VM changes for the sensing_of block. This handles lists properly (by ignoring them like Scratch2 and makes the attribute menu update based on what was chosen in the target menu.

src/blocks/scratch3_sensing.js

@@ -313,12 +313,11 @@ class Scratch3SensingBlocks {
             }
         }
 
-        // Variables
+        // Local variables (not lists).
         const varName = args.PROPERTY;
-        for (const id in attrTarget.variables) {
-            if (attrTarget.variables[id].name === varName) {
-                return attrTarget.variables[id].value;
-            }
+        const variable = attrTarget.lookupVariableByNameAndType(varName, '', true);
+        if (variable) {
+            return variable.value;
         }
 
         // Otherwise, 0

src/engine/blocks.js

@@ -567,6 +567,16 @@ class Blocks {
                 if (!optRuntime){
                     break;
                 }
+                // The selected item in the sensing of block menu needs to change based on the
+                // selected target.  Set it to the first item in the menu list.
+                if (block.opcode === 'sensing_of_object_menu') {
+                    if (block.fields.OBJECT.value === '_stage_') {
+                        this._blocks[block.parent].fields.PROPERTY.value = 'backdrop #';
+                    } else {
+                        this._blocks[block.parent].fields.PROPERTY.value = 'x position';
+                    }
+                    optRuntime.requestBlocksUpdate();
+                }
 
                 const flyoutBlock = block.shadow && block.parent ? this._blocks[block.parent] : block;
                 if (flyoutBlock.isMonitored) {

src/engine/runtime.js

@@ -613,6 +613,14 @@ class Runtime extends EventEmitter {
         return 'RUNTIME_STARTED';
     }
 
+    /**
+     * Event name for reporting that a block was updated and needs to be rerendered.
+     * @const {string}
+     */
+    static get BLOCKS_NEED_UPDATE () {
+        return 'BLOCKS_NEED_UPDATE';
+    }
+
     /**
      * How rapidly we try to step threads by default, in ms.
      */
@@ -2169,6 +2177,13 @@ class Runtime extends EventEmitter {
         this._refreshTargets = true;
     }
 
+    /**
+     * Emit an event that indicate that the blocks on the workspace need updating.
+     */
+    requestBlocksUpdate () {
+        this.emit(Runtime.BLOCK_NEED_UPDATE);
+    }
+
     /**
      * Set up timers to repeatedly step in a browser.
      */

src/virtual-machine.js

@@ -108,6 +108,9 @@ class VirtualMachine extends EventEmitter {
         this.runtime.on(Runtime.BLOCKSINFO_UPDATE, blocksInfo => {
             this.emit(Runtime.BLOCKSINFO_UPDATE, blocksInfo);
         });
+        this.runtime.on(Runtime.BLOCKS_NEED_UPDATE, () => {
+            this.emitWorkspaceUpdate();
+        });
         this.runtime.on(Runtime.PERIPHERAL_LIST_UPDATE, info => {
             this.emit(Runtime.PERIPHERAL_LIST_UPDATE, info);
         });

test/unit/blocks_sensing.js

@@ -229,6 +229,33 @@ test('loud? boolean', t => {
     t.end();
 });
 
+test('get attribute of sprite variable', t => {
+    const rt = new Runtime();
+    const sensing = new Sensing(rt);
+    const s = new Sprite();
+    const target = new RenderedTarget(s, rt);
+    const thing = {
+        name: 'cars',
+        value: 'trucks'
+    };
+    rt.getSpriteTargetByName = () => target;
+    target.lookupVariableByNameAndType = () => thing;
+    t.equal(sensing.getAttributeOf({PROPERTY: 'cars'}), 'trucks');
+
+    t.end();
+});
+test('get attribute of variable that does not exist', t => {
+    const rt = new Runtime();
+    const sensing = new Sensing(rt);
+    const s = new Sprite();
+    const stage = new RenderedTarget(s, rt);
+    rt.getTargetForStage = () => stage;
+    stage.lookupVariableByNameAndType = () => null;
+    t.equal(sensing.getAttributeOf({PROPERTY: 'stage'}), 0);
+
+    t.end();
+});
+
 test('username block', t => {
     const rt = new Runtime();
     const sensing = new Sensing(rt);