Github user martin-g commented on a diff in the pull request:
https://github.com/apache/wicket/pull/258#discussion_r165631611
--- Diff:
wicket-core/src/main/java/org/apache/wicket/markup/head/filter/JavaScriptDeferHeaderResponse.java
---
@@ -0,0 +1,141 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.wicket.markup.head.filter;
+
+import org.apache.wicket.core.util.string.JavaScriptUtils;
+import org.apache.wicket.markup.head.AbstractJavaScriptReferenceHeaderItem;
+import org.apache.wicket.markup.head.HeaderItem;
+import org.apache.wicket.markup.head.IHeaderResponse;
+import org.apache.wicket.markup.head.IWrappedHeaderItem;
+import org.apache.wicket.markup.head.JavaScriptContentHeaderItem;
+import org.apache.wicket.markup.head.JavaScriptHeaderItem;
+import org.apache.wicket.markup.head.OnDomReadyHeaderItem;
+import org.apache.wicket.markup.head.OnLoadHeaderItem;
+import org.apache.wicket.markup.html.DecoratingHeaderResponse;
+import org.apache.wicket.page.PartialPageUpdate;
+import org.apache.wicket.request.Response;
+import org.apache.wicket.util.string.Strings;
+
+/**
+ * A header response that defers all {@link
AbstractJavaScriptReferenceHeaderItem}s.
+ * <p>
+ * To prevent any error because of possible dependencies to referenced
JavaScript files
+ * *all* {@link JavaScriptHeaderItem}s are replaced with suitable
implementations that
+ * delay any execution until {@link
AbstractJavaScriptReferenceHeaderItem}s have been loaded.
+ *
+ * @author svenmeier
++ */
+public class JavaScriptDeferHeaderResponse extends DecoratingHeaderResponse
+{
+ /**
+ * Decorate the given response.
+ *
+ * @param response
+ */
+ public JavaScriptDeferHeaderResponse(IHeaderResponse response)
+ {
+ super(response);
+ }
+
+ @Override
+ public void render(HeaderItem item)
+ {
+ if (item instanceof IWrappedHeaderItem) {
+ item = ((IWrappedHeaderItem)item).getWrapped();
+ }
+
+ if (item instanceof AbstractJavaScriptReferenceHeaderItem) {
+
((AbstractJavaScriptReferenceHeaderItem)item).setDefer(true);
+ } else if (item instanceof JavaScriptContentHeaderItem) {
+ item = new
NativeOnDomContentLoadedHeaderItem(((JavaScriptContentHeaderItem)item).getJavaScript());
+ } else if (item instanceof OnDomReadyHeaderItem) {
+ item = new
NativeOnDomContentLoadedHeaderItem(((OnDomReadyHeaderItem)item).getJavaScript());
+ } else if (item instanceof OnLoadHeaderItem) {
+ item = new
NativeOnLoadHeaderItem(((OnLoadHeaderItem)item).getJavaScript());
+ }
+
+ super.render(item);
+ }
+
+ /**
+ * A specialization that uses native "DOMContentLoaded" events without
dependency to external JavaScript.
+ * <p>
+ * For Ajax requests we utilize the fact, that {@link
PartialPageUpdate} renders {@link #getJavaScript()} only,
+ * executing the JavaScript directly without any event registration.
+ */
+ private class NativeOnDomContentLoadedHeaderItem extends
OnDomReadyHeaderItem
+ {
+ private static final long serialVersionUID = 1L;
+
+ /**
+ * Construct.
+ *
+ * @param javaScript
+ */
+ public NativeOnDomContentLoadedHeaderItem(CharSequence
javaScript)
+ {
+ super(javaScript);
+ }
+
+ /**
+ * Overriden to use native {@code
addEventListener('DOMContentLoaded')} instead.
+ */
+ @Override
+ public void render(Response response)
+ {
+ CharSequence js = getJavaScript();
+ if (Strings.isEmpty(js) == false)
+ {
+ JavaScriptUtils.writeJavaScript(response,
"document.addEventListener('DOMContentLoaded', function() { " + js + "; });");
+ }
+ }
+ }
+
+ /**
+ * A specialization that uses native "load" events without dependency
to external JavaScript
+ * <p>
+ * For Ajax requests we utilize the fact, that {@link
PartialPageUpdate} renders {@link #getJavaScript()} only,
+ * executing the JavaScript directly without any event registration.
+ */
+ private class NativeOnLoadHeaderItem extends OnLoadHeaderItem
--- End diff --
`static` ?!
---
