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Activation Code Paths
As we saw in Chapter 2, new projects created in Visual Studio or Blend give you the following code in
js/default.js (a few comments have been removed):
(function () {
"use strict";
var app = WinJS.Application;
var activation = Windows.ApplicationModel.Activation;
app.onactivated = function (args) {
if (args.detail.kind === activation.ActivationKind.launch) {
if (args.detail.previousExecutionState !==
activation.ApplicationExecutionState.terminated) {
// TODO: This application has been newly launched. Initialize
// your application here.
} else {
// TODO: This application has been reactivated from suspension.
// Restore application state here.
}
args.setPromise(WinJS.UI.processAll());
}
};
app.oncheckpoint = function (args) {
};
app.start();
})();
Let’s go through this piece by piece to review what we already learned and complete our
understanding of this essential code structure:
• (function () { … })(); surrounding everything is again the JavaScript module pattern.
• "use strict" instructs the JavaScript interpreter to apply Strict Mode, a feature of ECMAScript 5.
This checks for sloppy programming practices, like using implicitly declared variables, so it’s a
good idea to leave it in place.
• var app = WinJS.Application; and var activation = Windows.ApplicationMode.Activation;
both create substantially shortened aliases for commonly used namespaces. This is a common
practice to simplify multiple references to the same part of WinJS or WinRT.
• app.onactivated = function (args) {…} assigns a handler for the WinJS.UI.onactivated event,
which is a wrapper for Windows.UI.WebUI.WebUIApplication.onactivated. In this handler:
• args.detail.kind identifies the type of activation.
• args.detail.previousExecutionState identifies the state of the app prior to this activation,
which determines whether to reload session state.
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