.float-ball {

    position: fixed;

    width: 50px;

    height: 50px;

    bottom: 0px;

    right: 0px;

}

<cover-view class="float-ball" style="bottom: {{ballBottom}}px; right: {{ballRight}}px;"

    bindtap="ballClickEvent"catchtouchmove="catchBallMove">

    <cover-image src="/images/icon" style="width: 100%; height: 100%;" mode="aspectFill" />

</cover-view>

catchBallMove: function (e) {

    let touchs = e.touches[0];

    let pageX = touchs.pageX;

    let pageY = touchs.pageY;

    let offSetY = _this.data.offSetY;

    let x = this.data.screenWidth - pageX - 25;

    let y = this.data.screenHeight - (pageY - offSetY) + 25;

    if(x < 0) {

        x = 0;

    }

    if(x > _this.data.screenWidth - 50) {

        x = _this.data.screenWidth - 50;

    }

    if(y < 45) {

        y = 45;

    }

    if(y > _this.data.screenHeight) {

        y = _this.data.screenHeight;

    }

    this.setData({

        ballBottom: y,

        ballRight: x,

    });

},