Various clean-ups regarding the handling of activeLayer, JSON deserialization, DOM insertion and and layer reusing.

JSON deserialization does no longer insert items into DOM twice now, and should therefore be faster.

src/project/Project.js

@@ -37,10 +37,11 @@ var Project = PaperScopeItem.extend(/** @lends Project# */{
 
 	// TODO: Add arguments to define pages
 	/**
-	 * Creates a Paper.js project.
+	 * Creates a Paper.js project containing one empty {@link Layer}, referenced
+	 * by {@link Project#activeLayer}.
 	 *
-	 * When working with PaperScript, a project is automatically created for us
-	 * and the {@link PaperScope#project} variable points to it.
+	 * Note that when working with PaperScript, a project is automatically
+	 * created for us and the {@link PaperScope#project} variable points to it.
 	 *
 	 * @param {View|HTMLCanvasElement} view Either a view object or an HTML
 	 * Canvas element that should be wrapped in a newly created view.
@@ -154,6 +155,29 @@ var Project = PaperScopeItem.extend(/** @lends Project# */{
 		return this._index;
 	},
 
+	// Helper function used in Item#copyTo and Layer#initialize
+	// It's called the same as Item#addChild so Item#copyTo does not need to
+	// make the distinction.
+	// TODO: Consider private function with alias in Item?
+	addChild: function(child) {
+		if (child instanceof Layer) {
+			Base.splice(this.layers, [child]);
+			// Also activate this layer if there was none before
+			if (!this.activeLayer)
+				this.activeLayer = child;
+		} else if (child instanceof Item) {
+			// Anything else than layers needs to be added to a layer first
+			(this.activeLayer
+				// NOTE: If there is no layer and this project is not the active
+				// one, passing insert: false and calling addChild on the
+				// project will handle it correctly.
+				|| this.addChild(new Layer({ insert: false }))).addChild(child);
+		} else {
+			child = null;
+		}
+		return child;
+	},
+
 	/**
 	 * The selected items contained within the project.
 	 *
@@ -316,7 +340,10 @@ var Project = PaperScopeItem.extend(/** @lends Project# */{
 	 */
 	importJSON: function(json) {
 		this.activate();
-		return Base.importJSON(json);
+		// Provide the activeLayer as a possible target for layers, but only if
+		// it's empty.
+		var layer = this.activeLayer;
+		return Base.importJSON(json, layer && layer.isEmpty() && layer);
 	},
 
 	/**