Frekzok Frekzok - 2 years ago 250
Android Question

ConstraintLayout background

Hi, I was trying to change background by clicking a button, but I can't identify Constraint Layout in java code.
activity_main .xml


<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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:background="@color/red"
tools:context="frekzok.trafficlight.MainActivity">


//here is some button options

</android.support.constraint.ConstraintLayout>

MainActivity.java:

package frekzok.trafficlight;

import android.support.constraint.ConstraintLayout;
import android.support.v4.content.ContextCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {
private ConstraintLayout mConstraintLayout;
private TextView mInfoTextView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mConstraintLayout =
(ConstraintLayout)findViewById(R.id.constraintLayout);
mInfoTextView = (TextView)findViewById(R.id.textView2);}

public void onRedButtonClick(View view) {
mInfoTextView.setText(R.string.red);
mConstraintLayout.setBackgroundColor(ContextCompat.getColor(this, R.color.red));
}
public void onYellowButtonClick(View view) {
mInfoTextView.setText(R.string.yellow);
mConstraintLayout.setBackgroundColor(ContextCompat.getColor(this, R.color.yellow));
}
public void onGreenButtonClick(View view) {
mInfoTextView.setText(R.string.green);
mConstraintLayout.setBackgroundColor(ContextCompat.getColor(this, R.color.green));
}
}


in Main Activity on line 18 is a error on "constraintLayout": Cannot resolve symbol 'constraintLayout'
How can I change background color?

Answer Source

You didn't add the ID to your constraint layout. Try modifying your xml file to include it:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
  android:id="@+id/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:background="@color/red"
  tools:context="frekzok.trafficlight.MainActivity">
</android.support.constraint.ConstraintLayout>
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