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<!-- facebook app id from developer site -->
<string name="facebook_app_id" translatable="false">560040454153884</string>

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.hungvu.testpack">

    <uses-permission android:name="android.permission.INTERNET" />
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme">
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <!-- facebook activity for login -->
        <activity
            android:name="com.facebook.FacebookActivity"
            android:configChanges="keyboard|keyboardHidden|screenLayout|screenSize|orientation"
            android:label="@string/app_name"
            android:theme="@android:style/Theme.Translucent.NoTitleBar" />
        <!-- fb app id -->
        <meta-data android:name="com.facebook.sdk.ApplicationId"
            android:value="@string/facebook_app_id"/>

    </application>

</manifest>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <com.facebook.login.widget.LoginButton
        android:id="@+id/login_fb"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
</LinearLayout>

- https://developers.facebook.com/apps/ and click Create a New App
- keytool -exportcert -alias androiddebugkey -keystore ~/.android/debug.keystore | openssl sha1 -binary | openssl base64 (pass: android)
- compile 'com.facebook.android:facebook-android-sdk:4.8.2'
=> facebook app id

public class MainActivity extends AppCompatActivity {

    private CallbackManager fbCallbackManager;
    private LoginButton loginButton;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        /// remember after setContentView
        FacebookSdk.sdkInitialize(getApplicationContext());
        setContentView(R.layout.activity_main);

        fbCallbackManager = CallbackManager.Factory.create();
        loginButton = (LoginButton)findViewById(R.id.login_fb);

        /// permission to get email from user
        loginButton.setReadPermissions(Arrays.asList("email"));

        loginButton.registerCallback(fbCallbackManager, new FacebookCallback<LoginResult>() {
            @Override
            public void onSuccess(LoginResult loginResult) {
                // login success

                /// get user info (must have access token)
                GraphRequest request = GraphRequest.newMeRequest(loginResult.getAccessToken(), new GraphRequest.GraphJSONObjectCallback() {
                    @Override
                    public void onCompleted(JSONObject object, GraphResponse response) {

                        // json user info
                        // {"id":"xxxxxxxxxxx","picture":{"data":{"url":"https:\/\/fbcdn-profile-a.akamaihd.net\/hprofile-ak-xpf1\/v\/t1.0-1\/p200x200\/12039505_124605024564769_2183097001496262180_n.jpg?oh=b6e2ee1c81655e7aedfa64e1211fec4c&oe=573E50E1&__gda__=1463803399_7a0115d5b2214ade64225af994c29c88","is_silhouette":false}},"email":"xxxxxx@gmail.com","name":"xxxx"}
                        Log.i("log_tag", "onCompleted: " + object.toString());
                    }
                });
                Bundle bundle = new Bundle();

                /// get name, email, avatar uri
                bundle.putString("fields", "name,email,picture.type(large)");
                request.setParameters(bundle);

                // start request
                request.executeAsync();
            }

            @Override
            public void onCancel() {
                // user cancel
            }

            @Override
            public void onError(FacebookException error) {
                // login error
            }
        });
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        fbCallbackManager.onActivityResult(requestCode, resultCode, data);
    }
}