android学习笔记 1.Activity

Activity的创建

项目中的任何Activity都应重写onCreate()方法

class FirstActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
}

默认的实现调用父类onCreate()即可

所有Activity都要在AndroidMainfest.xml中注册

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

    <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=".FirstActivity"></activity>
    </application>

</manifest>

在application标签内通过</activity>注册，其中 .FirstActivity 省略了包名

配置主Activity

加入<intent-filter>标签

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

<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=".FirstActivity">
        android:label:="First Activity">
        <intent-filter>
            <action android:name="android.intent.action.MAIN"/>
            <category android:name="android.intent.category.LAUNCHER"/>
        </intent-filter>
    </activity>
</application>

</manifest>

可以用label属性设置标题栏内容

在Activity中使用Toast

class FirstActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.first_layout)
        button1.setOnClickListener {
            Toast.makeText(this, "you are ugly", Toast.LENGTH_SHORT).show()
        }

    }
}

this为上下文，传入当前Activity。Toast.LENGTH_SHORT为显示时长。获得Toast对象调用show()方法显示出来

在Activity中使用Menu

res目录下新建menu文件夹，创建menu resource file

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
    <item
        android:id="@+id/add_item"
        android:title="Add"/>
    <item
        android:id="@+id/remove_item"
        android:title="Remove"/>
</menu>

使用<item 添加菜单项，使用title属性指定名称

为了使用menu，在Activity中重写onCreateOptionsMenu方法

 override fun onCreateOptionsMenu(menu: Menu?): Boolean {
        menuInflater.inflate(R.menu.main, menu)
        return true;
    }

使用R.menu.main资源文件添加到menu对象

为了让菜单响应事件，重写onOptionsItemSelected

override fun onOptionsItemSelected(item: MenuItem): Boolean {
        when(item.itemId){
            R.id.add_item -> Toast.makeText(this, "sb", Toast.LENGTH_SHORT).show()
            R.id.remove_item -> Toast.makeText(this,"hah",Toast.LENGTH_SHORT).show()
        }
        return super.onOptionsItemSelected(item)
    }

使用finish()销毁Activity

布局文件的加载

在res/layout目录下创建Activity的布局文件，布局编写完成后须在Activity的onCreate中加载

class FirstActivity : AppCompatActivity() {

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

setContentView(R.layout.first_layout)加载布局文件

使用intent切换Activity
1.显式intent

button1.setOnClickListener {
            Toast.makeText(this, "you are ugly", Toast.LENGTH_SHORT).show()
            val intent = Intent(this, SecondActivity::class.java)
            startActivity(intent)
        }

2.隐式intent

指定action和category，交由系统找出合适的activity启动

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

向下一个activity传递数据

通过putExtra将数据暂存intent中

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

接收

class SecondActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_second)
        val data = intent.getStringExtra("data")
        Toast.makeText(this, "data is $data", Toast.LENGTH_SHORT).show()
    }
}

返回数据给上一个Activity

使用startActivityForResult

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

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

 override fun onActivityResult(requestCode:Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)
        when(requestCode){
            1 -> if(resultCode == Activity.RESULT_OK){
                val returned_data = data?.getStringExtra("data_return")
                Toast.makeText(this, "$returned_data", Toast.LENGTH_SHORT).show()
            }
        }
    }