单击一个按钮，TextView不会更新

Question

This, being my first attempt, may look naive. I have the activity_main.xml as below:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.buch.pranav.andy.hisabkitab.MainActivity" >

<Button
    android:id="@+id/button1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="30dp"
    android:onClick="clickHandler"
    android:text="@string/connect" />

<TextView
    android:id="@+id/textView2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/button1"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="65dp"
    android:text="@string/testVal2"
    android:textAppearance="?android:attr/textAppearanceMedium" />

</RelativeLayout>

and my MainActivity.java would look like this:

public class MainActivity extends ActionBarActivity {

//Auto generated code here

public void clickHandler(View view){
    View header = (View)getLayoutInflater().inflate(R.layout.activity_main, null);
    TextView tv = (TextView) header.findViewById(R.id.textView2);
    System.out.println("tv = "+tv.getText());
    tv.setText("lol");
}

//Auto generated code here 
}

Now when I click the button, I see the sysout in logcat window so there are no issues with the binding of clickHandler, but the textview on my app screen will not update! Logcat doesn't even show any exception being thrown.

What am I missing here?

Answer 1

As far as I can see you use the wrong layout because you're inflating it inside of your click-handle instead of using the layout which you should have inflated in "onCreate" or "onCreateView".

Initialize your view inside of onCreate, keep a reference to the TextView and modify only this TextView from the reference.

What you currently do is: Create a view - modify it - remove it (it's never attached to your window)

Do something like this instead:

public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);

  final TextView textView2 = (TextView) findViewById(R.id.textView2);
  final Button button = (Button) findViewById(R.id.button1);
  button.setOnClickListener(new OnClickListener(){
        public void onClick(View v) {
           textView2.setText("lol");
        }

    });
}

Answer 2

You are inflating the layout XML, but not attaching it anywhere in your activity's view hierarchy nor setting it as the content view of your activity. So you are updating the text in your TextView, but it is not actually visible on the screen.

You will want to do something like:

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

Or, if you don't want the layout to be your main content view, attach it appropriately within your view hierarchy.

Answer 3

Try this,

activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.newproject.MainActivity" >

<Button
    android:id="@+id/button1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="30dp"
    android:onClick="clickHandler"
    android:text="@string/connect" />

<TextView
    android:id="@+id/textView2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/button1"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="65dp"
    android:text="@string/textView2"
    android:textAppearance="?android:attr/textAppearanceMedium" />

MainActivity.java

import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends ActionBarActivity {

    TextView tv;
    Button button;

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

    }

    public void clickHandler(View v) {
        tv = (TextView) findViewById(R.id.textView2);
        button = (Button) findViewById(R.id.button1);
        System.out.println("tv = " + tv.getText());
        tv.setText("lol");

    }
    }

strings.xml

 <?xml version="1.0" encoding="utf-8"?>
<resources>

    <string name="app_name">NewProject</string>
    <string name="hello_world">Hello world!</string>
    <string name="action_settings">Settings</string>
    <string name="connect">Connect</string>
    <string name="textView2">This is the text</string>

</resources>

Output :

When the app starts,

After clicking connect button,

In Logcat ,you can see the text in green color which gets from TextView .