# Firebase with Android Studio part-4 : Using Google accunt sign-in Firebase

## 開始吧

### 前置作業
* 參考[Part-1](https://hackmd.io/OVJ6GLpPR_Kb5kA-85kdgQ?view)

### gradle
* 增加一個play-services:auth

 implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-play-services:1.1.1'
    implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-core:1.4.3'
    implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-android:1.4.3'
    implementation 'com.google.android.gms:play-services-auth:19.0.0'

### UI
* 這裡只使用一顆按鈕..

### 找到你的auth client id
* 把目錄切換到Project，並在app底下找到google-services.json並打開
* 我們會使用到client_id
* 並將ID加入資源檔，可以放在strings.xml也可以放在local.properties

### 設定Firebase
* 在專案設定加入SHA-A
* 登入方式打開Google

### 撰寫登入功能
* 宣告GoogleSignInOptions並帶入我們的webClient_id
* 並用Intent方式啟動google登入頁面
* OnActivityResult已經停用了，改用registerForActivityResult
* 注意到程式碼第12行，ActivityResultLauncher必須在生命週期onStarted之前，相當於在onCreate就要註冊，但是這個例子有按鈕的點擊監聽，所以當執行app時候，是在發生click事件才進行註冊，所以app會噴掉，以至於這個例子要把它移出click listener。

class MainActivity : AppCompatActivity() {

    private lateinit var auth: FirebaseAuth

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        auth = FirebaseAuth.getInstance()

        val registerLauncher =
            registerForActivityResult(ActivityResultContracts.StartActivityForResult()) { ActivityResult ->
                if (ActivityResult.resultCode == Activity.RESULT_OK) {
                    getIntentData(ActivityResult.data)
                }
            }

        findViewById<Button>(R.id.button).setOnClickListener {
            val options = GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
                .requestIdToken(BuildConfig.webclinet_id)
                .requestEmail()
                .build()

            val signInClient = GoogleSignIn.getClient(this, options)

            registerLauncher.launch(signInClient.signInIntent)
        }
    }


    private fun getIntentData(data: Intent?) {
        GoogleSignIn.getSignedInAccountFromIntent(data).result?.let {
            googleAuthForFirebase(it)
        }
    }

    private fun googleAuthForFirebase(account: GoogleSignInAccount) {
        val credentials = GoogleAuthProvider.getCredential(account.idToken, null)
        CoroutineScope(Dispatchers.IO).launch {
            try {
                auth.signInWithCredential(credentials).await()
                withContext(Dispatchers.Main) {
                    Toast.makeText(this@MainActivity, "登入成功", Toast.LENGTH_LONG).show()
                }
            } catch (e: Exception) {
                withContext(Dispatchers.Main) {
                    Toast.makeText(this@MainActivity, e.message, Toast.LENGTH_LONG).show()
                }
            }
        }
    }
}

參考資料
[Philipp Lackner's channel](https://www.youtube.com/watch?v=mhLlbWQ0p4s)

###### tags: `Firebase` `kotlin` `Android`