Learning Android from scratch -- Introduction

Reference books ： First line of code Android

3.18 Introduction to project structure

app Code in project 、 Resources and other contents are placed in this directory , Our later development work is basically carried out under this directory .

• libs If you use a third party in your project jar package , That's what we need to do jar Bags are all placed in libs Under the table of contents , In this directory jar The package is automatically added to the project's build path .
• java The catalog is where we put all our Java Where to code (Kotlin The code is also here ), Expand the Catalog , You will see that the system automatically generates a MainActivity file .
• res There are a lot of contents in this directory . To put it simply , It's all the pictures you use in the project 、 Layout 、 word String and other resources should be stored in this directory . Of course, there are many subdirectories in this directory , Picture on drawable Under the table of contents , Layout in layout Under the table of contents , String in values Under the table of contents , So you don't have to worry about putting the whole res The catalogue is in a mess .
• AndroidManifest.xml
  This is the whole thing Android The configuration file for the project , All four components you define in the program need to be noted in this file book , You can also add a permission declaration to the application in this file .

Android- Manifest.xml file
To register , Not in the AndroidManifest.xml Registered in Activity It can't be used

<activity android:name=".MainActivity">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>

Write the interface in the layout file , And then in Activity To bring in , Layout files are defined in res/layout In the catalog

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

res Catalog ： Put all the pictures in drawable-xxhdpi It's just in the catalog , Because this is the most popular device resolution Directory .

• all With “drawable” The first directory is used to put pictures
• All with “mipmap” The first directory is used to put application icons
• All with “values” The first directory is used to put strings 、 style 、 Color, etc
• all With “layout” The first directory is used to put layout files .
  

In the code through R.string.app_name You can get a reference to the string .
stay XML Pass through @string/app_name You can get a reference to the string .

build.gradle in ：dependencies Closure
This closure is very powerful , It can specify all dependencies of the current project . Usually Android Studio There are 3 Ways of dependence : Local dependence 、 Library dependency and remote dependency . Local dependence can be on local jar Package or directory add dependency , Library dependencies can add dependencies to library modules in a project , Remote dependencies can jcenter Open source projects on the warehouse Add dependencies .

3.19 Logging tools

• Log.v(). For printing the most trivial 、 Least significant log information . Corresponding level verbose, yes Android The lowest level in the log .
• Log.d(). Used to print some debugging information , This information should be helpful for debugging programs and analyzing problems . Corresponding level debug, Than verbose Higher level .
• Log.i(). Used to print some important data , These data should be what you really want to see 、 I can divide it for you Analyze the data of user behavior . Corresponding level info, Than debug Higher level .
• Log.w(). Used to print some warning messages , Tip programs may have potential risks in this place , You'd better fix it Repeat these warnings . Corresponding level warn, Than info Higher level .
• Log.e(). Used to print error messages in programs , For example, the program enters catch In the sentence . When there is an error message, type When printed , Generally speaking, there is a serious problem with your program , It must be repaired as soon as possible . Corresponding level error, Than warn Higher level .

//example
class MainActivity : AppCompatActivity() {
    
    override fun onCreate(savedInstanceState: Bundle?) {
    
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        Log.d("MainActivity", "onCreate execute")
} }

Log.d() Method passed in two parameters : The first parameter is tag, It's usually better to pass in the current class name , Mainly used in relation to Print information for filtering ; The second parameter is msg, That is, the specific content you want to print .
Don't use println() Why ： The log switch is not controllable 、 Cannot add log tag 、 There is no level difference between logs …

3.20 kotlin introduction

Type declaration

• val Constant
• var Variable

val a: Int = 10

Function definition

fun main() {
    
    val a = 10
    println("a = " + a)
}

fun methodName(param1: Int, param2: Int): Int {
    
    return 0
}

// One line of code 
fun largerNumber(num1: Int, num2: Int) = if (num1 > num2) num1 else num2

//when
fun getScore(name: String) = when (name) {
    
    "Tom" -> 86
    "Jim" -> 77
    "Jack" -> 95
    "Lily" -> 100
    else -> 0
}

//is keyword 
fun checkNumber(num: Number) {
    
    when (num) {
    
        is Int -> println("number is Int")
        is Double -> println("number is Double")
        else -> println("number not support")
} }

Range val range = 0…10 Closed interval [0,10]

for (i in 0..10) {
    
	println(i) 
}

for (i in 10 downTo 1) {
    
	println(i) 
}

Initialization list 、 aggregate 、Map

//listof() Construct immutable list 
val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape")  
for (fruit in list) {
    
	println(fruit)
    }

//mutableListof() Construct variable list 
var list = mutableListOf("Apple", "Banana", "Orange", "Pear", "Grape")  
list.add("Watermelon")
for (fruit in list) {
    
	println(fruit)
	}
//lambda
val maxLengthFruit = list.maxBy {
     it.length }
//lambda
val newList = list.map {
     it.toUpperCase() }  //map The function is very powerful , It can arbitrarily map and transform the elements in the collection according to our needs , Top only   It's just a simple example . besides , You can also convert all fruit names to lowercase , Or just take the first word   Letter , Even a set of numbers converted into word length 

// aggregate 
val set = setOf("Apple", "Banana", "Orange", "Pear", "Grape")

//map
// Method 1
val map = HashMap<String, Int>()
map["Apple"] = 1
map["Banana"] = 2
map["Orange"] = 3
map["Pear"] = 4
map["Grape"] = 5
// Method 2
val map = mapOf("Apple" to 1, "Banana" to 2, "Orange" to 3, "Pear" to 4, "Grape" to 5)
//map Traverse 
for ((fruit, number) in map) {
    
        println("fruit is " + fruit + ", number is " + number)
}

Functional expression API

//filter function  
val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")
val newList = list.filter {
     it.length <= 5 }
                  .map {
     it.toUpperCase() }

//any,all function 
val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")
val anyResult = list.any {
     it.length <= 5 }  //true
val allResult = list.all {
     it.length <= 5 }  //false

// Register click events for buttons 
button.setOnClickListener {
    
}

Nullable type
Add a question mark after the class name . such as ,Int surface Indicates a non nullable integer , and Int? Represents an integer that can be null ;String Represents a non nullable string , and String? Represents an empty string .

// Equivalent to making judgment 
fun doStudy(study: Study?) {
    
    study?.readBooks()
    study?.doHomework()
}

val c = a ?: b

//let
fun doStudy(study: Study?) {
    
    study?.let {
     stu ->
        stu.readBooks()
        stu.doHomework()
    }
}
// improvement 
fun doStudy(study: Study?) {
    
    study?.let {
    
        it.readBooks()
        it.doHomework()
    }
}

String inline expression
${}, When there is only one variable in the expression , You can also omit curly braces on both sides .