在视图上使用画布绘制，但未绘制任何内容

Question

I'm a beginner. I got a problem. This is an example about canvas on a view.

A circle and text on it are supposed to be seen.

( http://goo.gl/6ZPvQ ) My reputation isn't enough to get a picture.

But Nothing happened.

This is the view I draw canvas on.

public class TestCanvasActivity extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(new MyGraphics(this));
    }
}

class MyGraphics extends View {
    private Paint cPaint,tPaint;
    private Path circle;
    private String text;

    @Override
    protected void onDraw(Canvas canvas) {
        // TODO Auto-generated method stub
        super.onDraw(canvas);
        cPaint = new Paint(Color.GRAY);
        tPaint = new Paint(Color.BLACK);
        circle = new Path();
        text = "Welcome to Android!!";

        circle.addCircle(150, 150, 100, Direction.CW);
        canvas.drawPath(circle, cPaint);
        canvas.drawTextOnPath(text, circle, 0, 20, tPaint);
    }

    public MyGraphics(Context context) {
        super(context);
        // TODO Auto-generated constructor stub
        setBackgroundColor(R.drawable.background);
    }

}

This is the background image code!!

background.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android">
<gradient
android:startColor="#FFFFFF"
android:endColor="#808080"
android:angle="270" />
</shape>

Thank you!!

I Made a low level mistake. I change cPaint = new Paint(Color.GRAY) to the next two line. It works!!

 cPaint = new Paint();
 cPaint.setColor(Color.WHITE);

Thank you all the same.

Answer 1

I saw an example in 《Hello Android 3rd》. I found that cPaint = new Paint(Color.GRAY); is wrong. I changed it to cPaint = new Paint(); cPaint.setColor(Color.WHITE); cPaint = new Paint(); cPaint.setColor(Color.WHITE); and it worked.

Answer 2

Its just because you pass values in paint Construtor *cPaint = new Paint(VALUES) * it returns null! when you pass null in canvas functions as paint object, it gives u default color (BLACK). Correct way of initializing paint object is as below...

    cPaint = new Paint();
    cPaint.setColor(Color.GRAY);
    tPaint = new Paint();
    tPaint.setColor(Color.BLACK);

Answer 3

1. Put invalidate(); after the line canvas.drawTextOnPath(text, circle, 0, 20, tPaint);

2. Try not to set Background in the constructor and see if it draws.