xml代码：

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="@color/bg_gray"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="意见反馈"
        android:textColor="#000000"
        android:textSize="18sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/et_content"
        android:layout_width="match_parent"
        android:layout_height="130dp"
        android:background="@drawable/shape_et_bg"
        android:gravity="left|top"
        android:hint="说点什么吧~我们会做得更好~"
        android:maxLength="200"
        android:padding="8dp"
        android:textSize="15sp"
        android:layout_margin="10dp"
        app:layout_constraintTop_toBottomOf="@+id/textView"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        />

    <TextView
        android:id="@+id/text_count"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="right"
        android:text="0/200"
        app:layout_constraintBottom_toBottomOf="@+id/et_content"
        app:layout_constraintEnd_toEndOf="@+id/et_content" />


    <Button
        android:id="@+id/btn_submit"
        android:layout_width="match_parent"
        android:layout_height="45dp"
        android:text="提交"
        android:textColor="#FFFFFF"
        android:textSize="16sp"
        android:layout_margin="30dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/et_content" />

</androidx.constraintlayout.widget.ConstraintLayout>

activity代码：

class MainActivity : AppCompatActivity() {
    @SuppressLint("MissingInflatedId", "ResourceType")
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val edtext = findViewById<EditText>(R.id.et_content)
        val tvtext = findViewById<TextView>(R.id.text_count)

        edtext.addTextChangedListener {
//            Log.d("wang","===>/${it!!.length}/$it")
            tvtext.text= it!!.length.toString()+"/200"
        }
    }
}