实现功能：Activity之间的切换与传递值
MainActivity：
val greetingText = findViewById<TextView>(R.id.printer)val inputField = findViewById<EditText>(R.id.inputer)val submitButton = findViewById<Button>(R.id.btn)submitButton.setOnClickListener {val message = inputField.text.toString() //获取输入内容if (message == "") { //当输入内容为空//Toast为系统提示，第一个参数为Context(Activity亦为一种Context)，//第二个参数为提示文本，第三个参数为显示时长Toast.makeText(this,"You haven't entered anything!",Toast.LENGTH_SHORT).show()} else {greetingText.text = "Welcomed! " + messageinputField.text.clear()val intent = Intent(this, SecondActivity::class.java)intent.putExtra("USER", message) //向下一个Activity传递数据startActivity(intent)}}SecondActivity：
//获取上一个Activity传递的数据val name = intent.getStringExtra("USER")val textView = findViewById<TextView>(R.id.secTxt)textView.text = "This is the second activity!\nWelcome $name!"
