Learning Android II from scratch - activity

Basic knowledge

• Any... In the project Activity All should be rewrite onCreate() Method , And now our FirstActivity This method has been overridden in , This is a Android Studio It's automatic

• Android The design of program stresses the separation of logic and view , It's better for each one Activity Can correspond to a layout . Layout is used to display the content of the interface .

• Check Generate Layout File Indicates that it will automatically be FirstActivity Create a corresponding Layout file , Check Launcher Activity Indicates that the FirstActivity Set as... Of the current project Lord Activity

• Destroy one activity,finish() Method

• startActivity() Method , Used to start Activity

• Android Medium Activity It can be layered . Every time we start one new Activity, Will cover the original Activity above , And then click Back The key will destroy the top Activity, Below One Activity It will reappear .

• activity Four kinds of state ： function 、 Pause 、 stop it 、 The destruction

button

activity in ：

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


        val button1: Button = findViewById<Button>(R.id.button1)
        button1.setOnClickListener {
    
            Toast.makeText(this, "You clicked Button 1", Toast.LENGTH_SHORT).show()
        }
    }

• Toast yes Android A very good reminder provided by the system Toast.makeText(this, "You clicked Button 1", Toast.LENGTH_SHORT).show() The first parameter is Context, That is to say Toast Required context , because Activity Is itself a Context object , therefore It's coming in directly here this that will do . The second parameter is Toast Displayed text content . The third parameter is Toast When displayed Long , There are two built-in constants to choose from :Toast.LENGTH_SHORT and Toast.LENGTH_LONG.
• **setOnClickListener()** Method register a listener for the button , When you click the button, the onClick() Method .button.setOnClickListener {}

menu

• res New under the directory menu Catalog
• rewrite onCreateOptionsMenu() Method
• rewrite onOptionsItemSelected() Method

Intent（ Jump 、 To transfer data ）

• Show intent
  Intent Overload with multiple constructors , One of them is Intent(Context packageContext, Class<? > cls). This constructor takes two arguments : The first parameter Context Ask for a startup Activity Up and down writing ; The second parameter Class Used to specify the target you want to start Activity, With this constructor, you can build Intent Of “ Intention ”.

val intent = Intent(this, SecondActivity::class.java)
startActivity(intent)

• Implicit Intent

button1.setOnClickListener {
    
    val intent = Intent("com.example.activitytest.ACTION_START")
    intent.addCategory("com.example.activitytest.MY_CATEGORY")
    startActivity(intent)
}

<activity android:name=".SecondActivity" >
    <intent-filter>
        <action android:name="com.example.activitytest.ACTION_START" />
        <category android:name="android.intent.category.DEFAULT" />
        <category android:name="com.example.activitytest.MY_CATEGORY"/>
    </intent-filter>
</activity>

• Show a web page

button1.setOnClickListener {
    
    val intent = Intent(Intent.ACTION_VIEW)
    intent.data = Uri.parse("https://www.baidu.com")
    startActivity(intent)
}

• Call dialing

button1.setOnClickListener {
    
    val intent = Intent(Intent.ACTION_DIAL)
    intent.data = Uri.parse("tel:10086")
    startActivity(intent)
}

• Next Activity To transfer data

button1.setOnClickListener {
    
    val data = "Hello SecondActivity"
    val intent = Intent(this, SecondActivity::class.java)
    intent.putExtra("extra_data", data)
    startActivity(intent)
}

val extraData = intent.getStringExtra("extra_data")
Log.d("SecondActivity", "extra data is $extraData")

• Returns data to the previous Activity,startActivityForResult(intent, 1) Method

button1.setOnClickListener {
    
    val intent = Intent(this, SecondActivity::class.java)
    startActivityForResult(intent, 1)
}

override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
    
    super.onActivityResult(requestCode, resultCode, data)
    when (requestCode) {
    
        1 -> if (resultCode == RESULT_OK) {
    
            val returnedData = data?.getStringExtra("data_return")
            Log.d("FirstActivity", "returned data is $returnedData")
} }
}

button2.setOnClickListener {
    
	val intent = Intent()
	intent.putExtra("data_return", "Hello FirstActivity")
	setResult(RESULT_OK, intent)
	finish()
}

override fun onBackPressed() {
    
    val intent = Intent()
    intent.putExtra("data_return", "Hello FirstActivity")
    setResult(RESULT_OK, intent)
    finish()
}

activity The survival period of 7 Callback methods

• onCreate(). It will be Activity Called the first time it is created . In this method Activity Of Initialization operation , Such as loading layout 、 Binding events, etc .
• onStart(). This method is Activity Call when it changes from invisible to visible .
• onResume(). This method is Activity Called when you are ready to interact with the user . At this time Activity One Locate at the top of the return stack , And in operation .
• onPause(). This method is used when the system is ready to Start or restore another Activity Called when . We usually Will consume some... In this method CPU Resources released , And save some key data , But the execution speed of this method must be fast , Otherwise, it will affect the new stack top Activity Use .
• onStop(). This method is Activity Called when it is completely invisible . It and onPause() The main area of the method Don't lie in , If you start new Activity It's a dialog like Activity, that onPause() The method will be implemented That's ok , and onStop() Method does not execute .
• onDestroy(). This method is Activity By Call before destruction , after Activity The state of will change to destroyed state .
• onRestart(). This method is Activity The call is made from the stop state to the running state , That is to say Activity Has been rebooted .

3 Species survival time

• Complete life span .Activity stay onCreate() Methods and onDestroy() Method What you're going through is complete survival . In general , One Activity Will be in onCreate() Method to complete various initialization operations , And in the onDestroy() Method to release memory .
• Visible lifetime .Activity stay onStart() Methods and onStop() Method What you're going through is visible survival . Within the visible lifetime ,Activity Always visible to users , Even though it may not be possible to interact with users . I Through these two methods, we can reasonably manage the resources visible to users . For example onStart() Method to load resources , And in the onStop() Method to release resources , So as to ensure that it is in a stopped state Activity It won't take up too much memory .
• Front desk lifetime .Activity stay onResume() Methods and onPause() Method What we're going through is the life span of the front desk . Within the life of the front desk ,Activity Always running , At this time Activity It can interact with users , What we usually see and contact most is in this state Activity.

Activity Best practices

• Know which one you are now Activity, There is no need to let BaseActivity stay AndroidManifest.xml Register in , So choose to create a normal Kotlin Class 了 . And then let BaseActivity Inherited from AppCompatActivity, And rewrite onCreate() Method . Give Way BaseActivity Become ActivityTest All... In the project Activity Parent class of , because BaseActivity It's inherited from AppCompatActivity Of , So all in the project Activity The existing functionality of is not affected , They still inherit Activity All properties in .

open class BaseActivity : AppCompatActivity() {
    
    override fun onCreate(savedInstanceState: Bundle?) {
    
        super.onCreate(savedInstanceState)
        Log.d("BaseActivity", javaClass.simpleName)
} }

• sign out activity,controller class
• To transfer data

companion object {
    
	fun actionStart(context: Context, data1: String, data2: String) {
    
	val intent = Intent(context, SecondActivity::class.java)
	intent.putExtra("param1", data1)
	intent.putExtra("param2", data2)
	context.startActivity(intent)
} }

Activity2.actionStart(this, "data1", "data2")