Shulou(Shulou.com)06/02 Report--

This article mainly introduces "how to use Kotlin to develop Android applications". In daily operation, I believe many people have doubts about how to use Kotlin to develop Android applications. The editor consulted all kinds of materials and sorted out simple and easy-to-use methods of operation. I hope it will be helpful to answer the doubts of "how to use Kotlin to develop Android applications". Next, please follow the editor to study!

Configuration

Project gradle file

Apply plugin:' com.android.application' apply plugin:'kotlin-android' apply plugin:'kotlin-android-extensions' dependencies {classpath 'org.jetbrains.kotlin:kotlin-gradle-plugin:1.1.1'}

App Gradle file:

Compile 'org.jetbrains.kotlin:kotlin-stdlib:1.1.1' compile' org.jetbrains.anko:anko-sdk25:0.10.0-beta-1'// sdk15, sdk19, sdk21, sdk23 are also available compile 'org.jetbrains.anko:anko-appcompat-v7:0.10.0-beta-1'

Anko

Through the above configuration, you will find that there are anko dependencies introduced. Anko is a powerful library developed by JetBrains, when it comes to JetBrains, the Kotlin language is developed by them, intellij idea is developed by them, and AS is also based on IDEA. All right, back to the point, Anko is a Kotlin library officially developed by Kotlin that makes it faster and easier to develop Android applications, and makes the code we write simpler, clearer and easier to read. It includes several parts, as follows

Anko Commons: a lightweight library full of helpers for intents, dialogs, logging and so on; Anko Layouts: a fast and type-safe way to write dynamic Android layouts; Anko SQLite: a query DSL and parser collection for Android SQLite; Anko Coroutines: utilities based on the kotlinx.coroutines library

So next, we will understand the advantages of developing Android with Kotlin language through code.

No more findViewById.

Anyone who has done Android development knows that layout files are written too much, and findViewById is also a lot of work, and we have to declare variables in findViewById and then forcefully convert them into our controls. The general way to use them is as follows

TextView username; username= (TextView) findViewById (R.id.user); username.setText ("I am a TextView")

Sometimes when you write about vomiting, some people may say that there are no libraries with annotations, such as butterknife. When we use annotations, we can use findViewById instead, as follows

@ BindView (R.id.user) TextView username; username.setText ("I am a TextView")

Indeed, using annotations does save us some work, but it's still not the simplest. The simplest thing is that we can directly assign values to id's user controls, which may seem a little weird to you. But Kotlin did. We can just write like this.

User.text= "I am a TextView"

Do you have a feeling that it is too late to meet each other when you see this? it's too simple. User is the id,.text declared in our layout file, which we want to do with setText (). In the Kotlin language, we can't see the set/get method like Java. It is important to note that when we want to use it this way (instead of findViewById, we need to add apply plugin: 'kotlin-android-extensions' to gradle instead of xml control), we need to add the following code

/ / activity_login is our layout import kotlinx.android.synthetic.main.activity_login.*

Anko Layout

Usually we use xml files to write our layout, but it has some disadvantages such as not type-safe, not empty-safe, parsing xml files consuming more CPU and power, and so on. And Anko Layout can use DSL to dynamically create our UI, and it is much easier than we use Java to dynamically create layouts, mainly more concise, and it has a hierarchical relationship with xml to create layouts, which makes it easier for us to read.

VerticalLayout {val textView=textView ("I am a TextView") val name = editText ("EditText") button ("Button") {onClick {toast ("${name.text}!")}

We can remove setContentView in the OnCreate method, and then add the above code to show the effect of the following figure, that is, a vertical linear layout with a TextView, an EditText, and a Button. And there is a click event in Button, which will EditText the content when clicked.

Displayed in toast.

The above code is not very easy to understand, of course, the default controls can not meet our needs, for example, we will change the font color and size, will set width and height, will set margin, padding values, then how to implement, of course, very simple, because its logic and xml writing layout is a routine. For example, the following implementation

Val textView=textView ("I am a TextView") {textSize = sp (17). ToFloat () textColor=context.resources.getColor (R.color.red)} .lparams {margin=dip (10) height= dip (40) width= matchParent}

I don't think I need to explain the above code to see the effect of the control implementation. Because its property corresponds to the name of the property we set in xml.

In the process of creating the UI above, we wrote the code to create the UI directly in the onCreate method, and of course, there is another way to write it. We create an inner class that implements the AnkoComponent interface and overrides the createView method, which returns a View, the layout we created. Modify as follows

Inner class UI: AnkoComponent {override fun createView (ui: AnkoContext): View {return with (ui) {verticalLayout {val textView=textView ("I am a TextView") {textSize = sp (17) .toFloat () textColor=context.resources.getColor (R.color.red) .lparams {margin=dip (10) height= dip (40) width= matchParent} val name = editText ("EditText") button ("Button") {onClick {view-> Toast ("Hello" ${name.text}! ")}}

Then add a line of code to the onCreate method to create our layout page. As follows

UI () setContentView (this@LoginActivity)

Now that we compile and run, we find that the effect is the same as the interface written in the layout file. But its performance has an advantage, in fact, it did not find the performance advantage. In any case, this kind of DSL is really easy to read, but also easy to use, in the above code, you may have noticed dip (10), which means to convert 10dp into pixels, which is the extension function of Anko, that is, extension function. If you read the source code of Anko, we find that there are a lot of extension functions used, which is also one of the advantages of Kotlin language. It's really powerful, such as the dip extension (extract the View extension)

Inline fun View.dip (value: Int): Int = context.dip (value) fun Context.dip (value: Int): Int = (value * resources.displayMetrics.density) .toInt ()

Resources.displayMetrics.density and we Java getResources (). GetDisplayMetrics (). Density is the same effect, but see if you will feel much more comfortable than Java writing, anyway, I feel so.

In the above we added a click event to Button, and we found that it supports lambda expressions. We want to display a Toast, all we need is toast ("content"), isn't it very concise? In fact, it is also an extension function that implements the

Inline fun AnkoContext.toast (message: CharSequence) = ctx.toast (message) fun Context.toast (message: CharSequence) = Toast.makeText (this, message, Toast.LENGTH_SHORT). Show ()

Of course, creating a dialog is still very simple, as follows

Alert ("I am Dialog") {yesButton {toast ("yes")} noButton {toast ("no")}. Show ()

The more you look at it, the more comfortable you feel. Haha. Let's have a powerful and simple code implementation.

DoAsync {/ / background execution code uiThread {/ / UI thread toast ("thread ${Thread.currentThread (). Name}")}}

This code implements the effect of AsyncTask, but you should find that it is much simpler than the implementation of Java, unless you are color-blind, of course.

If you use Kotlin to develop Android for a period of time, you will find that it gives us a lot less code, of course, more advantages and usage need to be explored by ourselves. I believe it will surprise you after exploration.

Implement a simple login interface

The interface is simple, pseudo code.

It doesn't look complicated, so the code for the xml implementation will not be posted here. If you want to see the xml implementation can be seen and clicked, then just look at the Anko implementation of this layout in the Kotlin code.

Lateinit var et_account: EditText lateinit var et_password: EditText inner class LoginUi: AnkoComponent {override fun createView (ui: AnkoContext) = with (ui) {verticalLayout {backgroundColor = context.resources.getColor (android.R.color.white) gravity = Gravity.CENTER_HORIZONTAL imageView (R.mipmap.ic_launcher). Lparams { Width = dip height = dip topMargin = dip (64)} linearLayout {gravity = Gravity.CENTER_VERTICAL orientation = HORIZONTAL backgroundResource = R.drawable.bg_frame_corner ImageView {image = resources.getDrawable (R.mipmap.ic_username)} .lparams (width = wrapContent Height = wrapContent) {leftMargin = dip (12) rightMargin = dip (15)} et_account = editText {hint = "login account" hintTextColor = Color.parseColor ("# 666666") TextSize = 16f background = null} .lparams (width = dip) Height = dip (40)) {topMargin = dip (45)} linearLayout {orientation = HORIZONTAL backgroundResource = R.drawable.bg_frame_corner gravity = Gravity.CENTER_VERTICAL imageView { Image = resources.getDrawable (R.mipmap.ic_password)}. Lparams {leftMargin = dip (12) rightMargin = dip (15)} et_password = editText {hint = "login secret" Code "hintTextColor = Color.parseColor (" # 666666 ") textSize = 16f background = null}}. Lparams {width = dip (666666) height = dip (40) TopMargin = dip (10)} button ("login") {gravity = Gravity.CENTER background = resources.getDrawable (R.drawable.bg_login_btn) textColor = Color.parseColor ("# ffffff") onClick {if (et_account.text.toString (). IsNotEmpty () & & et_password.text.toString (). IsNotEmpty ()) startActivity () else toast ("Please enter your account or password")} .lparams (width = dip Height = dip (44) {topMargin = dip (18)} linearLayout {orientation = HORIZONTAL gravity = Gravity.CENTER_VERTICAL checkBox ("remember password") {textColor = Color.parseColor ("# 666666 ") textSize = 16f leftPadding = dip (5)} textView (Privacy Agreement) {textColor = Color.parseColor (" # 1783e3 ") gravity = Gravity.RIGHT TextSize = 16f} .lparams (width = matchParent)} .lparams (width = dip (300)) {topMargin = dip (18)} textView ("Copyright ©Code4Android") {textSize = 14f Gravity = Gravity.CENTER or Gravity.BOTTOM} .lparams {bottomMargin = dip (35) weight = 1f}

See how the above code looks, doesn't it? even if you can't write it now, you can still read it. Above we set an event to open MainActivity for the login button. What is written in startActivity is the Activity we want to jump to. If you pass parameters to the open interface, write it directly in (). For example, if we pass the entered account and password to the redirected interface, it will be implemented as

StartActivity ("account" to et_account.text.toString (), "password" to et_password.text.toString ()) ends the study of "how to use Kotlin to develop Android applications", hoping to solve everyone's doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.