Android--called invalidate() but view does not refresh

Question

Hi I'm doing a practice project which could draw a line with the coordinates I put. The screen only have two textfields and one button. For example if I put "20" and "30" in those two textfields and click the "draw" button, I want the app to draw a line from (0,0) to (20,30) in ANOTHER VIEW. The problem is when I click the button, the values in those two textfields are passed into the setCoordinates() function, but the view just doesn't show the line. I tried change coordinates[0] and coordinates[1] to 50 and 50 in the canvas.drawLine() function then the line shows up, so I guess may be the invalidate() function is not working? Anyway please help find where my problem is, thanks!!!!! Here is my code:

MazeBuilder.java

package com.example.maze;

import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.app.Activity;
import android.graphics.Color;

public class MazeBuilder extends Activity {

private DrawMaze drawMaze ;
private EditText editTextX;
private EditText editTextY;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);


    drawMaze = new DrawMaze(this);
    setContentView(R.layout.activity_maze_builder);
    editTextX = (EditText) findViewById(R.id.editText1);
    editTextY = (EditText) findViewById(R.id.editText2);



}

public void buildMaze(View view){

    final int x = getValue(editTextX);
    final int y = getValue(editTextY);

    drawMaze.setCoordinates(x, y);

}

private static int getValue (EditText text) {
    try {
        return Integer.parseInt(text.getText().toString());
    } catch (NumberFormatException ex) {
        return 0;
    }
}

}

DrawMaze.java

package com.example.maze;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.View;

public class DrawMaze extends View{

private Paint paint = new Paint();

private int[] coordinates = new int[2];

//===============================Constructors==============================================
//
//  
public DrawMaze(Context context) {

    super( context );

}

public DrawMaze(Context context, AttributeSet attrs) {

    super( context, attrs );
}

public DrawMaze(Context context, AttributeSet attrs, int defStyle) {

    super( context, attrs, defStyle );
}

//===============================Initialize the color of line and background==========================================
//
//      
public void init(){

    paint.setColor(Color.BLACK);
    this.setBackgroundColor(Color.WHITE);
}

public void setCoordinates(int x, int y){
    coordinates[0] = x;
    coordinates[1] = y;

    invalidate();
}

@Override
protected void onDraw(Canvas canvas) {

    System.out.println("=============coordinates[0]:"+coordinates[0]+"================coordinates[1]:"+coordinates[1]+"========");
    canvas.drawLine(0, 0, coordinates[0] , coordinates[1] , paint);
}



}

activity_maze_builder.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=".MazeBuilder" >

<com.example.maze.DrawMaze

    android:id="@+id/relativeLayout2"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_below="@+id/relativeLayout1" />



<RelativeLayout
    android:id="@+id/relativeLayout1"
    android:layout_width="500dp"
    android:layout_height="50dp"
    android:layout_alignParentLeft="true"
    android:layout_alignParentTop="true" >

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/editText2"
        android:layout_alignBottom="@+id/editText2"
        android:layout_centerHorizontal="true"
        android:text="Columns: "
        android:textAppearance="?android:attr/textAppearanceMedium" />

    <EditText
        android:id="@+id/editText2"
        android:layout_width="60dp"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_toLeftOf="@+id/button1"
        android:ems="10" >

        <requestFocus />
    </EditText>

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBottom="@+id/editText2"
        android:layout_alignParentRight="true"
        android:layout_alignParentTop="true"
        android:onClick="buildMaze"
        android:text="Build" />

    <EditText
        android:id="@+id/editText1"
        android:layout_width="60dp"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_toLeftOf="@+id/textView1"
        android:ems="10" />

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignBaseline="@+id/editText1"
        android:layout_alignBottom="@+id/editText1"
        android:layout_toLeftOf="@+id/editText1"
        android:text="Rows: "
        android:textAppearance="?android:attr/textAppearanceMedium" />

</RelativeLayout>

Answer 1

instead of

drawMaze = new DrawMaze(this);

use

drawMaze = (DrawMaze)findViewById(R.id.relativeLayout2); 

but under

setContentView()

the reason is that when you set the content view with your layout, it is using the DrawMaze from your layout and not the

drawMaze = new DrawMaze(this);

which was instantiated but never set into the content view