How can I draw a polyline on a canvas in android?

Question

I've been trying to draw a polyline on a canvas. There are no errors in my codes, but the connected sequence of line segments are not showing.Please find code snippet below;

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

public class MyView extends View {
    private Paint redPaint;

    public MyView(Context context) {
        super(context, null);
        redPaint=new Paint(Paint.ANTI_ALIAS_FLAG);
        redPaint.setStyle(Paint.Style.STROKE);
        redPaint.setColor(0xffff0000);
        redPaint.setStrokeWidth(5);

    }
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.drawRect(10,30,200,200,redPaint);
        canvas.drawCircle(300,300,250,redPaint);

        Path mylines=new Path();
        mylines.moveTo(0,0);
        mylines.lineTo(1,1);
        mylines.lineTo(2,2);
        mylines.lineTo(3,3);
        mylines.lineTo(4,4);
        Paint GreenPaint=new Paint();
        GreenPaint.setARGB(255,0,255,0);
        canvas.drawPath(mylines,GreenPaint);

    }
}

Answer 1

Your code is largely correct. You do not completely initialized GreenPaint like you do redPaint and that is a problem. The second issue, although it may not be a problem, is that your polyline shape is so small that you may miss it even with a fully initialized GreenPaint.

Here is an updated version of your custom view with an additional constructor and an initialized greenPaint. I also changed the shape of the polyline and made it larger to be easily seen - it is just a speck in your code. In addition I moved object allocation out of onDraw().

    public class MyView extends View {
    Path mylines = new Path();
    private Paint redPaint;
    private Paint greenPaint;

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

    public MyView(Context context) {
        super(context, null);
        init();
    }

    private void init() {
        redPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
        redPaint.setStyle(Paint.Style.STROKE);
        redPaint.setColor(0xffff0000);
        redPaint.setStrokeWidth(5);

        greenPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
        greenPaint.setStyle(Paint.Style.STROKE);
        greenPaint.setARGB(255, 0, 255, 0);
        greenPaint.setStrokeWidth(5);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.drawRect(10, 30, 200, 200, redPaint);
        canvas.drawCircle(300, 300, 250, redPaint);

        mylines.moveTo(0, 0);
        mylines.lineTo(200, 50);
        mylines.lineTo(300, 150);
        mylines.lineTo(400, 250);
        mylines.lineTo(500, 300);

        canvas.drawPath(mylines, greenPaint);

    }
}

Here is the display. (I added a gray background for visibility of the view but it is not needed.)