Kotlin在Android端的基本用法详解

妖狐艹你老母 2022-06-01 02:45 140阅读 0赞

![SouthEast][]

作者：谭东

Kotlin相信现在大家都不陌生了，它是谷歌在5月18日，它的安卓团队在Google I/O 2017 大会上正式宣布 Kotlin 成为官方头等支持语言。最近一段时间我学习和研究了下Kotlin的特点和基本用法。大概用了一天时间，把Android的一些主要的APP功能，用Kotlin语言和结构重新写了一遍，体会就是：上手和学习很快、语法简洁、代码少写了很多、**不用很麻烦的写控件绑定了（自动导包）、**兼容性不错、空指针处理都帮你想好了、Java和Android的原来的框架和库都可以正常使用、支持Java和Kotlin混合编写等等。一些语法糖很好用。

Kotlin 是一个基于 JVM 的新的编程语言，由 JetBrains 开发。Kotlin可以编译成Java字节码，也可以编译成JavaScript，方便在没有JVM的设备上运行。

先看下官网的一个最形象简单的例子。

![SouthEast 1][]

好吧，看到这里你会说没什么简洁的，感觉都差不多。

官方使用文档地址：[http://kotlinlang.org/docs/reference/android-overview.html][http_kotlinlang.org_docs_reference_android-overview.html]

那我先总结下Kotlin中必要有特点的几个地方：

1、方法名fun开头、语句和声明结尾无需加分号了（当然加了也不报错）、方法参数和变量声明是反过来的：前面是名称，后面是类型，例如

var name:String?=null

2、对象的创建没有new了，直接对象名+括号。例如创建Utils这个类，然后调用它的foo方法：

var utils = Utils();
    utils.foo(this);

3、控件直接不用findViewById了，直接对应布局里的控件id名字使用，会自动导包。

import kotlinx.android.synthetic.main.activity_main.*

4、重写父类方法由原来的@override注解改成了前缀。

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

5、继承变成了冒号：分隔代替了extend，实现接口直接逗号，分隔代替了immplement。

class MainActivity : BaseActivity(), View.OnClickListener {
    
    }

6、没有了switch case语句，取而代之是when  ->语句，例如：

override fun onClick(v: View?) {
            when (v!!.id) {
                R.id.tv_getdata -> {
                    getData()
                }
                R.id.tv_intent -> {
                    var intent = Intent(this@MainActivity, SencondActivity::class.java);
                    intent.putExtra("name", "名字")
                    intent.putExtra("id", 12)
                    startActivity(intent)
                }
            }
        }

7、非空控制严格，更安全方便。!!、?。例如在使用!!后，使用的对象不可为空，空的时候直接抛出空指针异常；使用?后，使用的对象可以为空，空的时候返回Null。

private var users: Call<List<User>>? = null
        private var userList: List<User>? = null

8、Intent意图的接收，直接intent接收，不用getIntent了。已经封装好，例如

name = intent.getStringExtra("name")
            id = intent.getIntExtra("id", 1)

9、for循环的不同。例如i从0到9循环，添加到ArrayList

for (i in 0..9) {
                list!!.add("" + i)
            }

也有这种，for(i in 对象集合.indices)，例如

for (i in userList!!.indices) {
                    Log.i("info", "用户：" + userList!!.get(i).full_name);
                }

10、构造方法的不同，构造方法可以卸载类名后，如空的构造方法，后面会直接执行init方法，例如

class ApiClient constructor() {
        private var apiservice: ApiService? = null;
        private var retrofit: Retrofit? = null;
    
        init {
            retrofit = Retrofit.Builder().baseUrl(Conf.BASE_URL)
                    .addConverterFactory(GsonConverterFactory.create())
                    .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                    .build();
            apiservice = retrofit!!.create(ApiService::class.java)
        }

当然有参的多个构造方法，也可以写在类里，例如

constructor(resultListener: ResultListener<T>) {
            this.resultListener = resultListener
        }
    
    constructor(context: Context, progress: Boolean, resultListener: ResultListener<T>) {
            this.progress = progress
            this.resultListener = resultListener
            if (progress) {
                loadingView = LoadingView(context)
                loadingView!!.setOnCancelListener(this)
            }
        }

11、接口的定义。

interface ResultListener<T> {
        fun complete(t: T)
    
        fun onError(e: Throwable)
    }

12、对象实体的构建，不用setter和getter了，直接定义或者data class+对象名，例如

class User {
        var id: Int = 0
        var name: String? = null
        var full_name: String? = null
        var owner: OwnerBean? = null
        @SerializedName("private")
        var isPrivateX: Boolean = false
        var html_url: String? = null
        var description: String? = null
        var isFork: Boolean = false
        var url: String? = null
        var forks_url: String? = null
        var keys_url: String? = null
        var collaborators_url: String? = null
        var teams_url: String? = null
        var hooks_url: String? = null
        var issue_events_url: String? = null
        var events_url: String? = null
        var assignees_url: String? = null
        var branches_url: String? = null
        var tags_url: String? = null
        var blobs_url: String? = null
        var git_tags_url: String? = null
        var git_refs_url: String? = null
        var trees_url: String? = null
        var statuses_url: String? = null
        var languages_url: String? = null
        var stargazers_url: String? = null
        var contributors_url: String? = null
        var subscribers_url: String? = null
        var subscription_url: String? = null
        var commits_url: String? = null
        var git_commits_url: String? = null
        var comments_url: String? = null
        var issue_comment_url: String? = null
        var contents_url: String? = null
        var compare_url: String? = null
        var merges_url: String? = null
        var archive_url: String? = null
        var downloads_url: String? = null
        var issues_url: String? = null
        var pulls_url: String? = null
        var milestones_url: String? = null
        var notifications_url: String? = null
        var labels_url: String? = null
        var releases_url: String? = null
        var deployments_url: String? = null
        var created_at: String? = null
        var updated_at: String? = null
        var pushed_at: String? = null
        var git_url: String? = null
        var ssh_url: String? = null
        var clone_url: String? = null
        var svn_url: String? = null
        var homepage: String? = null
        var size: Int = 0
        var stargazers_count: Int = 0
        var watchers_count: Int = 0
        var language: String? = null
        var isHas_issues: Boolean = false
        var isHas_projects: Boolean = false
        var isHas_downloads: Boolean = false
        var isHas_wiki: Boolean = false
        var isHas_pages: Boolean = false
        var forks_count: Int = 0
        var mirror_url: Any? = null
        var isArchived: Boolean = false
        var open_issues_count: Int = 0
        var license: Any? = null
        var forks: Int = 0
        var open_issues: Int = 0
        var watchers: Int = 0
        var default_branch: String? = null
    
        class OwnerBean {
            var login: String? = null
            var id: Int = 0
            var avatar_url: String? = null
            var gravatar_id: String? = null
            var url: String? = null
            var html_url: String? = null
            var followers_url: String? = null
            var following_url: String? = null
            var gists_url: String? = null
            var starred_url: String? = null
            var subscriptions_url: String? = null
            var organizations_url: String? = null
            var repos_url: String? = null
            var events_url: String? = null
            var received_events_url: String? = null
            var type: String? = null
            var isSite_admin: Boolean = false
        }
    }

13、BaseActivity的写法大家也应该可以大概猜到什么样式的。

open class BaseActivity : AppCompatActivity() {
    
        override fun onCreate(savedInstanceState: Bundle?) {
            super.onCreate(savedInstanceState)
        }
    
        fun showToast(context: Context, text: String) {
            Toast.makeText(context, text, Toast.LENGTH_SHORT).show();
        }
    
        /*
    * show toast in activity
    * */
        fun Activity.toast(msg: String){
            Toast.makeText(this, msg, Toast.LENGTH_SHORT).show()
        }
    
        override fun onResume() {
            super.onResume()
        }
    
    }

14、常量的定义，val表示常量，类似java里的final，常量定义后不能修改，var定义的可以修改。例如

object Conf {
        val BASE_URL: String? = "https://api.github.com/"
    }

15、多了Any这个任意数据类型，字符串支持三个引号"""，也支持换行。例如

/*
    * kotlin对字符串的加强，三个引号"""中可以包含换行、反斜杠等等特殊字符
    * */
        fun testString() {
            val str1 = "abc"
            val str2 = """line1\n
            line2
            line3
            """
            val js = """
            function myFunction()
            {
                document.getElementById("demo").innerHTML="My First JavaScript Function";
            }
            """.trimIndent()
            println(str1)
            println(str2)
            println(js)
        }

16、单例模式。可以这样写。

package com.tandong.kotlin.utils
    
    import android.util.Log
    
    /**
     * Created by Tandong on 2018/1/19.
     */
    class TestInstance private constructor() {
    
        init {
            Log.i("info", "构造方法")
        }
    
        private object SingletonHolder {
            val instance = TestInstance()
        }
    
        fun method() {
            Log.i("info", "构造方法SingletonInner")
        }
    
        companion object {
    
            val instance: TestInstance
                @Synchronized get() = SingletonHolder.instance
        }
    }

调用时候这样声明调用。

private var testInstance: TestInstance? = null
    ...
    testInstance = TestInstance.instance;
    testInstance!!.method();
    ...

好了，我总结的关键的大概这么多。都是实际实践操作中总结的比较突出的。

下面我粘贴部分我的写的例子的代码。完整的去Github上看。

Android Studio或者IntelliJ IDEA创建是选择Kotlin的，当然如果Android也可以，只不过需要自己手动在build.gradle等文件里添加一些Kotlin配置库。

先看BaseApplication.kt，很简单，里面其他逻辑没写。

package com.tandong.kotlin.base
    
    import android.app.Application
    
    /**
     * Created by Tandong on 2018/1/17.
     */
    class BaseApplication : Application() {
    
        override fun onCreate() {
            super.onCreate()
        }
    }

BaseActivity.kt

package com.tandong.kotlin.base
    
    import android.app.Activity
    import android.content.Context
    import android.os.Bundle
    import android.support.v7.app.AppCompatActivity
    import android.widget.Toast
    
    /**
     * Created by Tandong on 2018/1/15.
     */
    open class BaseActivity : AppCompatActivity() {
    
        override fun onCreate(savedInstanceState: Bundle?) {
            super.onCreate(savedInstanceState)
        }
    
        fun showToast(context: Context, text: String) {
            Toast.makeText(context, text, Toast.LENGTH_SHORT).show();
        }
    
        /*
    * show toast in activity
    * */
        fun Activity.toast(msg: String){
            Toast.makeText(this, msg, Toast.LENGTH_SHORT).show()
        }
    
        override fun onResume() {
            super.onResume()
        }
    
    }

MainActivity.kt，里面接口网络请求数据使用了Retrofit和Rxjava

package com.tandong.kotlin.ui
    
    import android.content.Intent
    import android.os.Bundle
    import android.util.Log
    import android.view.View
    import com.bumptech.glide.Glide
    import com.tandong.kotlin.R
    import com.tandong.kotlin.base.BaseActivity
    import com.tandong.kotlin.entity.User
    import com.tandong.kotlin.net.ApiClient
    import com.tandong.kotlin.net.ResultListener
    import com.tandong.kotlin.net.ResultObserver
    import com.tandong.kotlin.utils.Utils
    import kotlinx.android.synthetic.main.activity_main.*
    import retrofit2.Call
    
    class MainActivity : BaseActivity(), View.OnClickListener {
        private var users: Call<List<User>>? = null
        private var userList: List<User>? = null
    
        override fun onCreate(savedInstanceState: Bundle?) {
            super.onCreate(savedInstanceState)
            setContentView(R.layout.activity_main)
            initView();
        }
    
        private fun initView() {
            var utils = Utils();
            utils.foo(this);
            tv_tips.setText("测试控件");
            utils.testString3();
            utils.testApply(this);
            utils.getPoint('B');
            Glide.with(this).load("https://www.baidu.com/img/bd_logo1.png").into(iv_img);
            Log.i("info", "测试输出Log");
            runOnUiThread(Runnable {
                kotlin.run {
                    toast("测试弹出提示")
                }
            })
            tv_getdata.setOnClickListener(this);
            tv_intent.setOnClickListener(this)
            Utils().foo(this);
            ApiClient().getListRepo("1", ResultObserver(object : ResultListener<List<User>> {
                override fun complete(t: List<User>) {
                    showToast(this@MainActivity, t.size.toString() + "  " + t.get(0).full_name)
                }
    
                override fun onError(e: Throwable) {
    
                }
    
            }));
        }
    
        override fun onClick(v: View?) {
            when (v!!.id) {
                R.id.tv_getdata -> {
                    getData()
                }
                R.id.tv_intent -> {
                    var intent = Intent(this@MainActivity, SencondActivity::class.java);
                    intent.putExtra("name", "名字")
                    intent.putExtra("id", 12)
                    startActivity(intent)
                }
            }
        }
    
        fun getData() {
            Thread(Runnable {
                users = ApiClient().getListRepo("1");
                userList = users!!.execute().body();
                for (i in userList!!.indices) {
                    Log.i("info", "用户：" + userList!!.get(i).full_name);
                }
            }).start();
        }
    }

SecondActivity.kt

package com.tandong.kotlin.ui
    
    import android.os.Bundle
    import android.support.v7.app.AppCompatActivity
    import android.support.v7.widget.LinearLayoutManager
    import android.support.v7.widget.OrientationHelper
    import com.tandong.kotlin.R
    import com.tandong.kotlin.adapter.ListAdapter
    import kotlinx.android.synthetic.main.activity_second.*
    
    
    class SencondActivity : AppCompatActivity() {
        private var name: String? = null
        private var id: Int? = null
        private var adapter: ListAdapter? = null
        private var layoutManager: LinearLayoutManager? = null
        private var list: ArrayList<String>? = null
    
        override fun onCreate(savedInstanceState: Bundle?) {
            super.onCreate(savedInstanceState)
            setContentView(R.layout.activity_second)
            initView()
        }
    
        private fun initView() {
            name = intent.getStringExtra("name")
            id = intent.getIntExtra("id", 1)
            tv_intent.setText(name + " " + id)
            layoutManager = LinearLayoutManager(this)
            rv.setLayoutManager(layoutManager)
            layoutManager!!.orientation = OrientationHelper.VERTICAL
            list = ArrayList<String>()
            for (i in 0..9) {
                list!!.add("" + i)
            }
            adapter = ListAdapter(this@SencondActivity, list!!);
            rv.setAdapter(adapter)
        }
    }

ListAdapter.kt

package com.tandong.kotlin.adapter
    
    import android.content.Context
    import android.support.v7.widget.RecyclerView
    import android.support.v7.widget.RecyclerView.ViewHolder
    import android.view.LayoutInflater
    import android.view.View
    import android.view.ViewGroup
    import android.widget.TextView
    import com.tandong.kotlin.R
    
    
    /**
     * Created by Tandong on 2018/1/17.
     */
    class ListAdapter(private val mContext: Context, private val mDatas: List<String>) : RecyclerView.Adapter<ListAdapter.MyViewHolder>() {
        private var inflater: LayoutInflater? = null
    
        override fun getItemCount(): Int {
            return mDatas!!.size
        }
    
        override fun onBindViewHolder(holder: MyViewHolder, position: Int) {
            holder.tv.text = mDatas!![position]
        }
    
        override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
            inflater = LayoutInflater.from(mContext)
            val view = inflater!!.inflate(R.layout.item_list, parent, false)
            return MyViewHolder(view)
        }
    
        class MyViewHolder(view: View) : ViewHolder(view) {
            var tv: TextView
    
            init {
                tv = view.findViewById(R.id.tv_text)
            }
        }
    }

Conf.kt

package com.tandong.kotlin.base
    
    /**
     * Created by Tandong on 2018/1/17.
     */
    object Conf {
        val BASE_URL: String? = "https://api.github.com/"
    }

LoadingView.kt

package com.tandong.kotlin.views
    
    import android.app.Dialog
    import android.content.Context
    import android.content.DialogInterface
    import android.view.Gravity
    import android.view.WindowManager
    import com.tandong.kotlin.R
    
    /**
     * Created by Tandong on 2017/7/19.
     */
    class LoadingView : Dialog {
    
        constructor(context: Context) : super(context) {
            init(context)
        }
    
        constructor(context: Context, themeResId: Int) : super(context, R.style.Loading) {
            init(context)
        }
    
        protected constructor(context: Context, cancelable: Boolean, cancelListener: DialogInterface.OnCancelListener) : super(context, cancelable, cancelListener) {
            init(context)
        }
    
        private fun init(context: Context) {
            setContentView(R.layout.layout_loading)
            setCanceledOnTouchOutside(false)
            val lp = window!!.attributes
            lp.width = WindowManager.LayoutParams.WRAP_CONTENT
            lp.height = WindowManager.LayoutParams.WRAP_CONTENT
            lp.gravity = Gravity.CENTER
            lp.dimAmount = 0f
            window!!.attributes = lp
        }
    }

ApiClient.kt，retrofit和rxjava写法

package com.tandong.kotlin.net
    
    import com.tandong.kotlin.base.Conf
    import com.tandong.kotlin.entity.User
    import io.reactivex.Observable
    import io.reactivex.Observer
    import io.reactivex.android.schedulers.AndroidSchedulers
    import io.reactivex.schedulers.Schedulers
    import retrofit2.Call
    import retrofit2.Retrofit
    import retrofit2.adapter.rxjava2.RxJava2CallAdapterFactory
    import retrofit2.converter.gson.GsonConverterFactory
    
    /**
     * Created by Tandong on 2018/1/15.
     */
    class ApiClient constructor() {
        private var apiservice: ApiService? = null;
        private var retrofit: Retrofit? = null;
    
        init {
            retrofit = Retrofit.Builder().baseUrl(Conf.BASE_URL)
                    .addConverterFactory(GsonConverterFactory.create())
                    .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
                    .build();
            apiservice = retrofit!!.create(ApiService::class.java)
        }
    
        fun getListRepo(id: String): Call<List<User>> {
            return apiservice!!.listRepos(id);
        }
    
        private fun <T> subscribeOnobserveOn(observable: Observable<T>, observer: Observer<T>) {
            observable.subscribeOn(Schedulers.io()).observeOn(AndroidSchedulers.mainThread()).subscribe(observer)
        }
    
        fun getListRepo(id: String, resultObserver: ResultObserver<List<User>>) {
            subscribeOnobserveOn(apiservice!!.getUserList(id), resultObserver)
        }
    
    }

ApiService.kt写法

package com.tandong.kotlin.net
    
    import com.tandong.kotlin.entity.User
    import io.reactivex.Observable
    import retrofit2.Call
    import retrofit2.http.GET
    import retrofit2.http.Path
    
    /**
     * Created by Tandong on 2018/1/17.
     */
    interface ApiService {
    
        @GET("users/{user}/repos")
        fun listRepos(@Path("user") user: String): Call<List<User>>
    
        @GET("users/{user}/repos")
        fun getUserList(@Path("user") user: String): Observable<List<User>>
    }

Utils.kt，这里面的测试例子借用的别人的

package com.tandong.kotlin.utils
    
    import android.app.Activity
    import android.content.Context
    import android.content.pm.PackageInfo
    import android.content.pm.PackageManager
    import android.widget.ImageView
    import android.widget.TextView
    import android.widget.Toast
    
    /**
     * Created by Tandong on 2018/1/16.
     */
    class Utils {
    
        fun foo(context: Context) {
            Toast.makeText(context, "文本", Toast.LENGTH_LONG).show();
            print("成员函数Foo")
        } // 成员函数
    
        fun demo(x: Any) {
            if (x is String) {
                print(x.length) // x 自动转换为字符串
            }
        }
    
        /*
    * kotlin对字符串的加强，三个引号"""中可以包含换行、反斜杠等等特殊字符
    * */
        fun testString() {
            val str1 = "abc"
            val str2 = """line1\n
            line2
            line3
            """
            val js = """
            function myFunction()
            {
                document.getElementById("demo").innerHTML="My First JavaScript Function";
            }
            """.trimIndent()
            println(str1)
            println(str2)
            println(js)
        }
    
    
        /*
        * kotlin字符串模版，可以用$符号拼接变量和表达式
        * */
        fun testString2() {
            val strings = arrayListOf("abc", "efd", "gfg")
            println("First content is $strings")
            println("First content is ${strings[0]}")
            println("First content is ${if (strings.size > 0) strings[0] else "null"}")
        }
    
        /*
        *Kotlin中，美元符号$是特殊字符，在字符串中不能直接显示，必须经过转义，方法1是用反斜杠，方法二是${'$'}
        * */
        fun testString3() {
            println("First content is \$strings")
            println("First content is ${'$'}strings")
        }
    
        /*
    * 用apply语句简化类的初始化，在类实例化的时候，就可以通过apply把需要初始化的步骤全部实现，非常的简洁
    * */
        fun testApply(context: Context) {
            var imgView = ImageView(context).apply {
                setBackgroundColor(0)
                setImageBitmap(null)
            }
    
            var textView = TextView(context).apply {
                text = "content"
                textSize = 20.0f
                setPadding(10, 0, 0, 0)
            }
        }
    
        fun test01() {
            val list = listOf(2, 5, 10)
            /*
            * 传人函数来过滤
            * */
            println(list.filter { it > 4 })
        }
    
        /*
    * kotlin中，when是表达式，可以取代Java 中的switch，when的每个分支的最后一行为当前分支的值
    * */
        fun getPoint(grade: Char) = when (grade) {
            'A' -> "GOOD"
            'B', 'C' -> {
                println("test when")
                "OK"
            }
            'D' -> "BAD"
            else -> "UN_KNOW"
        }
    
        /*
    * show toast in activity
    * */
        fun Activity.toast(msg: String) {
            Toast.makeText(this, msg, Toast.LENGTH_SHORT).show()
        }
    
        /**
         * 获取版本号VersionCode
         */
        fun getVersionCode(context: Context): Int {
            val packageManager = context.packageManager
            val packageInfo: PackageInfo
            var versionCode = ""
            try {
                packageInfo = packageManager.getPackageInfo(context.packageName, 0)
                versionCode = packageInfo.versionCode.toString() + ""
            } catch (e: PackageManager.NameNotFoundException) {
                e.printStackTrace()
            }
            return Integer.parseInt(versionCode)
        }
    }

针对混淆的话，加入

-dontwarn kotlin.**

主要的就这些，完整KotlinDemo可以在Github上体验。 [https://github.com/jaychou2012/KotlinDemo][https_github.com_jaychou2012_KotlinDemo]

后续继续完善。

**参考文献：**

###### \[1\]Kotlin for Android - Kotlin Programming Language\[OL\].[http://kotlinlang.org/docs/reference/android-overview.html][http_kotlinlang.org_docs_reference_android-overview.html]，2018-1-17 ######

######     ######

[SouthEast]: /images/20220601/b3e38268292a42c7896c84f48bfa5c2e.png
[SouthEast 1]: /images/20220601/f63261ae87a44fdfad5277b7c4b92edd.png
[http_kotlinlang.org_docs_reference_android-overview.html]: http://kotlinlang.org/docs/reference/android-overview.html
[https_github.com_jaychou2012_KotlinDemo]: https://github.com/jaychou2012/KotlinDemo