Replace the component bounded cachedValue by a global UnsavedEdits

This new draft saving system is currently only implemented for the
card description and comment. We need better a component
inheritance/composition model to support this for all editable fields.

Fixes #186

client/lib/unsavedEdits.js

@@ -0,0 +1,82 @@
+Meteor.subscribe('unsaved-edits');
+
+// `UnsavedEdits` is a global key-value store used to save drafts of user
+// inputs. We used to have the notion of a `cachedValue` that was local to a
+// component but the global store has multiple advantages:
+// 1. When the component is unmounted (ie, destroyed) the draft isn't lost
+// 2. The drafts are synced across multiple computers
+// 3. The drafts are synced across multiple browser tabs
+//    XXX This currently doesn't work in purely offline mode since the sync is
+//    handled with the DDP connection to the server. To solve this, we could use
+//    something like GroundDB that syncs using localstorage.
+//
+// The key is a dictionary composed of two fields:
+// * a `fieldName` which identifies the particular field. Since this is a global
+//   identifier a good practice would be to compose it with the collection name
+//   and the document field, eg. `boardTitle`, `cardDescription`.
+// * a `docId` which identifies the appropriate document. In general we use
+//   MongoDB `_id` field.
+//
+// The value is a string containing the draft.
+
+UnsavedEdits = {
+  // XXX Wanted to have the collection has an instance variable, but
+  // unfortunately the collection isn't defined yet at this point. We need ES6
+  // modules to solve the file order issue!
+  //
+  // _collection: UnsavedEditCollection,
+
+  get({ fieldName, docId }, defaultTo = '') {
+    let unsavedValue = this._getCollectionDocument(fieldName, docId);
+    if (unsavedValue) {
+      return unsavedValue.value
+    } else {
+      return defaultTo;
+    }
+  },
+
+  has({ fieldName, docId }) {
+    return Boolean(this.get({fieldName, docId}));
+  },
+
+  set({ fieldName, docId }, value) {
+    let currentDoc = this._getCollectionDocument(fieldName, docId);
+    if (currentDoc) {
+      UnsavedEditCollection.update(currentDoc._id, {
+        $set: {
+          value: value
+        }
+      });
+    } else {
+      UnsavedEditCollection.insert({
+        fieldName,
+        docId,
+        value,
+      });
+    }
+  },
+
+  reset({ fieldName, docId }) {
+    let currentDoc = this._getCollectionDocument(fieldName, docId);
+    if (currentDoc) {
+      UnsavedEditCollection.remove(currentDoc._id);
+    }
+  },
+
+  _getCollectionDocument(fieldName, docId) {
+    return UnsavedEditCollection.findOne({fieldName, docId});
+  }
+}
+
+Blaze.registerHelper('getUnsavedValue', (fieldName, docId, defaultTo) => {
+  // Workaround some blaze feature that ass a list of keywords arguments as the
+  // last parameter (even if the caller didn't specify any).
+  if (! _.isString(defaultTo)) {
+    defaultTo = '';
+  }
+  return UnsavedEdits.get({ fieldName, docId }, defaultTo);
+});
+
+Blaze.registerHelper('hasUnsavedValue', (fieldName, docId) => {
+  return UnsavedEdits.has({ fieldName, docId });
+});