Android programming - How to acces [to draw on] XML view in main.xml layout, using code

Question

Ok I'm a newbie at Android programming, have a hard time with the graphics part. Understand the beauty of creating layout in XML file, but lost how to access various elements, especially a View element to draw on it.

See example of my layout main.xml here;

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:id="@+id/root" 
     android:orientation="vertical"
     android:layout_width="fill_parent" 
     android:layout_height="fill_parent">

     <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
           android:layout_width="fill_parent"   
           android:layout_height="wrap_content">

           <TextView xmlns:android="http://schemas.android.com/apk/res/android"
               android:id="@+id/Title" android:text="App title"
               android:layout_width="fill_parent" 
               android:layout_height="wrap_content"
               android:textColor="#000000"
               android:background="#A0A0FF"/>
     </LinearLayout>

     <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
          android:id="@+id/PaperLayout" 
          android:layout_width="fill_parent" 
          android:layout_height="0dp" 
          android:layout_weight="1" 
          android:orientation="horizontal" 
          android:focusable="true">

          <View 
              android:id="@+id/Paper" 
              android:layout_width="fill_parent" 
              android:layout_height="fill_parent" />
     </LinearLayout>


     <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
          android:layout_width="fill_parent" 
          android:layout_height="wrap_content">

          <Button 
               android:id="@+id/File" 
               android:layout_width="fill_parent"
               android:layout_weight="1" 
               android:layout_height="34dp"
               android:layout_alignParentTop="true"            
               android:layout_centerInParent="true"
               android:clickable="true" android:textSize="10sp" 
               android:text="File" />

         <Button 
              android:id="@+id/Edit" 
              android:layout_width="fill_parent"
              android:layout_weight="1" 
              android:layout_height="34dp"
              android:clickable="true" 
              android:textSize="10sp" android:text="Edit" />
     </LinearLayout>
</LinearLayout>

As you can see I have a custom app title bar, then a View filling middle, and finally two buttons in the bottom. Catching buttons' events and responding to, for example changing title bar text and changing View background color works fine, but how the heck do I access and more importantly draw on the view defined in main.xml

UPDATE: Thank you very much for your suggestion, however besides that I need a View, not ImageView and you are missing a parameter on canvas.drawText() and an ending bracket, it does not work.

Now this is most likely because you missed the fact that I am a newbie and assuming I can fill in any blanks.

Now first of all I do NOT understand why in my main.xml layout file I can create a View or even a SurfaceView element, which is what I need, but according to your solution I don't even specify the View like <View ....../>

Anyways I edited my main.xml according to your solution, and slimmed it down a bit for simplicity;

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:id="@+id/root" 
     android:orientation="vertical"
     android:layout_width="fill_parent" 
     android:layout_height="fill_parent">

     <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
           android:layout_width="fill_parent" 
           android:layout_height="wrap_content">

      <TextView xmlns:android="http://schemas.android.com/apk/res/android"
           android:id="@+id/Title" android:text="App title"
           android:layout_width="fill_parent" 
           android:layout_height="wrap_content"
           android:textColor="#000000"
           android:background="#A0A0FF"/>
     </LinearLayout>

     <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
           android:id="@+id/PaperLayout" 
           android:layout_width="fill_parent" 
           android:layout_height="0dp" 
           android:layout_weight="1" 
           android:orientation="horizontal" 
           android:focusable="true">

           <com.example.MyApp.CustomView 
                android:id="@+id/Paper" 
                android:layout_width="fill_parent" 
                android:layout_height="fill_parent" />
           <com.example.colorbook.CustomView/>

     </LinearLayout>

     <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
           android:layout_width="fill_parent" 
           android:layout_height="wrap_content">

          <Button 
               android:id="@+id/File" 
               android:layout_width="fill_parent"
               android:layout_weight="1" 
               android:layout_height="34dp"
               android:layout_alignParentTop="true" 
               android:layout_centerInParent="true"
               android:clickable="true" 
               android:textSize="10sp"
               android:text="File" />
     </LinearLayout>
</LinearLayout>

In my main java file MyApp.java. I added this after onCreate();

public class CustomView extends ImageView {
     @Override
     protected void onDraw(Canvas canvas) {
          super.onDraw(canvas);
          canvas.drawText("Your Text", 1, 1, null);
     }
}

But I get error on the CustomView part; "Implicit super constructor ImageView() is undefined for default constructor.Must define an explicit constructor"

Eclipse suggests 3 quick fixes about adding constructor, but none helps, well it removes error but gives error on app when running.

I hope somebody can break this down for me and provide a solution, and perhaps explain why I can't just create a View element in my main.xml layout file and draw on it in code.

UPDATE: Ok I am still trying to use your solution, which is still giving me problems and various errors. But first and foremost I still do NOT understand why I have to create a custom class to use in my layout, when I have to option to even use the wysiwyg layout editor to insert a View element and "just" draw on that in my main, well for now only code file. So pleeeeeease someone answer me that one.

Anyways now I have;

package com.example.myapp;

import com.example.myapp.CustomView;
//and here comes other import

public class MyApp extends Activity { 
//rest of code including OnCreate()

And I created a new class, new file CustomView.Java, where I directly pasted your code so it looks like;

public class CustomView extends ImageView {
     @Override
     protected void onDraw(Canvas canvas) {
         super.onDraw(canvas);
         Paint paint = new Paint();
         paint.setARGB(255, 150, 205, 129);
         paint.setTextSize(20);
         canvas.drawText("Your Text", 1, 1, paint);
     }
}

Ok so this class code wants me to add imports for ImageView, Canvas and Paint, which I did, and then again I am back to the error the public class *CustomView* part; "Implicit super constructor ImageView() is undefined for default constructor.Must define an explicit constructor".

I appreciate you trying to help me, THANKS!

Answer 1

To draw on a view, creat your own custom view.

To do so, you need to add a new class. With eclipse it's just File->New->Class and there you go. Override the onDraw()-Method to fit your needs and add your CustomView to your Layout:

public class CustomView extends ImageView {
    // The needed constructors:
    public CustomView(Context context) {
        super(context);
    }
    public CustomView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }
    public CustomView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }


@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    Paint paint = new Paint();
    paint.setARGB(255, 150, 205, 129);
paint.setTextSize(20);
    canvas.drawText("Your Text", xposition, yposition, paint);
}


}

In your xml-File you add your CustomView like this:

<com.example.CustomView
            android:id="@+id/yourID"
            android:layout_alignParentRight="true"
            android:layout_alignParentBottom="true"
            android:layout_margin="10dp"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content">
        </com.example.CustomView>

If you wan't to draw onto a drawable from your res-Folder, specifiy the "src"-parameter of your customView.

Answer 2

to use the view defined in main.xml in an Activity, override Activity's onCreate method such as:

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

}

edit: sorry, I misread "draw on view" as "draw view" in your question :) See L0rdAli3n's answer for that.

Answer 3

override the onCreate method and then use the method findViewById. For example to use and modify the view background color use the following :

View v = (View)findViewById(R.id.Paper);
v.setBackgroundColor(myColor);

Similarly modify any other view/Widget defined in your layout