Add implementation for ifElse

src/blocks/scratch3_control.js

@@ -16,6 +16,7 @@ Scratch3ControlBlocks.prototype.getPrimitives = function() {
         'control_forever': this.forever,
         'control_wait': this.wait,
         'control_if': this.if,
+        'control_if_else': this.ifElse,
         'control_stop': this.stop
     };
 };
@@ -55,6 +56,19 @@ Scratch3ControlBlocks.prototype.if = function(args, util) {
     }
 };
 
+Scratch3ControlBlocks.prototype.ifElse = function(args, util) {
+    // Only execute one time. `ifElse` will be returned to
+    // when the substack finishes, but it shouldn't execute again.
+    if (util.stackFrame.executed === undefined) {
+        util.stackFrame.executed = true;
+        if (args.CONDITION) {
+            util.startSubstack(1);
+        } else {
+            util.startSubstack(2);
+        }
+    }
+};
+
 Scratch3ControlBlocks.prototype.stop = function() {
     // @todo - don't use this.runtime
     this.runtime.stopAll();

src/engine/execute.js

@@ -69,8 +69,8 @@ var execute = function (sequencer, thread) {
             },
             timeout: YieldTimers.timeout,
             stackFrame: currentStackFrame,
-            startSubstack: function () {
-                sequencer.stepToSubstack(thread);
+            startSubstack: function (substackNum) {
+                sequencer.stepToSubstack(thread, substackNum);
             }
         });
     }

src/engine/sequencer.js

@@ -104,11 +104,17 @@ Sequencer.prototype.startThread = function (thread) {
 /**
  * Step a thread into a block's substack.
  * @param {!Thread} thread Thread object to step to substack.
- * @param {string} currentBlockId Block which owns a substack to step to.
+ * @param {Number} substackNum Which substack to step to (i.e., 1, 2).
  */
-Sequencer.prototype.stepToSubstack = function (thread) {
+Sequencer.prototype.stepToSubstack = function (thread, substackNum) {
+    if (!substackNum) {
+        substackNum = 1;
+    }
     var currentBlockId = thread.peekStack();
-    var substackId = this.runtime.blocks.getSubstack(currentBlockId);
+    var substackId = this.runtime.blocks.getSubstack(
+        currentBlockId,
+        substackNum
+    );
     if (substackId) {
         // Push substack ID to the thread's stack.
         thread.pushStack(substackId);
@@ -121,7 +127,6 @@ Sequencer.prototype.stepToSubstack = function (thread) {
 /**
  * Finish stepping a thread and proceed it to the next block.
  * @param {!Thread} thread Thread object to proceed.
- * @param {string} currentBlockId Block we are finished with.
  */
 Sequencer.prototype.proceedThread = function (thread) {
     var currentBlockId = thread.peekStack();