draw a curve line with arrow in android canvas

问题:

draw a curved line with arrow from point1(x1,y1) to point2(x2,y2) like this

I'm developing an application in android canvas. automata like application and I'm having trouble drawing a curved line with an arrow at the end. pointing the next circle.

can u give me a code or suggestion about this?

回答1:

I think what You need is a mix out of draw on path and line drawing. Declare this method inside Your onDraw:

    private void drawOvalAndArrow(Canvas canvas){


    Paint circlePaint = new Paint();
    circlePaint.setStyle(Paint.Style.FILL_AND_STROKE);
    circlePaint.setAntiAlias(true);
    circlePaint.setStrokeWidth(2);
    circlePaint.setColor(Color.CYAN);

    float centerWidth = canvas.getWidth()/2; //get center x of display
    float centerHeight = canvas.getHeight()/2; //get center y of display
    float circleRadius = 20; //set radius 
    float circleDistance = 200; //set distance between both circles

    //draw circles
    canvas.drawCircle(centerWidth, centerHeight, circleRadius, circlePaint);
    canvas.drawCircle(centerWidth+circleDistance, centerHeight, circleRadius, circlePaint);


    //to draw an arrow, just lines needed, so style is only STROKE
    circlePaint.setStyle(Paint.Style.STROKE);       
    circlePaint.setColor(Color.RED);

    //create a path to draw on
    Path arrowPath = new Path();

    //create an invisible oval. the oval is for "behind the scenes" ,to set the path´
    //area. Imagine this is an egg behind your circles. the circles are in the middle of this egg
    final RectF arrowOval = new RectF();
    arrowOval.set(centerWidth, 
            centerHeight-80, 
            centerWidth + circleDistance, 
            centerHeight+80);

    //add the oval to path
    arrowPath.addArc(arrowOval,-180,180);

    //draw path on canvas
    canvas.drawPath(arrowPath, circlePaint);


    //draw arrowhead on path start
     arrowPath.moveTo(centerWidth,centerHeight ); //move to the center of first circle
     arrowPath.lineTo(centerWidth-circleRadius, centerHeight-circleRadius);//draw the first arrowhead line to the left
     arrowPath.moveTo(centerWidth,centerHeight );//move back to the center
     arrowPath.lineTo(centerWidth+circleRadius, centerHeight-circleRadius);//draw the next arrowhead line to the right

     //same as above on path end
     arrowPath.moveTo(centerWidth+circleDistance,centerHeight );
     arrowPath.lineTo((centerWidth+circleDistance)-circleRadius, centerHeight-circleRadius);
     arrowPath.moveTo(centerWidth+circleDistance,centerHeight );
     arrowPath.lineTo((centerWidth+circleDistance)+circleRadius, centerHeight-circleRadius);

     //draw the path
     canvas.drawPath(arrowPath,circlePaint);

}

This is just a poor example, but itshould show where to start with.

回答2:

I know i should leave comment but the code in the comment is hard to read, so i put up another answer. The answer Opiatefuchs it's basally right. but there's 1 thing you should notice, if you want to test his code.

float centerWidth = canvas.getWidth()/2; //get center x of display
float centerHeight = canvas.getHeight()/2; //get center y of display

the centerWidth and the centerHeight should be obtained like below, or nothing would painted to your screen. and the circleDistance = 200 is a bit large for a regular phone's screen( as for my device samsung i9300, 200 is too large, the second circle positioned out of the screen range. you change it to a smaller value 80 for example.)

    @Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
    super.onSizeChanged(w, h, oldw, oldh);
    centerWidth = w / 2;
    centerHeight = h / 2;
}

the screenshot.