当我使用Kotlin更改为Android Studio中的另一个活动时，如何停止崩溃

Question

Everytime I try and switch from LoginActivity to RegisterActivity it crashes, and I can't figure out why

This is my MainActivity.kt

package com.test.kotlinfirstapp

import android.content.Intent
import android.os.Bundle
import android.support.v7.app.AppCompatActivity;
import android.widget.Button
import com.test.kotlinfirstapp.R.layout.activity_main
import kotlinx.android.synthetic.main.activity_login.*
import kotlinx.android.synthetic.main.activity_register.*
import kotlinx.android.synthetic.main.content_main.*

class MainActivity : AppCompatActivity() {


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

        val button2 = findViewById<Button>(R.id.button2) as Button
        button2.setOnClickListener{
        val intent = Intent(this, LoginActivity::class.java)
        startActivity(intent)
    }


    }

}

This is my LoginActivity.kt

package com.test.kotlinfirstapp

import android.animation.Animator
import android.animation.AnimatorListenerAdapter
import android.annotation.TargetApi
import android.content.pm.PackageManager
import android.support.design.widget.Snackbar
import android.support.v7.app.AppCompatActivity
import android.app.LoaderManager.LoaderCallbacks
import android.content.CursorLoader
import android.content.Loader
import android.database.Cursor
import android.net.Uri
import android.os.AsyncTask
import android.os.Build
import android.os.Bundle
import android.provider.ContactsContract
import android.text.TextUtils
import android.view.View
import android.view.inputmethod.EditorInfo
import android.widget.ArrayAdapter
import android.widget.TextView

import java.util.ArrayList
import android.Manifest.permission.READ_CONTACTS
import android.content.Intent
import android.widget.Button

import kotlinx.android.synthetic.main.activity_login.*

/**
 * A login screen that offers login via email/password.
 */
class LoginActivity : AppCompatActivity(), LoaderCallbacks<Cursor> {
    /**
     * Keep track of the login task to ensure we can cancel it if requested.
     */
    private var mAuthTask: UserLoginTask? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_login)
        // Set up the login form.
        populateAutoComplete()
        password.setOnEditorActionListener(TextView.OnEditorActionListener { _, id, _ ->
            if (id == EditorInfo.IME_ACTION_DONE || id == EditorInfo.IME_NULL) {
                attemptLogin()
                return@OnEditorActionListener true
            }
            false


        })
        val button = findViewById<Button>(R.id.button4) as Button
        button4.setOnClickListener{
            val intent = Intent(this, RegisterActivity::class.java)
            startActivity(intent)

        button3.setOnClickListener { attemptLogin() }

    }


    private fun populateAutoComplete() {
        if (!mayRequestContacts()) {
            return
        }

        loaderManager.initLoader(0, null, this)
    }

    private fun mayRequestContacts(): Boolean {
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.M) {
            return true
        }
        if (checkSelfPermission(READ_CONTACTS) == PackageManager.PERMISSION_GRANTED) {
            return true
        }
        if (shouldShowRequestPermissionRationale(READ_CONTACTS)) {
            Snackbar.make(email, R.string.permission_rationale, Snackbar.LENGTH_INDEFINITE)
                .setAction(android.R.string.ok,
                    { requestPermissions(arrayOf(READ_CONTACTS), REQUEST_READ_CONTACTS) })
        } else {
            requestPermissions(arrayOf(READ_CONTACTS), REQUEST_READ_CONTACTS)
        }
        return false
    }

    /**
     * Callback received when a permissions request has been completed.
     */
    override fun onRequestPermissionsResult(
        requestCode: Int, permissions: Array<String>,
        grantResults: IntArray
    ) {
        if (requestCode == REQUEST_READ_CONTACTS) {
            if (grantResults.size == 1 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                populateAutoComplete()
            }
        }
    }


    /**
     * Attempts to sign in or register the account specified by the login form.
     * If there are form errors (invalid email, missing fields, etc.), the
     * errors are presented and no actual login attempt is made.
     */
    private fun attemptLogin() {
        if (mAuthTask != null) {
            return
        }

        // Reset errors.
        email.error = null
        password.error = null

        // Store values at the time of the login attempt.
        val emailStr = email.text.toString()
        val passwordStr = password.text.toString()

        var cancel = false
        var focusView: View? = null

        // Check for a valid password, if the user entered one.
        if (!TextUtils.isEmpty(passwordStr) && !isPasswordValid(passwordStr)) {
            password.error = getString(R.string.error_invalid_password)
            focusView = password
            cancel = true
        }

        // Check for a valid email address.
        if (TextUtils.isEmpty(emailStr)) {
            email.error = getString(R.string.error_field_required)
            focusView = email
            cancel = true
        } else if (!isEmailValid(emailStr)) {
            email.error = getString(R.string.error_invalid_email)
            focusView = email
            cancel = true
        }

        if (cancel) {
            // There was an error; don't attempt login and focus the first
            // form field with an error.
            focusView?.requestFocus()
        } else {
            // Show a progress spinner, and kick off a background task to
            // perform the user login attempt.
            showProgress(true)
            mAuthTask = UserLoginTask(emailStr, passwordStr)
            mAuthTask!!.execute(null as Void?)
        }
    }

    private fun isEmailValid(email: String): Boolean {
        //TODO: Replace this with your own logic
        return email.contains("@")
    }

    private fun isPasswordValid(password: String): Boolean {
        //TODO: Replace this with your own logic
        return password.length > 4
    }

    /**
     * Shows the progress UI and hides the login form.
     */
    @TargetApi(Build.VERSION_CODES.HONEYCOMB_MR2)
    private fun showProgress(show: Boolean) {
        // On Honeycomb MR2 we have the ViewPropertyAnimator APIs, which allow
        // for very easy animations. If available, use these APIs to fade-in
        // the progress spinner.
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB_MR2) {
            val shortAnimTime = resources.getInteger(android.R.integer.config_shortAnimTime).toLong()

            login_form.visibility = if (show) View.GONE else View.VISIBLE
            login_form.animate()
                .setDuration(shortAnimTime)
                .alpha((if (show) 0 else 1).toFloat())
                .setListener(object : AnimatorListenerAdapter() {
                    override fun onAnimationEnd(animation: Animator) {
                        login_form.visibility = if (show) View.GONE else View.VISIBLE
                    }
                })

            login_progress.visibility = if (show) View.VISIBLE else View.GONE
            login_progress.animate()
                .setDuration(shortAnimTime)
                .alpha((if (show) 1 else 0).toFloat())
                .setListener(object : AnimatorListenerAdapter() {
                    override fun onAnimationEnd(animation: Animator) {
                        login_progress.visibility = if (show) View.VISIBLE else View.GONE
                    }
                })
        } else {
            // The ViewPropertyAnimator APIs are not available, so simply show
            // and hide the relevant UI components.
            login_progress.visibility = if (show) View.VISIBLE else View.GONE
            login_form.visibility = if (show) View.GONE else View.VISIBLE
        }
    }

    override fun onCreateLoader(i: Int, bundle: Bundle?): Loader<Cursor> {
        return CursorLoader(
            this,
            // Retrieve data rows for the device user's 'profile' contact.
            Uri.withAppendedPath(
                ContactsContract.Profile.CONTENT_URI,
                ContactsContract.Contacts.Data.CONTENT_DIRECTORY
            ), ProfileQuery.PROJECTION,

            // Select only email addresses.
            ContactsContract.Contacts.Data.MIMETYPE + " = ?", arrayOf(
                ContactsContract.CommonDataKinds.Email
                    .CONTENT_ITEM_TYPE
            ),

            // Show primary email addresses first. Note that there won't be
            // a primary email address if the user hasn't specified one.
            ContactsContract.Contacts.Data.IS_PRIMARY + " DESC"
        )
    }

    override fun onLoadFinished(cursorLoader: Loader<Cursor>, cursor: Cursor) {
        val emails = ArrayList<String>()
        cursor.moveToFirst()
        while (!cursor.isAfterLast) {
            emails.add(cursor.getString(ProfileQuery.ADDRESS))
            cursor.moveToNext()
        }

        addEmailsToAutoComplete(emails)
    }

    override fun onLoaderReset(cursorLoader: Loader<Cursor>) {

    }

    private fun addEmailsToAutoComplete(emailAddressCollection: List<String>) {
        //Create adapter to tell the AutoCompleteTextView what to show in its dropdown list.
        val adapter = ArrayAdapter(
            this@LoginActivity,
            android.R.layout.simple_dropdown_item_1line, emailAddressCollection
        )

        email.setAdapter(adapter)
    }

    object ProfileQuery {
        val PROJECTION = arrayOf(
            ContactsContract.CommonDataKinds.Email.ADDRESS,
            ContactsContract.CommonDataKinds.Email.IS_PRIMARY
        )
        val ADDRESS = 0
        val IS_PRIMARY = 1
    }

    /**
     * Represents an asynchronous login/registration task used to authenticate
     * the user.
     */
    inner class UserLoginTask internal constructor(private val mEmail: String, private val mPassword: String) :
        AsyncTask<Void, Void, Boolean>() {

        override fun doInBackground(vararg params: Void): Boolean? {
            // TODO: attempt authentication against a network service.

            try {
                // Simulate network access.
                Thread.sleep(2000)
            } catch (e: InterruptedException) {
                return false
            }

            return DUMMY_CREDENTIALS
                .map { it.split(":") }
                .firstOrNull { it[0] == mEmail }
                ?.let {
                    // Account exists, return true if the password matches.
                    it[1] == mPassword
                }
                ?: true
        }

        override fun onPostExecute(success: Boolean?) {
            mAuthTask = null
            showProgress(false)

            if (success!!) {
                finish()
            } else {
                password.error = getString(R.string.error_incorrect_password)
                password.requestFocus()
            }
        }

        override fun onCancelled() {
            mAuthTask = null
            showProgress(false)
        }
    }

    companion object {

        /**
         * Id to identity READ_CONTACTS permission request.
         */
        private val REQUEST_READ_CONTACTS = 0

        /**
         * A dummy authentication store containing known user names and passwords.
         * TODO: remove after connecting to a real authentication system.
         */
        private val DUMMY_CREDENTIALS = arrayOf("foo@example.com:hello", "bar@example.com:world")
    }
}

This is my RegisterActivity.kt

    package com.test.kotlinfirstapp

import android.animation.Animator
import android.animation.AnimatorListenerAdapter
import android.annotation.TargetApi
import android.content.pm.PackageManager
import android.support.design.widget.Snackbar
import android.support.v7.app.AppCompatActivity
import android.app.LoaderManager.LoaderCallbacks

import android.content.CursorLoader
import android.content.Loader
import android.database.Cursor
import android.net.Uri
import android.os.AsyncTask

import android.os.Build
import android.os.Bundle
import android.provider.ContactsContract
import android.text.TextUtils
import android.view.KeyEvent
import android.view.View
import android.view.View.OnClickListener
import android.view.inputmethod.EditorInfo
import android.widget.ArrayAdapter
import android.widget.AutoCompleteTextView
import android.widget.Button
import android.widget.EditText
import android.widget.TextView

import java.util.ArrayList

import android.Manifest.permission.READ_CONTACTS

/**
 * A login screen that offers login via email/password.
 */
class RegisterActivity : AppCompatActivity(), LoaderCallbacks<Cursor> {
    /**
     * Keep track of the login task to ensure we can cancel it if requested.
     */
    private var mAuthTask: UserLoginTask? = null

    // UI references.
    private var mEmailView: AutoCompleteTextView? = null
    private var mPasswordView: EditText? = null
    private var mProgressView: View? = null
    private var mLoginFormView: View? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_register)
        // Set up the login form.
        mEmailView = findViewById<View>(R.id.email) as AutoCompleteTextView
        populateAutoComplete()

        mPasswordView = findViewById<View>(R.id.password) as EditText
        mPasswordView!!.setOnEditorActionListener(TextView.OnEditorActionListener { textView, id, keyEvent ->
            if (id == EditorInfo.IME_ACTION_DONE || id == EditorInfo.IME_NULL) {
                attemptLogin()
                return@OnEditorActionListener true
            }
            false
        })

        val mEmailSignInButton = findViewById<View>(R.id.button4) as Button
        mEmailSignInButton.setOnClickListener { attemptLogin() }

        mLoginFormView = findViewById(R.id.login_form)
        mProgressView = findViewById(R.id.login_progress)
    }

    private fun populateAutoComplete() {
        if (!mayRequestContacts()) {
            return
        }

        loaderManager.initLoader(0, null, this)
    }

    private fun mayRequestContacts(): Boolean {
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.M) {
            return true
        }
        if (checkSelfPermission(READ_CONTACTS) == PackageManager.PERMISSION_GRANTED) {
            return true
        }
        if (shouldShowRequestPermissionRationale(READ_CONTACTS)) {
            Snackbar.make(mEmailView!!, R.string.permission_rationale, Snackbar.LENGTH_INDEFINITE)
                .setAction(android.R.string.ok) { requestPermissions(arrayOf(READ_CONTACTS), REQUEST_READ_CONTACTS) }
        } else {
            requestPermissions(arrayOf(READ_CONTACTS), REQUEST_READ_CONTACTS)
        }
        return false
    }

    /**
     * Callback received when a permissions request has been completed.
     */
    override fun onRequestPermissionsResult(
        requestCode: Int, permissions: Array<String>,
        grantResults: IntArray
    ) {
        if (requestCode == REQUEST_READ_CONTACTS) {
            if (grantResults.size == 1 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                populateAutoComplete()
            }
        }
    }


    /**
     * Attempts to sign in or register the account specified by the login form.
     * If there are form errors (invalid email, missing fields, etc.), the
     * errors are presented and no actual login attempt is made.
     */
    private fun attemptLogin() {
        if (mAuthTask != null) {
            return
        }

        // Reset errors.
        mEmailView!!.error = null
        mPasswordView!!.error = null

        // Store values at the time of the login attempt.
        val email = mEmailView!!.text.toString()
        val password = mPasswordView!!.text.toString()

        var cancel = false
        var focusView: View? = null

        // Check for a valid password, if the user entered one.
        if (!TextUtils.isEmpty(password) && !isPasswordValid(password)) {
            mPasswordView!!.error = getString(R.string.error_invalid_password)
            focusView = mPasswordView
            cancel = true
        }

        // Check for a valid email address.
        if (TextUtils.isEmpty(email)) {
            mEmailView!!.error = getString(R.string.error_field_required)
            focusView = mEmailView
            cancel = true
        } else if (!isEmailValid(email)) {
            mEmailView!!.error = getString(R.string.error_invalid_email)
            focusView = mEmailView
            cancel = true
        }

        if (cancel) {
            // There was an error; don't attempt login and focus the first
            // form field with an error.
            focusView!!.requestFocus()
        } else {
            // Show a progress spinner, and kick off a background task to
            // perform the user login attempt.
            showProgress(true)
            mAuthTask = UserLoginTask(email, password)
            mAuthTask!!.execute(null as Void?)
        }
    }

    private fun isEmailValid(email: String): Boolean {
        //TODO: Replace this with your own logic
        return email.contains("@")
    }

    private fun isPasswordValid(password: String): Boolean {
        //TODO: Replace this with your own logic
        return password.length > 4
    }

    /**
     * Shows the progress UI and hides the login form.
     */
    @TargetApi(Build.VERSION_CODES.HONEYCOMB_MR2)
    private fun showProgress(show: Boolean) {
        // On Honeycomb MR2 we have the ViewPropertyAnimator APIs, which allow
        // for very easy animations. If available, use these APIs to fade-in
        // the progress spinner.
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB_MR2) {
            val shortAnimTime = resources.getInteger(android.R.integer.config_shortAnimTime)

            mLoginFormView!!.visibility = if (show) View.GONE else View.VISIBLE
            mLoginFormView!!.animate().setDuration(shortAnimTime.toLong()).alpha(
                (if (show) 0 else 1).toFloat()
            ).setListener(object : AnimatorListenerAdapter() {
                override fun onAnimationEnd(animation: Animator) {
                    mLoginFormView!!.visibility = if (show) View.GONE else View.VISIBLE
                }
            })

            mProgressView!!.visibility = if (show) View.VISIBLE else View.GONE
            mProgressView!!.animate().setDuration(shortAnimTime.toLong()).alpha(
                (if (show) 1 else 0).toFloat()
            ).setListener(object : AnimatorListenerAdapter() {
                override fun onAnimationEnd(animation: Animator) {
                    mProgressView!!.visibility = if (show) View.VISIBLE else View.GONE
                }
            })
        } else {
            // The ViewPropertyAnimator APIs are not available, so simply show
            // and hide the relevant UI components.
            mProgressView!!.visibility = if (show) View.VISIBLE else View.GONE
            mLoginFormView!!.visibility = if (show) View.GONE else View.VISIBLE
        }
    }

    override fun onCreateLoader(i: Int, bundle: Bundle): Loader<Cursor> {
        return CursorLoader(
            this,
            // Retrieve data rows for the device user's 'profile' contact.
            Uri.withAppendedPath(
                ContactsContract.Profile.CONTENT_URI,
                ContactsContract.Contacts.Data.CONTENT_DIRECTORY
            ), ProfileQuery.PROJECTION,

            // Select only email addresses.
            ContactsContract.Contacts.Data.MIMETYPE + " = ?", arrayOf(
                ContactsContract.CommonDataKinds.Email
                    .CONTENT_ITEM_TYPE
            ),

            // Show primary email addresses first. Note that there won't be
            // a primary email address if the user hasn't specified one.
            ContactsContract.Contacts.Data.IS_PRIMARY + " DESC"
        )
    }

    override fun onLoadFinished(cursorLoader: Loader<Cursor>, cursor: Cursor) {
        val emails = ArrayList<String>()
        cursor.moveToFirst()
        while (!cursor.isAfterLast) {
            emails.add(cursor.getString(ProfileQuery.ADDRESS))
            cursor.moveToNext()
        }

        addEmailsToAutoComplete(emails)
    }

    override fun onLoaderReset(cursorLoader: Loader<Cursor>) {

    }

    private fun addEmailsToAutoComplete(emailAddressCollection: List<String>) {
        //Create adapter to tell the AutoCompleteTextView what to show in its dropdown list.
        val adapter = ArrayAdapter(
            this@RegisterActivity,
            android.R.layout.simple_dropdown_item_1line, emailAddressCollection
        )

        mEmailView!!.setAdapter(adapter)
    }


    private interface ProfileQuery {
        companion object {
            val PROJECTION = arrayOf(
                ContactsContract.CommonDataKinds.Email.ADDRESS,
                ContactsContract.CommonDataKinds.Email.IS_PRIMARY
            )

            val ADDRESS = 0
            val IS_PRIMARY = 1
        }
    }

    /**
     * Represents an asynchronous login/registration task used to authenticate
     * the user.
     */
    inner class UserLoginTask internal constructor(private val mEmail: String, private val mPassword: String) :
        AsyncTask<Void, Void, Boolean>() {

        override fun doInBackground(vararg params: Void): Boolean? {
            // TODO: attempt authentication against a network service.

            try {
                // Simulate network access.
                Thread.sleep(2000)
            } catch (e: InterruptedException) {
                return false
            }

            for (credential in DUMMY_CREDENTIALS) {
                val pieces = credential.split(":".toRegex()).dropLastWhile { it.isEmpty() }.toTypedArray()
                if (pieces[0] == mEmail) {
                    // Account exists, return true if the password matches.
                    return pieces[1] == mPassword
                }
            }

            // TODO: register the new account here.
            return true
        }

        override fun onPostExecute(success: Boolean?) {
            mAuthTask = null
            showProgress(false)

            if (success!!) {
                finish()
            } else {
                mPasswordView!!.error = getString(R.string.error_incorrect_password)
                mPasswordView!!.requestFocus()
            }
        }

        override fun onCancelled() {
            mAuthTask = null
            showProgress(false)
        }
    }

    companion object {

        /**
         * Id to identity READ_CONTACTS permission request.
         */
        private val REQUEST_READ_CONTACTS = 0

        /**
         * A dummy authentication store containing known user names and passwords.
         * TODO: remove after connecting to a real authentication system.
         */
        private val DUMMY_CREDENTIALS = arrayOf("foo@example.com:hello", "bar@example.com:world")
    }
}

And here is my AndroidManifest.xml if it is needed

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

    <!-- To auto-complete the email text field in the login form with the user's emails -->
    <uses-permission android:name="android.permission.GET_ACCOUNTS"/>
    <uses-permission android:name="android.permission.READ_PROFILE"/>
    <uses-permission android:name="android.permission.READ_CONTACTS"/>

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

                <category android:name="android.intent.category.LAUNCHER"/>
            </intent-filter>
        </activity>
        <activity
                android:name=".LoginActivity"
                android:label="@string/title_activity_login">
        </activity>
        <activity
                android:name=".RegisterActivity"
                android:label="@string/title_activity_register">
        </activity>
    </application>

</manifest>

All I am trying to do is have one of my buttons on LoginActivity to go to RegisterActivity, and then from there return back to LoginActivity.

Answer 1

I don't see any implementation in your LoginActivity that starts the RegisterActivity, so you won't be able to get there. In addition to that if you don't want to get back to MainActivity after logging in you would have to replace your success logic in UserLoginTask:onPostExecute() from finish() to what you want to do.