Question

New to Android development, and I’m having trouble doing a simple drawing to a view using a canvas.

From what I’ve understood, something like this:

import android.content.Context;
import android.graphics.Canvas;
import android.view.View;

public class DrawView extends View
{
    public DrawView(Context context)
    {
        super(context);
    }

    @Override
    protected void onDraw(Canvas canvas)
    {
        super.onDraw(canvas);
        canvas.drawRGB(255,0,0);
    }
}

With this as my Activity:

public class Prototype1 extends Activity
{
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }
}

And this for the layout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
    <com.dhs2.prototype1.DrawView
        android:id="@+id/map"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        />
</LinearLayout>

Should just draw red everywhere, but instead I just get a blank screen.
Any idea where I’m going wrong?

Answer 1

Since you added your custom view in an XML layout file, you should add 2 more constructors:

public DrawView(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
}


public DrawView(Context context, AttributeSet attrs) {
    super(context, attrs);
}

This is because you pass some attributes to the view, like fill_parent, wrap_content, so the constructor public DrawView(Context context) won’t be called.

This would work however, if you won’t declare your custom view in the XML layout file, but setting it directly from the onCreate() like this:

setContentView(new DrawView(this));