判断移动端系统

  //判断系统
            let u = navigator.userAgent;
            let isAndroid = u.indexOf('Android') > -1 || u.indexOf('Linux') > -1; //g
            let isIOS = !!u.match(/\(i[^;]+;( U;)? CPU.+Mac OS X/); //ios终端
            if (isAndroid) {
                this.system = 'Android';
            } else if (isIOS) {
                this.system = 'IOS';
            } else {
                this.system = 'pc';
            }
            
   download(){
                //如果是ios用户则提示，否则直接下载
                if( this.system == 'IOS'){
                    this.showAlert = true;
                    this.alertText = 'IOS用户请前往AppStore下载'
                }else{
                    try {
                        let elemIF = document.createElement("iframe");
                        elemIF.src = 'http://cangdu.org/files/elm.apk';
                        elemIF.style.display = "none";
                        document.body.appendChild(elemIF);
                    } catch (e) {
                        alert('下载失败')
                    }
                }
            }