Add support extensions to define custom field types.

This is done by adding a new element "customFieldTypes" to the extension info structure. Ex:
```
customFieldTypes: {
   angleField: {
      implementation: { fromJson: options => new AngleField(options) },
      output: 'number',
      outputShape: 2,
   }
}
```
Field types are defined by an implementation that has to match what is expected by ScratchBlocks.Field.register and its output and shape.

src/engine/runtime.js has been updated to handle the new "customFieldTypes"-field:
- Existing (global) field types cannot be overridden
- New fields are "namespaced" to the extension in the same way as opcodes are.

Once the custom field type has been picked up by scratch-vm a "EXTENSION_FIELD_ADDED" event is emitted.
It is then up to the hosting app to call ScratchBlocks.Field.register to register the field type with ScratchBlocks. Ex:
```
vm.addListener('EXTENSION_FIELD_ADDED', fieldInfo => {
   this.ScratchBlocks.Field.register(fieldInfo.name, fieldInfo.implementation);
});
```

src/engine/runtime.js

@@ -556,6 +556,14 @@ class Runtime extends EventEmitter {
         return 'EXTENSION_ADDED';
     }
 
+    /**
+     * Event name for reporting that an extension as asked for a custom field to be added
+     * @const {string}
+     */
+    static get EXTENSION_FIELD_ADDED () {
+        return 'EXTENSION_FIELD_ADDED';
+    }
+
     /**
      * Event name for updating the available set of peripheral devices.
      * This causes the peripheral connection modal to update a list of
@@ -779,6 +787,7 @@ class Runtime extends EventEmitter {
             color1: extensionInfo.colour || '#0FBD8C',
             color2: extensionInfo.colourSecondary || '#0DA57A',
             color3: extensionInfo.colourTertiary || '#0B8E69',
+            customFieldTypes: {},
             blocks: [],
             menus: []
         };
@@ -787,7 +796,23 @@ class Runtime extends EventEmitter {
 
         this._fillExtensionCategory(categoryInfo, extensionInfo);
 
-        this.emit(Runtime.EXTENSION_ADDED, categoryInfo.blocks.concat(categoryInfo.menus));
+        const fieldTypeDefinitionsForScratch = [];
+        for (const fieldTypeName in categoryInfo.customFieldTypes) {
+            if (extensionInfo.customFieldTypes.hasOwnProperty(fieldTypeName)) {
+                const fieldTypeInfo = categoryInfo.customFieldTypes[fieldTypeName];
+                fieldTypeDefinitionsForScratch.push(fieldTypeInfo.scratchBlocksDefinition);
+
+                // Emit events for custom field types from extension
+                this.emit(Runtime.EXTENSION_FIELD_ADDED, {
+                    name: `field_${fieldTypeInfo.extendedName}`,
+                    implementation: fieldTypeInfo.fieldImplementation
+                });
+            }
+        }
+
+        const allBlocks = fieldTypeDefinitionsForScratch.concat(categoryInfo.blocks).concat(categoryInfo.menus);
+
+        this.emit(Runtime.EXTENSION_ADDED, allBlocks);
     }
 
     /**
@@ -811,7 +836,8 @@ class Runtime extends EventEmitter {
     }
 
     /**
-     * Read extension information, convert menus and blocks, and store the results in the provided category object.
+     *  Read extension information, convert menus, blocks and custom field types
+     * and store the results in the provided category object.
      * @param {CategoryInfo} categoryInfo - the category to be filled
      * @param {ExtensionMetadata} extensionInfo - the extension metadata to read
      * @private
@@ -824,6 +850,19 @@ class Runtime extends EventEmitter {
                 categoryInfo.menus.push(convertedMenu);
             }
         }
+        for (const fieldTypeName in extensionInfo.customFieldTypes) {
+            if (extensionInfo.customFieldTypes.hasOwnProperty(fieldTypeName)) {
+                const fieldType = extensionInfo.customFieldTypes[fieldTypeName];
+                const fieldTypeInfo = this._buildCustomFieldInfo(
+                    fieldTypeName,
+                    fieldType,
+                    extensionInfo.id,
+                    categoryInfo
+                );
+
+                categoryInfo.customFieldTypes[fieldTypeName] = fieldTypeInfo;
+            }
+        }
 
         for (const blockInfo of extensionInfo.blocks) {
             if (blockInfo === '---') {
@@ -897,6 +936,55 @@ class Runtime extends EventEmitter {
         };
     }
 
+    _buildCustomFieldInfo (fieldName, fieldInfo, extensionId, categoryInfo) {
+        const extendedName = `${extensionId}_${fieldName}`;
+        return {
+            fieldName: fieldName,
+            extendedName: extendedName,
+            argumentTypeInfo: {
+                shadowType: extendedName,
+                fieldType: `field_${extendedName}`
+            },
+            scratchBlocksDefinition: this._buildCustomFieldTypeForScratchBlocks(
+                extendedName,
+                fieldInfo.output,
+                fieldInfo.outputShape,
+                categoryInfo
+            ),
+            fieldImplementation: fieldInfo.implementation
+        };
+    }
+
+    /**
+     * Build the scratch-blocks JSON needed for a fieldType.
+     * Custom field types need to be namespaced to the extension so that extensions can't interfere with each other
+     * @param  {string} fieldName - The name of the field
+     * @param {string} output - The output of the field
+     * @param {number} outputShape - Shape of the field (from ScratchBlocksConstants)
+     * @param {object} categoryInfo - The category the field belongs to (Used to set its colors)
+     * @returns {object} - Object to be inserted into scratch-blocks
+     */
+    _buildCustomFieldTypeForScratchBlocks (fieldName, output, outputShape, categoryInfo) {
+        return {
+            json: {
+                type: fieldName,
+                message0: '%1',
+                inputsInline: true,
+                output: output,
+                colour: categoryInfo.color1,
+                colourSecondary: categoryInfo.color2,
+                colourTertiary: categoryInfo.color3,
+                outputShape: outputShape,
+                args0: [
+                    {
+                        name: `field_${fieldName}`,
+                        type: `field_${fieldName}`
+                    }
+                ]
+            }
+        };
+    }
+
     /**
      * Convert ExtensionBlockMetadata into scratch-blocks JSON & XML, and generate a proxy function.
      * @param {ExtensionBlockMetadata} blockInfo - the block to convert
@@ -1065,10 +1153,17 @@ class Runtime extends EventEmitter {
         };
 
         const argInfo = context.blockInfo.arguments[placeholder] || {};
-        const argTypeInfo = ArgumentTypeMap[argInfo.type] || {};
-        const defaultValue = (typeof argInfo.defaultValue === 'undefined' ?
-            '' :
-            escapeHtml(maybeFormatMessage(argInfo.defaultValue, this.makeMessageContextForTarget()).toString()));
+        let argTypeInfo = ArgumentTypeMap[argInfo.type] || {};
+
+        // Field type not a standard field type, see if extension has registered custom field type
+        if (!ArgumentTypeMap[argInfo.type] && context.categoryInfo.customFieldTypes[argInfo.type]) {
+            argTypeInfo = context.categoryInfo.customFieldTypes[argInfo.type].argumentTypeInfo;
+        }
+
+        const defaultValue =
+            typeof argInfo.defaultValue === 'undefined'
+                ? ''
+                : escapeHtml(maybeFormatMessage(argInfo.defaultValue, this.makeMessageContextForTarget()).toString());
 
         if (argTypeInfo.check) {
             argJSON.check = argTypeInfo.check;
@@ -1103,6 +1198,7 @@ class Runtime extends EventEmitter {
         blockArgs.push(argJSON);
         const argNum = blockArgs.length;
         context.argsMap[placeholder] = argNum;
+
         return `%${argNum}`;
     }
 

src/virtual-machine.js

@@ -109,6 +109,9 @@ class VirtualMachine extends EventEmitter {
         this.runtime.on(Runtime.EXTENSION_ADDED, blocksInfo => {
             this.emit(Runtime.EXTENSION_ADDED, blocksInfo);
         });
+        this.runtime.on(Runtime.EXTENSION_FIELD_ADDED, (fieldName, fieldImplementation) => {
+            this.emit(Runtime.EXTENSION_FIELD_ADDED, fieldName, fieldImplementation);
+        });
         this.runtime.on(Runtime.BLOCKSINFO_UPDATE, blocksInfo => {
             this.emit(Runtime.BLOCKSINFO_UPDATE, blocksInfo);
         });