Remembering Your User 記住你的使用者

Everyone likes it when you remember their name. One of the simplest, most effective things you can do to make your app more lovable is to remember who your user is—especially when the user upgrades to a new device or starts carrying a tablet as well as a phone. But how do you know who your user is? And how do you recognize them on a new device?

For many applications, the answer is the AccountManager APIs. With the user's permission, you can use Account Manager to fetch the account names that the user has stored on their device.   http://blog.csdn.net/sergeycao

Integration with the user's accounts allows you to do a variety of things such as:

Auto-fill forms with the user's email address.
Retrieve an ID that is tied to a user, not the device.
Determine if AccountManager for You
Applications typically try to remember the user using one of three techniques:

Ask the user to type in a username
Retrieve a unique device ID to remember the device
Retrieve a built-in account from AccountManager
Option (a) is problematic. First, asking the user to type something before entering your app will automatically make your app less appealing. Second, there's no guarantee that the username chosen will be unique.

Option (b) is less onerous for the user, but it's tricky to get right. More importantly, it only allows you to remember the user on one device. Imagine the frustration of someone who upgrades to a shiny new device, only to find that your app no longer remembers them.

Option (c) is the preferred technique. Account Manager allows you to get information about the accounts that are stored on the user's device. As we'll see in this lesson, using Account Manager lets you remember your user, no matter how many devices the user may own, by adding just a couple of extra taps to your UI.

Decide What Type of Account to Use
Android devices can store multiple accounts from many different providers. When you query AccountManager for account names, you can choose to filter by account type. The account type is a string that uniquely identifies the entity that issued the account. For instance, Google accounts have type "com.google," while Twitter uses "com.twitter.android.auth.login."

Request GET_ACCOUNT permission
In order to get a list of accounts on the device, your app needs the GET_ACCOUNTS permission. Add a <uses-permission> tag in your manifest file to request this permission:

<manifest ... >
    <uses-permission android:name="android.permission.GET_ACCOUNTS" />
    ...
</manifest>
Query AccountManager for a List of Accounts
Once you decide what account type you're interested in, you need to query for accounts of that type. Get an instance of AccountManager by calling AccountManager.get(). Then use that instance to call getAccountsByType().

AccountManager am = AccountManager.get(this); // "this" references the current Context

Account[] accounts = am.getAccountsByType("com.google");
This returns an array of Account objects. If there's more than one Account in the array, you should present a dialog asking the user to select one.

Use the Account Object to Personalize Your App
The Account object contains an account name, which for Google accounts is an email address. You can use this information in several different ways, such as:

As suggestions in forms, so the user doesn't need to input account information by hand.
As a key into your own online database of usage and personalization information.

Decide Whether an Account Name is Enough
An account name is a good way to remember the user, but the Account object by itself doesn't protect your data or give you access to anything besides the user's account name. If your app needs to allow the user to go online to access private data, you'll need something stronger: authentication. The next lesson explains how to authenticate to existing online services. The lesson after that deals with writing a custom authenticator so that you can install your own account types.