GWT Development with Activities and Places

The Activities and Places framework a built-in framework for browser history management. It builds on GWT's history mechanism  is introduced in GWT 2.1 allows you to create bookmarkable URLs within your application, allowing the browser's back button and bookmarks to work as users expect. may be used in conjunction with MVP development, though not required. Strictly speaking, MVP architecture is not concerned with browser history management, but Activities and Places may be used with MVP development as shown in this article.

Definitions An activity A place simply represents something the user is doing. contains no Widgets or UI code. typically restore state ("wake up"), perform initialization ("set up"), and load a corresponding UI ("show up"). can automatically display a warning confirmation when the Activity is about to be stopped (such as when the user navigates to a new Place). are started and stopped by an ActivityManager associated with a container Widget. the ActivityManager warns the user before the window is about to be closed. A place  is a Java object The object represents a particular state of the UI. can be converted to and from a URL history token by defining a PlaceTokenizer for each Place, see GWT's History mechanism) GWT's PlaceHistoryHandler automatically updates the browser URL corresponding to each Place in your app.

PlaceController PlaceChangeEvents PlaceHistoryMapper ActivityManager 2. fire PlaceController  PlaceChangeEvents  3. Listen to/updates  1. initiates navigation to a new Place, e.g., place2  PlaceHistoryMapper  PlaceHistoryHandler  URL1/Token1 Place1 URL2/Token2 Place2 declares  Place2/ State of UI_2/view User Activity1 Place1/ State of UI_1/view User Activity2 Managed by ActivityManager  Notify current_activity If current_activity is OK Start activity2 3.1 Activity2 := Get the activity corresponding to place2 ActivityMapper Activity1 Place1 Activity2 Pace2

Moving parts in a GWT 2.1 Views ClientFactory Activities Places PlaceHistoryMapper ActivityMapper

Views A view is simply the part of the UI is associated with an Activity. is defined by an interface, Interface allows multiple view implementations based on client characteristics (such as mobile vs. desktop) Interface facilitates lightweight unit testing by avoiding the time-consuming GWTTestCase. There is no View interface or class in GWT which views must implement or extend; however, GWT 2.1 introduces an IsWidget interface that is implemented by most Widgets as well as Composite. useful for views to extend IsWidget if they do in fact provide a Widget

The corresponding view implementation extends Composite, which keeps dependencies on a particular Widget from leaking out.

Activity The Presenter interface and setPresenter method allow for bi-directional communication between view and presenter, which simplifies interactions involving repeating Widgets and also allows view implementations to use UiBinder with @UiHandler methods that delegate to the presenter interface.

ClientFactory Widget creation It is good practice to A ClientFactory involves DOM operations, are relatively expensive to create. It is good practice to make them reusable, How? via a view factory, which might be part of a larger ClientFactory. A ClientFactory is not required to use Activities and Places ,however it is helpful to use a factory or dependency injection framework like GIN to obtain references to objects needed throughout your application like the event bus. Our example uses a ClientFactory to provide an EventBus, GWT PlaceController, and view implementations.

Another advantage of using a ClientFactory Deferred binding to use different implementation classes based on user.agent or other properties. For example, you might use a MobileClientFactory to provide different view implementations than the default DesktopClientFactory. How? instantiate your ClientFactory with GWT.create in onModuleLoad()

Activities Activity classes Example implement com.google.gwt.activity.shared.Activity. extend AbstractActivity, (for convenience) provides default (null) implementations of all required methods. Example simply says hello to a named user:

HelloActivity obtain HelloPlace’ states obtain a reference to the HelloView as well as the EventBus and PlaceController

HelloActivity.java The first thing to notice is that HelloActivity makes reference to HelloView, which is a view interface, not an implementation. One style of MVP coding defines the view interface in the presenter. This is perfectly legitimate; however, there is no fundamental reason why an Activity and it's corresponding view interface have to be tightly bound together. Note that HelloActivity also implements the view's Presenter interface. This is used to allow the view to call methods on the Activity, which facilitates the use of UiBinder as we saw above. The HelloActivity constructor takes two arguments: a HelloPlace and the ClientFactory. Neither is strictly required for an Activity. The HelloPlace simply makes it easy for HelloActivity to obtain properties of the state represented by HelloPlace (in this case, the name of the user we are greeting). Accepting an instance of a HelloPlace in the constructor implies that a new HelloActivity will be created for each HelloPlace. You could instead obtain an activity from a factory, but it's typically cleaner to use a newly constructed Activity so you don't have to clean up any prior state. Activities are designed to be disposable, whereas views, which are more expensive to create due to the DOM calls required, should be reusable. In keeping with this idea, ClientFactory is used by HelloActivity to obtain a reference to the HelloView as well as the EventBus and PlaceController. The start method is invoked by the ActivityManager and sets things in motion. It updates the view and then swaps the view back into the Activity's container widget by calling setWidget. The non-null mayStop() method provides a warning that will be shown to the user when the Activity is about to be stopped due to window closing or navigation to another Place. If it returns null, no such warning will be shown. Finally, the goTo() method invokes the PlaceController to navigate to a new Place. PlaceController in turn notifies the ActivityManager to stop the current Activity, find and start the Activity associated with the new Place, and update the URL in PlaceHistoryHandler.

Places One Activity -> one Place. A Place URL -> activity A Place extends com.google.gwt.place.shared. must have an associated PlaceTokenizer PlaceTokenizer knows how to serialize the Place's state to a URL token. By default, the URL consists of the Place's simple class name (like "HelloPlace") followed by a colon (:) and the token returned by the PlaceTokenizer. It is convenient (though not required) to declare the PlaceTokenizer as a static class inside the corresponding Place. However, you need not have a PlaceTokenizer for each Place. Many Places in your app might not save any state to the URL, so they could just extend a BasicPlace which declares a PlaceTokenizer that returns a null token.

PlaceHistoryMapper PlaceHistoryMapper You create an interface Declares all the Places available in your app Is the link between your PlaceTokenizers and GWT's PlaceHistoryHandler that synchronizes the browser URL with each Place. You create an interface that extends PlaceHistoryMapper and uses the annotation @WithTokenizers to list each of your tokenizer classes At GWT compile time, GWT generates (see PlaceHistoryMapperGenerator) a class based on your interface that extends AbstractPlaceHistoryMapper. For more control of the PlaceHistoryMapper, use the @Prefix annotation on a PlaceTokenizer to change the first part of the URL associated with the Place. For even more control, implement PlaceHistoryMapperWithFactory and provide a TokenizerFactory that, in turn, provides individual PlaceTokenizers.

ActivityMapper Maps It must implement Have code like each Place to its corresponding Activity. It must implement ActivityMapper, Have code like "if (place instanceof SomePlace) return new SomeActivity(place)". Must know about the ClientFactory so it can provide it to activities as needed

Putting it all together

How it all works The ActivityManager PlaceController keeps track of all Activities running within the context of one container widget. listens for PlaceChangeRequestEvents notifies the current activity when a new Place has been requested. If the current Activity allows the Place change (Activity.onMayStop() returns null) or the user allows it (by clicking OK in the confirmation dialog), the ActivityManager discards the current Activity and starts the new one. In order to find the new one, it uses your app's ActivityMapper to obtain the Activity associated with the requested Place. PlaceController initiates navigation to a new Place and is responsible for warning the user before doing so. PlaceHistoryHandler provides bi-directional mapping between Places and the URL. Whenever your app navigates to a new Place, the URL will be updated with the new token representing the Place so it can be bookmarked and saved in browser history. Likewise, when the user clicks the back button or pulls up a bookmark, PlaceHistoryHandler ensures that your application loads the corresponding Place.

How to navigate To navigate to a new Place in your application, call the goTo() method on your PlaceController. This is illustrated above in the goTo() method of HelloActivity. PlaceController warns the current Activity that it may be stopping (via a PlaceChangeRequest event) and once allowed, fires a PlaceChangeEvent with the new Place. The PlaceHistoryHandler listens for PlaceChangeEvents and updates the URL history token accordingly. The ActivityManager also listens for PlaceChangeEvents and uses your app's ActivityMapper to start the Activity associated with the new Place. Rather than using PlaceController.goTo(), you can also create a Hyperlink containing the history token for the new Place obtained by calling your PlaceHistoryMapper.getToken(). When the user navigates to a new URL (via hyperlink, back button, or bookmark), PlaceHistoryHandler catches the ValueChangeEvent from the History object and calls your app's PlaceHistoryMapper to turn the history token into its corresponding Place. It then calls PlaceController.goTo() with the new Place.