canvas 动态设置宽高问题 (apicloud)

apiready = function() {
    var winWidth = api.winWidth;
    var winHeight = api.winHeight;
    console.log(winHeight);
    var canvasList = document.getElementById('leftjoystick');
    var canvas = document.createElement('canvas');
    $api.css(canvasList,'margin-top:' + 2*winHeight/5 +'px;margin-left:'+winWidth/15 +'px;')
    // canvasList.marginLeft = winWidth / 10;
    // canvasList.marginTop = winHeight / 5;
    canvasList.appendChild(canvas);
    // if(api.systemType == 'ios')
    canvas.width = winHeight / 1.8; //☜
    canvas.height = winHeight / 1.8;
    //...
   //最后再执行画图操作  
}