子弹发射地方应该是从坦克面向的方向且炮口发出的。
    这里需要微调子弹x,y的坐标。因为graphics默认左上角.

    1. TankWar.prototype.fire = function () {
    2.   const { upIsDown, leftIsDown, downIsDown, rightIsDown } =
    3.         this.direction;
    4.   const { vx, vy } = defaultSpeed;
    5.   const {x,y,width,height} = this.player;
    6.   const gunX = x+width;
    7.   const gunY = y+height;
    8.   if ((this.gun.children.length === 0 && this.gun.children < 2) ) {
    9.     const graphics = new PIXI.Graphics();
    10.     graphics.beginFill(0xffffff);
    11.     graphics.drawRect(0, 0, 3, 3);
    12.     graphics.endFill();
    13.     this.gun.addChild(graphics);
    15.     if (!upIsDown) {
    16.       console.log(graphics);
    17.       graphics.vx = 0;
    18.       graphics.vy = 4;
    19.       graphics.x = gunX - width/2 - 2;
    20.       graphics.y = gunY;
    21.     }
    22.     else if (!leftIsDown){
    23.       graphics.vx = 4;
    24.       graphics.vy = 0;
    25.       graphics.x = gunX;
    26.       graphics.y = gunY - height/2 - 2;
    27.     }
    28.     else if(!rightIsDown){
    29.       graphics.vx = -4;
    30.       graphics.vy = 0;
    31.       graphics.x = gunX - width - 2;
    32.       graphics.y = gunY - height/2 - 2;
    33.     }
    34.     else if(!downIsDown){
    35.       graphics.vx = 0;
    36.       graphics.vy = -4;
    37.       graphics.x = gunX - width/2 - 3;
    38.       graphics.y = gunY - height - 2;
    39.     }
    40.   }
    41. };