TextView in ScrollView not updating

Question

I create textView in scrollView. Every time i setText the textView, the text not updating, but when i open the keyboard then close it, the text updated.. after googling, i got the solution is call textView.invalidate() and textView.requestLayout(). But i am currious why it's not updating without call invalidate and requestLayout? Is scrollView has somethis 'special' so i need to call invalidate and requestLayout?

here is the code

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    tools:context="com.zihadrizkyef.belajarinternalstorage.MainActivity"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="16dp"
    android:orientation="vertical">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="8"
        android:orientation="vertical">

        <EditText
            android:id="@+id/etWrite"
            android:layout_width="match_parent"
            android:layout_height="0dp"
            android:layout_weight="5"
            android:gravity="top"
            android:hint="write text here"/>
        <View android:id="@+id/separator1"
              android:layout_width="match_parent"
              android:layout_height="1px"
              android:layout_marginTop="20dp"
              android:layout_marginBottom="20dp"
              android:background="#aaa"/>
        <ScrollView
            android:layout_width="match_parent"
            android:layout_height="0dp"
            android:layout_weight="1">
            <TextView
                android:id="@+id/tvRead"
                android:layout_width="match_parent"
                android:layout_height="0dp"
                android:layout_weight="1"
                android:gravity="center"
                android:hint="(no text)"/>
        </ScrollView>
        <View android:id="@+id/separator2"
              android:layout_width="match_parent"
              android:layout_height="1px"
              android:layout_marginTop="20dp"
              android:layout_marginBottom="20dp"
              android:background="#aaa"/>

    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:orientation="horizontal"
        android:layout_weight="2"
        android:gravity="center">

        <Button
            android:id="@+id/btnSave"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginRight="10dp"
            android:onClick="onClick"
            android:text="save"/>
        <Button
            android:id="@+id/btnLoad"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="10dp"
            android:onClick="onClick"
            android:text="load"/>
    </LinearLayout>
</LinearLayout>

MainActivity.java

package com.zihadrizkyef.belajarinternalstorage;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

public class MainActivity extends AppCompatActivity {
    private final String FNAME = "mydata";
    EditText etWrite;
    TextView tvRead;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        etWrite = (EditText)findViewById(R.id.etWrite);
        tvRead = (TextView)findViewById(R.id.tvRead);
    }


    public void onClick(View v) {
        switch(v.getId()) {
            case R.id.btnSave:
                String data = etWrite.getText().toString();
                try {
                    FileOutputStream fOut = openFileOutput(FNAME, MODE_PRIVATE);
                    fOut.write(data.getBytes());
                    fOut.close();
                    Toast.makeText(MainActivity.this, "File saved", Toast.LENGTH_SHORT).show();
                } catch (FileNotFoundException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            break;
            case R.id.btnLoad:
                int cRead;
                String read="";

                try {
                    FileInputStream fIn = openFileInput(FNAME);
                    while((cRead=fIn.read())!=-1) {
                        read += Character.toString((char)cRead);
                    }
                    tvRead.setText(read);
                    tvRead.invalidate();
                    tvRead.requestLayout();
                    Toast.makeText(MainActivity.this, "File loaded", Toast.LENGTH_SHORT).show();
                } catch (FileNotFoundException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            break;
        }
    }
}

github : https://github.com/zihadrizkyef/FileWriteRead_InternalStorage

Answer 1

You can try updating the value using AsyncTask. As the method onPostExecute() runs in UI thread, it can make any kind of UI updates dynamically. Refer to this for more info:https://developer.android.com/reference/android/os/AsyncTask.html

Answer 2

you've set ScrollView's height and TextView's height to "0" and don't have any text set for TextView,so you have to invalidate your view in order to measure its layout parameters again.

you can use this link: Making TextView scrollable on Android

Answer 3

Its obvious, Whenever you change the data for a control it doesn't get reflected in the view, unless the control or its parent control or view is invalidated.

Its also mentioned in documentation of TextView https://developer.android.com/reference/android/widget/TextView.html#setText(char[], int, int)