当包含在gwt.xml中时如何跳过加载外部JavaScript库？

Question

Assume that I want to use the an external JavaScript library in my GWT application. I can include myjslib.js in my module xml file with:

 <script src="myjslib.js"></script>

Here is the problem: If the my GWT app is not a full page application, than it could be possible that the html page that contains my GWT app has allready loaded the myjslib.js file.

Is there a way to telll GWT that myjslib.js should not be loaded if it already exist on the page?

Answer 1

There is no way using the .gwt.xml file, you can though, do it in your code visiting all '<script>' tags of your document and checking whether the src attribute matches your library name. Then you can inject the script using the ScriptInjector helper.

    NodeList<Element> scripts = Document.get().getElementsByTagName("script");
    for (int i = 0; i < scripts.getLength(); i++) {
      Element e = scripts.getItem(i);
      if (e.getAttribute("src").matches(".*myjslib.js")) {
        return;
      }
    }
    ScriptInjector.fromUrl("myjslib.js").inject();

[EDITED]

As @ThomasBroyer says in his first comment, the easiest way to load the script is adding a <script> tag in the head of your page. And the most reliable way to be sure that the script has been loaded before using it in GWT is knowing that some property has been set in the window object. For instance if you want to load jquery.js you can test whether $wnd.jQuery is defined.

[EDITED]

In your Javascript libary include something like:

  window.myjslib = true;

In your Java code:

  public void OnModuleLoad() {
     if (isMyJsLibLoaded() == false) {
        ScriptInjector.fromUrl("myjslib.js").inject();
     }
  }

  private native boolean isMyJsLibLoaded() /*-{
     return !!$wnd.myjslib;
  }-*/;