通过内置账户自动补全邮箱注册，参见https://plus.google.com/+AndroidDevelopers/posts/DgLSWqZ7eVj

Register

public class Register extends Activity {
    private AutoCompleteTextView mEmailAutoTV;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.layout_register);
        mEmailAutoTV = (AutoCompleteTextView) this.findViewById(R.id.atv_sign);

        final Account[] accounts = AccountManager.get(this).getAccounts();
        final Set<String> emailSet = new HashSet<String>();
        for (Account account : accounts) {
            if (Patterns.EMAIL_ADDRESS.matcher(account.name).matches()) {
                emailSet.add(account.name);
            }
        }
        List<String> emails = new ArrayList<String>(emailSet);
        mEmailAutoTV.setAdapter(new ArrayAdapter<String>(this, android.R.layout.simple_dropdown_item_1line, emails));

    }
}

register_layout

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:text="Sign Up"
        android:textSize="30sp"
        android:layout_margin="20dp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

    <AutoCompleteTextView
        android:id="@+id/atv_sign"
        android:selectAllOnFocus="true"
        android:imeActionLabel="注册"
        android:inputType="textEmailAddress"
        android:hint="Enter Your Email"
        style="@style/TextAppearance.Autocomplete.Suggestion"
        android:layout_marginLeft="20dp"
        android:layout_marginRight="20dp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

</LinearLayout>