全局变量：
https://ask.dcloud.net.cn/article/35021
定义一个专用的模块，用来组织和管理这些全局的变量，在需要的页面引入。
注意这种方式只支持多个vue页面或多个nvue页面之间公用，vue和nvue之间不公用。
App.vue

<script>
        var serialport = uni.requireNativePlugin("wrs-serialport")
        var bluetoothcenter = uni.requireNativePlugin("wrs-bluetoothcenter")
    export default {
        globalData: {
            serialport: serialport,
            bluetoothcenter: bluetoothcenter
        },
        onLaunch: function() {
            console.log('App Launch')
        },
        onShow: function() {
            console.log('App Show')
        },
        onHide: function() {
            console.log('App Hide')
        }
    }
</script>

<style>
    /*每个页面公共css */
</style>

使用：

        data() {
            let bluetoothcenter = getApp().globalData.bluetoothcenter
            return {
                bluetoothcenter: bluetoothcenter
            }
        },

msg自动换行

<text class="msg">{{msg}}</text>

.msg {
         white-space: normal; /* 或者 pre-wrap，根据需求选择 */
         word-break: break-all;
         margin-left: 20upx;
         margin-right: 20upx;
}

判断是iOS还是Android

            var isAndroid = true
            switch (uni.getSystemInfoSync().platform) {
                // android
                case 'android':
                    isAndroid = true
                    break;
                    // ios
                default:
                    isAndroid = false
                    break;
            }

相对路径与绝对路径转换

    var url="_www/index.html";
    var path=plus.io.convertLocalFileSystemURL(url);
    var newurl = plus.io.convertAbsoluteFileSystem(path);

Android请求动态权限

                this.requestPermission([
                    "android.permission.BLUETOOTH_ADMIN",
                    "android.permission.BLUETOOTH",
                    "android.permission.ACCESS_FINE_LOCATION",
                    "android.permission.READ_EXTERNAL_STORAGE",
                    "android.permission.WRITE_EXTERNAL_STORAGE"
                ])

                requestPermission: function(permissions) {
                plus.android.requestPermissions(
                    permissions,
                    function(resultObj) {
                        for (var i = 0; i < resultObj.granted.length; i++) {
                            var grantedPermission = resultObj.granted[i];
                            console.log('已获取的权限：' + grantedPermission);
                        }
                        for (var i = 0; i < resultObj.deniedPresent.length; i++) {
                            var deniedPresentPermission = resultObj.deniedPresent[i];
                            console.log('拒绝本次申请的权限：' + deniedPresentPermission);
                        }
                        for (var i = 0; i < resultObj.deniedAlways.length; i++) {
                            var deniedAlwaysPermission = resultObj.deniedAlways[i];
                            console.log('永久拒绝申请的权限：' + deniedAlwaysPermission);
                        }
                        // 若所需权限被永久拒绝,则打开APP设置界面,可以在APP设置界面打开相应权限  
                        if (resultObj.deniedAlways.length > 0) {
                            // var Intent = plus.android.importClass("android.content.Intent");
                            // var Settings = plus.android.importClass("android.provider.Settings");
                            // var Uri = plus.android.importClass("android.net.Uri");
                            // var mainActivity = plus.android.runtimeMainActivity();
                            // var intent = new Intent();
                            // intent.setAction(Settings.ACTION_APPLICATION_DETAILS_SETTINGS);
                            // var uri = Uri.fromParts("package", mainActivity.getPackageName(), null);
                            // intent.setData(uri);
                            // mainActivity.startActivity(intent);
                        }
                    },
                    function(error) {
                        console.log('申请权限错误：' + error.code + " = " + error.message);
                    });
            },
            showModel: function(msg) {
                uni.showModal({
                    title: '提示',
                    content: msg,
                    success: function(res) {
                        if (res.confirm) {
                            console.log('用户点击确定');
                        } else if (res.cancel) {
                            console.log('用户点击取消');
                        }
                    }
                });
            },
            showToast: function(msg) {
                uni.showToast({
                    title: msg,
                    duration: 2000
                });
            },
            showActionSheet: function(itemList, clickFun) {
                uni.showActionSheet({
                    itemList: itemList,
                    success: function(res) {
                        if (clickFun) {
                            clickFun(res.tapIndex);
                        }
                    },
                    fail: function(res) {
                        console.log(res.errMsg);
                    }
                });
            },
            clear() {
                this.msg = null
            },
            showMsg(info) {
                console.log(info)
                if (this.msg) {
                    this.msg = this.msg + "\n" + info
                } else {
                    this.msg = info
                }
            }

全局变量：

main.js

var wrsSqlite = uni.requireNativePlugin("wrs-sqlite");
Vue.prototype.$wrsSqlite = wrsSqlite

使用：

            this.$wrsSqlite.setDBCallback((resp) => {
                var opt = resp.opt;
                if (opt == "onCreate") {
                    this.showMsg("创建数据库");
                } else if (opt == "onOpen") {
                    this.showMsg("打开数据库");
                    this.initTable();
                } else if (opt == "onUpgrade") {
                    var oldVersion = resp.oldVersion;
                    var newVersion = resp.newVersion;
                    this.showMsg("数据库升级，oldVersion:" + oldVersion + "  newVersion:" + newVersion);
                }
            });

user.js

var wrsSqlite = uni.requireNativePlugin("wrs-sqlite");
export function findAllUser() {
  wrsSqlite.findAllUser()
}

原生与uniapp的长度单位关系

            const {
                windowWidth,
                windowHeight,
                statusBarHeight
            } = uni.getSystemInfoSync();
            console.log("UNIApp windowWidth:" + windowWidth + " windowHeight:" + windowHeight)
            let info = getScreenInfo()
            console.log("原生:" + JSON.stringify(info))

iphone X:
 windowWidth:375 windowHeight:724 
 原生:{"height":812,"scale":3,"width":375} 

812  = 724 + 44(状态栏) + 34

android:
UNIApp windowWidth:362 windowHeight:714
 原生:{"widthPixels":1080,"density":2.987499952316284,"heightPixels":2265,"scaledDensity":2.987499952316284,"densityDpi":478}
1080 =  362 *  2.987499952316284(density)

结论：
uni.getSystemInfoSync() 获取的回来的数值单位是dp