围绕中心点旋转一条线

Question

I'm currently working on a game with a timer.

As time passes the redline is supposed to rotate about its center point and when times is over end up at the opposite side. I currently have the line rotating and it consistently stays the same length but it is moving sporadically and the animation isn't smooth. I'm not totally sure what I'm doing wrong here.

startX and Y is the starting end point of the line and endX and Y is the ending point for the line when its at 250 psi

Answer 1

Two points about smoothness

1) Use an animator of float type not int as I don't think the time on the Update calls is guaranteed to be millisecond accurate, therefore you are forcing it to truncate the value as the value is actually calculated using a fraction (float) between 0 and 1 of the duration

eg In a perfect world the timing sequence of 0 to 20 would begin like:-

• 0ms = 0
• 500ms = 1
• 1000ms = 2

But in reality the timing sequence might begin like:-

• 0ms = 0
• 499ms = 0.999 (when converted to an int would also be 0)
• 1000ms = 2

2) Recalculating the start and end point of the line to redraw it might not be the fastest and therefore the smoothest method.

It is probably faster and smoother to set the Rotation of a Canvas that contains the line onAnimationUpdate (with the Canvas pivot point X and Y set to the centre of the dial)

This is because this is most likely to be hardware accelerated transform

eg

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

public class Drawline extends View {
    Paint paint = new Paint();

    private void init() {
        paint.setColor(Color.BLACK);
    }

    public Drawline(Context context) {
        super(context);
        init();
    }

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

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

    @Override
    public void onDraw(Canvas canvas) {
        canvas.drawLine(100, 100, 150, 150, paint);
    }

}

public class MainActivity extends AppCompatActivity {

    Drawline drawLine;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        drawLine = new Drawline(this);
        drawLine.setPivotX(100f);
        drawLine.setPivotY(100f);
        setContentView(drawLine);

        ValueAnimator animator = ValueAnimator.ofFloat(0f, 20f);
        animator.setDuration(10000);
        animator.setInterpolator(new LinearInterpolator());
        animator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
            @Override
            public void onAnimationUpdate(ValueAnimator animation) {
                float x = (float) animation.getAnimatedValue();

                drawLine.setRotation(x);
                drawLine.invalidate();
            }
        });
        animator.start();
    }
}