当我按下返回按钮时，活动会发生什么？

Question

当我使用 Intent 进行活动 A（此处为 LoginActivity）并转到活动 B（此处为 MainActivity）并调用 finish() 函数时 意图功能后 示例：

 if (isloggedIn){
        val intent=Intent(this@LoginActivity,MainActivity::class.java)
        startActivity(intent)
        finish()
    }

当我按下返回按钮时，从 Activity B（此处为 MainActivity）会发生什么情况，Activity A 将再次被 onCreate() 调用，或者由于它不在堆栈中，我将被定向到我的手机主屏幕？ 我目前遇到的错误让我对此感到困惑： 我做了上面提到的两个活动（A 和 B），并为此使用了 AutoLogin 功能。当用户第一次登录时它工作正常，但是当用户停止应用程序并清除内存并再次重新打开应用程序时，它成功进入 Activity B（MainActivity）而不询问用户的登录凭据，但是当我按下应用程序时崩溃为什么？ 代码：

class LoginActivity : AppCompatActivity(){
lateinit var phone_num:EditText
lateinit var password_num:EditText
lateinit var button_login:Button
lateinit var txtForgotPassword:TextView
lateinit var RegisterYouself:TextView
val ValidMobileNumber="0123"
val ValidPassword= arrayOf("Iron","captain","hulk","avenger")
lateinit var SharedPreferences:SharedPreferences

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    SharedPreferences=getSharedPreferences(getString(R.string.Prefereces_file_name), Context.MODE_PRIVATE)

    val isloggedIn=SharedPreferences.getBoolean("isLoggedIn",false)

    if (isloggedIn){
        val intent=Intent(this@LoginActivity,MainActivity::class.java)
        startActivity(intent)
        finish()
    }
    else {
        setContentView(R.layout.activity_login)
    }




    title="Log In"

    phone_num=findViewById(R.id.phone_num)
    password_num=findViewById(R.id.password_num)
    button_login=findViewById(R.id.button_login)
    txtForgotPassword=findViewById(R.id.txtForgotPassword)
    RegisterYouself=findViewById(R.id.RegisterYouself)

    button_login.setOnClickListener{
        val mobilenumber=phone_num.text.toString()
        val password=password_num.text.toString()
        var nameOfAvenger="Avenger"
        val intent=Intent(this@LoginActivity,MainActivity::class.java)
        if (mobilenumber==ValidMobileNumber){
            if (password==ValidPassword[0]){

                nameOfAvenger="Iron Man"
                savePreferences(nameOfAvenger)

                startActivity(intent)
            }
            else if (password==ValidPassword[1]){

                nameOfAvenger="Captain America"
                savePreferences(nameOfAvenger)

                startActivity(intent)
            }
            else if(password==ValidPassword[2]){

                nameOfAvenger="Hulk"
                savePreferences(nameOfAvenger)

                startActivity(intent)
            }
            else if (password==ValidPassword[3]){

                nameOfAvenger="The Avenger"
                savePreferences(nameOfAvenger)

                startActivity(intent)
            }
            else {
                Toast.makeText(
                    this@LoginActivity, "Incorrect creditial", Toast.LENGTH_LONG
                ).show()
            }
        }
        else {
            Toast.makeText(
                this@LoginActivity, "Incorrect creditial", Toast.LENGTH_LONG
            ).show()
        }
    }
}
fun savePreferences(title:String){
    SharedPreferences.edit().putBoolean("isLoggedIn",true).apply()
    SharedPreferences.edit().putString("Title",title).apply()

}

}

错误：

Process: com.mustafa.activitylifecycle, PID: 26158
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.mustafa.activitylifecycle/com.mustafa.activitylifecycle.LoginActivity}: java.lang.NullPointerException: findViewById(R.id.phone_num) must not be null
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6669)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
 Caused by: java.lang.NullPointerException: findViewById(R.id.phone_num) must not be null
    at com.mustafa.activitylifecycle.LoginActivity.onCreate(LoginActivity.kt:43)
    at android.app.Activity.performCreate(Activity.java:7136)
    at android.app.Activity.performCreate(Activity.java:7127)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)

Answer 1

您的问题：

您编写的代码仅在用户未登录时调用setContentView。因此，当用户已登录时，没有可查找的视图。

if (isloggedIn){
    val intent=Intent(this@LoginActivity,MainActivity::class.java)
    startActivity(intent)
    finish() // JUST BECAUSE YOU CALL FINISH DOES NOT MEAN THE METHOD STOPS EXECUTING
}
else {
    // CONTENT ONLY BEING SET IF NOT LOGGED IN
    setContentView(R.layout.activity_login)
}

title="Log In"

// IF NOT LOGGED IN, THERE IS NO "phone_num" TO FIND!!!! CRASH! BOOM!
phone_num=findViewById(R.id.phone_num)
password_num=findViewById(R.id.password_num)

解决方案

要么总是调用setContentView，因此无论您是否登录，视图都可用：

if (isloggedIn){
    val intent=Intent(this@LoginActivity,MainActivity::class.java)
    startActivity(intent)
    finish()
}

// REMOVE THE "else" - ALWAYS SET VIEWS SO THEY'RE VALID
setContentView(R.layout.activity_login)

title="Log In"

phone_num=findViewById(R.id.phone_num)
password_num=findViewById(R.id.password_num)

或者......只是提前返回：

if (isloggedIn){
    val intent=Intent(this@LoginActivity,MainActivity::class.java)
    startActivity(intent)
    finish()
    return // EARLY RETURN TO PREVENT THE REST OF THE IRRELEVANT CODE FROM RUNNING
}
else {
    setContentView(R.layout.activity_login)
}

title="Log In"

phone_num=findViewById(R.id.phone_num)
password_num=findViewById(R.id.password_num)

OR ... 将您的视图设置逻辑提取到作为 else 块一部分的函数中：

fun onCreate(...) {
    if (isloggedIn) {
        val intent=Intent(this@LoginActivity,MainActivity::class.java)
        startActivity(intent)
        finish()
    }
    else {
        initializeActivity()
    }
}

fun initializeActivity() {
    setContentView(R.layout.activity_login)

    title="Log In"

    phone_num=findViewById(R.id.phone_num)
    password_num=findViewById(R.id.password_num)
    // ETC
}