Make DND feel much smoother by persisting the end position of drops, and more performance by properly memoizing some state values

packages/client/src/components/preview/DNDHandler.svelte

@@ -57,8 +57,10 @@
     }
 
     // Reset state
+    if (!$dndStore.dropped) {
       dndStore.actions.reset()
     }
+  }
 
   // Callback when initially starting a drag on a draggable component
   const onDragStart = e => {
@@ -251,7 +253,7 @@
   }
 
   // Callback when dropping a drag on top of some component
-  const onDrop = e => {
+  const onDrop = () => {
     if (!source || !drop?.parent || drop?.index == null) {
       return
     }
@@ -299,6 +301,7 @@
     }
 
     if (legacyDropTarget && legacyDropMode) {
+      dndStore.actions.markDropped()
       builderStore.actions.moveComponent(
         source.id,
         legacyDropTarget,

packages/client/src/index.js

@@ -24,10 +24,6 @@ loadSpectrumIcons()
 let app
 
 const loadBudibase = async () => {
-  if (get(builderStore).clearGridNextLoad) {
-    builderStore.actions.setDragging(false)
-  }
-
   // Update builder store with any builder flags
   builderStore.set({
     ...get(builderStore),
@@ -45,6 +41,11 @@ const loadBudibase = async () => {
     location: window["##BUDIBASE_LOCATION##"],
   })
 
+  // Reset DND state if we completed a successful drop
+  if (get(dndStore).dropped) {
+    dndStore.actions.reset()
+  }
+
   // Set app ID - this window flag is set by both the preview and the real
   // server rendered app HTML
   appStore.actions.setAppId(window["##BUDIBASE_APP_ID##"])

packages/client/src/stores/dnd.js

@@ -1,4 +1,5 @@
 import { writable, derived } from "svelte/store"
+import { computed } from "../utils/computed.js"
 
 const createDndStore = () => {
   const initialState = {
@@ -10,6 +11,9 @@ const createDndStore = () => {
 
     // Info about where the component would be dropped
     drop: null,
+
+    // Whether the current drop has been completed successfully
+    dropped: false,
   }
   const store = writable(initialState)
 
@@ -59,6 +63,13 @@ const createDndStore = () => {
     store.set(initialState)
   }
 
+  const markDropped = () => {
+    store.update(state => {
+      state.dropped = true
+      return state
+    })
+  }
+
   return {
     subscribe: store.subscribe,
     actions: {
@@ -67,6 +78,7 @@ const createDndStore = () => {
       updateTarget,
       updateDrop,
       reset,
+      markDropped,
     },
   }
 }
@@ -77,14 +89,12 @@ export const dndStore = createDndStore()
 // performance by deriving any state that needs to be externally observed.
 // By doing this and using primitives, we can avoid invalidating other stores
 // or components which depend on DND state unless values actually change.
-export const dndParent = derived(dndStore, $dndStore => $dndStore.drop?.parent)
-export const dndIndex = derived(dndStore, $dndStore => $dndStore.drop?.index)
-export const dndBounds = derived(
+export const dndParent = computed(dndStore, x => x.drop?.parent)
+export const dndIndex = computed(dndStore, x => x.drop?.index)
+export const dndDropped = computed(dndStore, x => x.dropped)
+export const dndBounds = computed(dndStore, x => x.source?.bounds)
+export const dndIsDragging = computed(dndStore, x => !!x.source)
+export const dndIsNewComponent = computed(
   dndStore,
-  $dndStore => $dndStore.source?.bounds
+  x => x.source?.newComponentType != null
 )
-export const dndIsNewComponent = derived(
-  dndStore,
-  $dndStore => $dndStore.source?.newComponentType != null
-)
-export const dndIsDragging = derived(dndStore, $dndStore => !!$dndStore.source)

packages/client/src/stores/index.js

@@ -22,6 +22,7 @@ export {
   dndBounds,
   dndIsNewComponent,
   dndIsDragging,
+  dndDropped,
 } from "./dnd"
 
 // Context stores are layered and duplicated, so it is not a singleton

packages/client/src/stores/screens.js

@@ -2,7 +2,13 @@ import { derived } from "svelte/store"
 import { routeStore } from "./routes"
 import { builderStore } from "./builder"
 import { appStore } from "./app"
-import { dndIndex, dndParent, dndIsNewComponent, dndBounds } from "./dnd.js"
+import {
+  dndIndex,
+  dndParent,
+  dndIsNewComponent,
+  dndBounds,
+  dndDropped,
+} from "./dnd.js"
 import { RoleUtils } from "@budibase/frontend-core"
 import { findComponentById, findComponentParent } from "../utils/components.js"
 import { Helpers } from "@budibase/bbui"
@@ -18,6 +24,7 @@ const createScreenStore = () => {
       dndIndex,
       dndIsNewComponent,
       dndBounds,
+      dndDropped,
     ],
     ([
       $appStore,
@@ -27,6 +34,7 @@ const createScreenStore = () => {
       $dndIndex,
       $dndIsNewComponent,
       $dndBounds,
+      $dndDropped,
     ]) => {
       let activeLayout, activeScreen
       let screens
@@ -64,39 +72,54 @@ const createScreenStore = () => {
 
       // Insert DND placeholder if required
       if (activeScreen && $dndParent && $dndIndex != null) {
-        // Remove selected component from tree if we are moving an existing
-        // component
         const { selectedComponentId } = $builderStore
-        if (!$dndIsNewComponent) {
+
+        // Extract and save the selected component as we need a reference to it
+        // later, and we may be removing it
         let selectedParent = findComponentParent(
           activeScreen.props,
           selectedComponentId
         )
-          if (selectedParent) {
+        const selectedComponent = selectedParent?._children?.find(
+          x => x._id === selectedComponentId
+        )
+
+        // Remove selected component from tree if we are moving an existing
+        // component
+        if (!$dndIsNewComponent && selectedParent) {
           selectedParent._children = selectedParent._children?.filter(
             x => x._id !== selectedComponentId
           )
         }
-        }
 
-        // Insert placeholder component
-        const placeholder = {
+        // Insert placeholder component if dragging, or artificially insert
+        // the dropped component in the new location if the drop completed
+        let componentToInsert
+        if ($dndDropped && !$dndIsNewComponent) {
+          componentToInsert = selectedComponent
+        } else {
+          componentToInsert = {
             _component: "@budibase/standard-components/container",
             _id: DNDPlaceholderID,
             _styles: {
               normal: {
-              width: `${$dndBounds?.width || 666}px`,
-              height: `${$dndBounds?.height || 666}px`,
+                width: `${$dndBounds?.width || 400}px`,
+                height: `${$dndBounds?.height || 200}px`,
                 opacity: 0,
               },
             },
             static: true,
           }
+        }
+        if (componentToInsert) {
           let parent = findComponentById(activeScreen.props, $dndParent)
+          if (parent) {
             if (!parent._children?.length) {
-          parent._children = [placeholder]
+              parent._children = [componentToInsert]
             } else {
-          parent._children.splice($dndIndex, 0, placeholder)
+              parent._children.splice($dndIndex, 0, componentToInsert)
+            }
+          }
         }
       }
 

packages/client/src/utils/computed.js

@@ -0,0 +1,26 @@
+import { writable } from "svelte/store"
+
+const getKey = value => {
+  if (value == null || typeof value !== "object") {
+    return value
+  } else {
+    return JSON.stringify(value)
+  }
+}
+
+export const computed = (store, getValue) => {
+  const initialValue = getValue(store)
+  const computedStore = writable(initialValue)
+  let lastKey = getKey(initialValue)
+
+  store.subscribe(state => {
+    const value = getValue(state)
+    const key = getKey(value)
+    if (key !== lastKey) {
+      lastKey = key
+      computedStore.set(value)
+    }
+  })
+
+  return computedStore
+}