Fix field_image click handler (#1320)

core/field_image.js

@@ -38,7 +38,8 @@ goog.require('goog.userAgent');
  * @param {number} width Width of the image.
  * @param {number} height Height of the image.
  * @param {string=} opt_alt Optional alt text for when block is collapsed.
- * @param {Function=} opt_onClick Optional function to be called when image is clicked
+ * @param {Function=} opt_onClick Optional function to be called when the image
+ *     is clicked.  If opt_onClick is defined, opt_alt must also be defined.
  * @extends {Blockly.Field}
  * @constructor
  */
@@ -92,6 +93,8 @@ Blockly.FieldImage.prototype.init = function() {
   // Configure the field to be transparent with respect to tooltips.
   this.setTooltip(this.sourceBlock_);
   Blockly.Tooltip.bindMouseEvents(this.imageElement_);
+
+  this.maybeAddClickHandler_();
 };
 
 /**
@@ -103,6 +106,19 @@ Blockly.FieldImage.prototype.dispose = function() {
   this.imageElement_ = null;
 };
 
+/**
+ * Bind events for a mouse down on the image, but only if a click handler has
+ * been defined.
+ * @private
+ */
+Blockly.FieldImage.prototype.maybeAddClickHandler_ = function() {
+  if (this.clickHandler_) {
+    this.mouseDownWrapper_ =
+        Blockly.bindEventWithChecks_(this.fieldGroup_, 'mousedown', this,
+        this.onMouseDown_);
+  }
+};
+
 /**
  * Change the tooltip text for this field.
  * @param {string|!Element} newTip Text for tooltip or a parent element to
@@ -171,8 +187,8 @@ Blockly.FieldImage.prototype.updateWidth = function() {
  * If field click is called, and click handler defined,
  * call the handler.
  */
- Blockly.FieldImage.prototype.showEditor = function() {
-   if (this.clickHandler_){
-     this.clickHandler_(this);
-   }
- };
+Blockly.FieldImage.prototype.showEditor_ = function() {
+  if (this.clickHandler_){
+    this.clickHandler_(this);
+  }
+};