' };
+  }
+};
+const parent = {
+  components: { editAttribute },
+  template: `<edit-attribute v-model="list[index].data" attribute="name">`,
+  props: {
+    index: {
+      type: Number,
+      default: 0,
+    },
+  },
+  data() {
+    return {
+      list: [
+        {
+          data: {
+            attributes: {
+              name: 'Foo',
+            },
+          },
+        },
+      ],
+    };
+  },
+};
+```
+
+This should be compiled into something equivalent like this:
+```js
+
+// helper which extracts the value based on the target components type,
+// basically `$event.target.value` for native elements and `arguments[0]` otherwise
+function EXTRACT_VALUE(/* whatever arguments needed, to extract the value */) {
+  return '';
+}
+
+const editAttribute = {
+  template: `
+    <component
+      :is="tag"
+      v-bind="tag === 'input' ? { type: "text" } : {}"
+      :value="model.attributes[attribute]"
+      @input="forward"
+    >
+  `,
+  model: {
+    prop: 'model',
+    event: 'attributeUpdated',
+  },
+  props: {
+    model: Object,
+    attribute: String,
+  },
+  data() {
+    return { tag: 'input' };
+  },
+  methods: {
+    forward(event) {
+      const value = EXTRACT_VALUE(/* whatever arguments needed, to extract the value */);
+      const $set = INNER_FORWARD ? arguments[1] : this.$set;
+
+      // a new 2nd argument to forwarded v-model events: "the setter thunk"
+      // it is only needed, if keyed models are used
+      this.$emit('attributeUpdated', value, (target, key, value) => {
+        if (KEYED_MODEL) { // decided at compile time, for this example, would go into the first branch
+          if (key != null) {
+            $set(target[key].attributes, this.attribute, value);
+          } else {
+            $set(target.attributes, this.attribute, value);
+          }
+        } else {
+          if (key != null) {
+            $set(target, key, value);
+          } else {
+            target = value; // THIS DOES NOT WORK, parent target is unkeyed
+          }
+        }
+      });
+    },
+  },
+};
+const parent = {
+  components: { editAttribute },
+  template: `
+    <edit-attribute
+      :model="list[index].data"
+      @attributeUpdated="attributeUpdated"
+      attribute="name"
+    >
+  `,
+  props: {
+    index: {
+      type: Number,
+      default: 0,
+    },
+  },
+  data() {
+    return {
+      list: [
+        {
+          data: {
+            attributes: {
+              name: 'Foo',
+            },
+          },
+        },
+      ],
+    };
+  },
+  methods: {
+    attributeUpdated(value, setter) {
+      const $set = setter || this.$set; // BC, in case the child is not compiled with this feature
+      $set(this.list[this.index], 'data', value);
+    },
+  },
+};
+```
+
+# Drawbacks
+
+- Increased complexity in `v-model`'s implementation
+
+# Alternatives
+
+In https://github.com/vuejs/vue/issues/8403 a separate modifier for the
+`v-model` directive was coined to enable the proposed behavior, but it is
+actually not needed.
+
+# Adoption strategy
+
+The adoption can be done seemless as none of the *supported* existing behavior
+is changed. Existing applications can mix and match between the new transparent
+`v-model` behavior and manual forwarding.
+
+# Unresolved questions
+
+- How to do EXTRACT_VALUE?
+- How to do INNER_FORWARD detection?
+- BC when only parent is compiled with this feature, but child is not?
+- BC when only child is compiled with this feature, but parent is not?