Today Achim Berg, vice president of business and marketing for Windows Phone in the interview with the Microsoft News Center told that the phone manufacturers have sold over 1.5 million Windows phones in the first six weeks. He also mentioned that there are 18,000 developers who are developing apps for the marketplace and there are more than 4000 apps available for Windows Phone 7.

I have been using Amazon Kindle software for PC a lot, I like reading books online and I have always wanted an App for Windows Mobile, but it was not available. Amazon has now announced that they would soon be releasing the App for Windows Phone 7.

Kindle has more that 725,000 Kindle Books available, which would become accessible through Windows Phone 7.

Amazon’s Whispersync technology syncs your phone across devices, so you can pick up where you left off. The Integrated shopping experience allows the customers to shop without leaving the app.

You can submit your email here(This links to Amazon site) to get notification by email when the App is available for Download.

In one of my previous Blog post we had a look at the Windows Phone 7 App Structure. In this post I will cover how the App runs in Windows Phone 7, we will also see the different layers of the WP7 UI.

Application Lifecycle

WP7 app has four stages:

1. When the App is not running
2. When the App starts
3. When the App is Running
4. When the App Exits

Phone call gets precedence over any running App. Running App is automatically paused and moved to the background. Shell UI Obscures application UI. Developer has to take care that when the interruption is over, App should be in the state it was prior to the interruption.

Events which could obscure an App could be a

• Phone call
• a Reminder
• Phone Battery low message.

Activation & Deactivation of the App

Deactivation of an App can happen by

• Hitting the Start Button
• Open Toast from the push notification service
• the Lock Screen

When you press start button the currently running App gets deactivated. When App is deactivated it gets killed as it cannot run in the background, hence the App state should be saved when it is deactivated and it should load that saved state, when the app gets activated again, the developer has to ensure that the app behaves properly when it is deactivated. This ensures that the resources are utilized optimally. The user should not feel that its a whole new instance of App and the experience should be seamless.

For example: You start IE from startup screen, do some website surfing and then again press start button to go to the home screen, you then start Twitter App, you post a Tweet and reply to couple of Tweets.

At this point, in the memory stack the top most app is Twitter App which is currently running, then it is IE, these are the active sessions. Though the Apps are not running in the background their last states are saved.

Now when you press the back button the current twitter app is terminated and you go back to the home screen, press back again and you are taken to the IE screen with the website you were surfing. you press back button again and you go to the home screen again, the memory stack is now empty.

Different layers of UI

Below I have tried to breakdown the UI of WP7 to look at how it functions. It is arranged in the Z-order as they exist.

1. Bottom layer – Direct 3D surface
2. Startup Screen
3. Running Application
4. Keyboard (if user input is required in a textbox like control)
5. App Bar (It is very specific to the application)
6. Incoming Call (takes the precedence over any running app)
7. Top layer - System Tray

Different Layers of the WP7 UI

In my previous Blog post we looked at the philosophy behind the new Windows Phone 7 UI, we also saw what are the minimum hardware specifications for the manufacturer. In this post I am going to cover the Windows Phone 7 App Model.

What is a Windows Phone 7 App?

Silverlight is the Core application Dev Framework for Windows Phone 7. Application according to Silverlight is a signed XAP which is injected by the Marketplace. Any App developed has to be submitted to the marketplace, which gives it a license (More details provided below). Application license is a Crypto-verifiable object issued to grant rights to an application. Please note, if a unlicensed app is loaded in the phone, the host process will not launch the App.

App Hub is the Marketplace website where Developers can submit applications for Windows phone and Xbox 360. For a $99 USD annual subscription, the apps & games can be submitted to the App Hub for sale or free download in the Windows Phone Marketplace or Xbox LIVE Marketplace. DreamSparkstudents need not pay any registration fee.

Certification Requirements

Document explaining the Windows Phone 7 Application Certification requirements can be downloaded from here. This document outlines the content policies required to be followed by Developers for Windows Phone 7 App. Please go through this document carefully if you are developing Applications for Windows Phone 7.

Submission Process

App has to be submitted through the Seller account you have created in the marketplace. There are three account types: Company, Individual and Student. You will have to select the account type when you create the account and pay the registration fee.

Once the App is submitted to the marketplace, it goes through the following steps:

• It is validated for proper functionality.
• On successful validation of the XAP, it has to pass the certification which involves the automated and manual verification.
• Once it passes certifications, it is signed and then it is eligible for publication which can be done immediately or on the date decided by the publisher.

Please note that any future updates to the already submitted and passed application has to pass through the same process to maintain the consistency.

App Manifest

When the user selects an app from the marketplace, Phone handles the installation of the XAP package. There is a App manifest in the package which defines what the App will require to run correctly, for example services like access to network, WIFI or GPS. The Apps can only use the capabilities declared in the Manifest.

Application Data is accessible only to that particular application. Other Apps cannot access the data of another application. They run in a host process which is sandboxed from other applications. If capabilities are not declared for the App, then it runs in the least privileged mode, hence developer should ensure that that the App manifest clearly declares what App will require to run correctly.

App development also includes the embeddable browsing control which enables you to develop in HTML, CSS and JavaScript, that allows you to embed HTML control inside the Windows Phone 7 App, so you can write in JavaScript and deploy it.

Apps Available

You can checkout the Apps currently available for Windows Phone through the Zune software. Apps are added regularly. Below is a screenshot of the Free Facebook App available at the Marketplace

Certain missing features like background processing and features like copy, paste are expected to be the part of the future updates in the Windows Phone 7 OS.

In my next Blog post I will cover how the App runs in the OS and what are the different layers of the UI.