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Incremental migration of existing GWT UI plugins

PolyGerrit plugin API is based on different concepts and provides a different type of API compared to the one available to GWT plugins. Depending on the plugin, it might require significant modifications to existing UI scripts to fully take advantage of the benefits provided by the PolyGerrit API.

To make migration easier, PolyGerrit recommends an incremental migration strategy. Starting with a .js file that works for GWT UI, plugin author can incrementally migrate deprecated APIs to the new plugin API.

The goal for this guide is to provide a migration path from .js-based UI script to a html based implementation

Note
 Web UI plugins distributed as a single .js file are not covered in this guide.

Let’s start with a basic plugin that has an UI module. Commonly, file tree should look like this:

├── BUILD
├── LICENSE
└── src
    └── main
        ├── java
        │   └── com
        │       └── foo
        │           └── SamplePluginModule.java
        └── resources
            └── static
                └── sampleplugin.js

For simplicity’s sake, let’s assume SamplePluginModule.java has following content:

public class SamplePluginModule extends AbstractModule {

  @Override
  protected void configure() {
    DynamicSet.bind(binder(), WebUiPlugin.class)
        .toInstance(new JavaScriptPlugin("sampleplugin.js"));
  }
}

Step 1: Create sampleplugin.html

As a first step, create sampleplugin.html and include the UI script in the module file.

Note
 GWT UI ignores html files which it doesn’t support. 
  @Override
  protected void configure() {
    DynamicSet.bind(binder(), WebUiPlugin.class)
        .toInstance(new JavaScriptPlugin("sampleplugin.js"));
    DynamicSet.bind(binder(), WebUiPlugin.class)
        .toInstance(new JavaScriptPlugin("sampleplugin.html"));
  }

Here’s recommended starter code for sampleplugin.html:

Note
 By specification, the id attribute of dom-module must contain a dash (-). 
<dom-module id="sample-plugin">
  <script>
    Gerrit.install(plugin => {
        // Setup block, is executed before sampleplugin.js
    });
  </script>

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

  <script>
    Gerrit.install(plugin => {
        // Cleanup block, is executed after sampleplugin.js
    });
  </script>
</dom-module>

Here’s how this works:

  • PolyGerrit detects migration scenario because UI scripts have same filename and different extensions

    • PolyGerrit will load sampleplugin.html and skip sampleplugin.js

    • PolyGerrit will reuse plugin (aka self) instance for Gerrit.install() callbacks

  • sampleplugin.js is loaded since it’s referenced in sampleplugin.html

  • setup script tag code is executed before sampleplugin.js

  • cleanup script tag code is executed after sampleplugin.js

This means the plugin instance is shared between .html-based and .js-based code. This allows to gradually and incrementally transfer code to the new API.

Step 2: Create cut-off marker in sampleplugin.js

Commonly, window.Polymer is being used to detect in GWT UI script if it’s being executed inside PolyGerrit. This could be used to separate code that was already migrated to new APIs from old not yet migrated code.

During incremental migration, some of the UI code will be reimplemented using the PolyGerrit plugin API. However, old code still could be required for the plugin to work in GWT UI.

To handle this case, add the following code at the end of the installation callback in sampleplugin.js

Gerrit.install(function(self) {

  // Existing code here, not modified.

  if (window.Polymer) { return; } // Cut-off marker

  // Everything below was migrated to PolyGerrit plugin API.
  // Code below is still needed for the plugin to work in GWT UI.
});

Step 3: Migrate!

The code that uses deprecated APIs should be eventually rewritten using non-deprecated counterparts. Duplicated pieces could be kept under cut-off marker to work in GWT UI.

If some data or functions needs to be shared between code in .html and .js, it could be stored in the plugin (aka self) object that’s shared between both

Step 4: Cleanup

Once deprecated APIs are migrated, sampleplugin.js will only contain duplicated code that’s required for GWT UI to work. As soon as GWT support is removed from Gerrit that file can be simply deleted, along with the script tag loading it.