vant-weapp/packages/mixins/safe-area.ts

let cache = null;

function getSafeArea() {
  return new Promise((resolve, reject) => {
    if (cache != null) {
      resolve(cache);
    } else {
      wx.getSystemInfo({
        success: ({ model, screenHeight, statusBarHeight }) => {
          const iphoneX = /iphone x/i.test(model);
          const iphoneNew = /iPhone11/i.test(model) && screenHeight === 812;

          cache = {
            isIPhoneX: iphoneX || iphoneNew,
            statusBarHeight
          };

          resolve(cache);
        },
        fail: reject
      });
    }
  });
}

export const safeArea = ({
  safeAreaInsetBottom = true,
  safeAreaInsetTop = false
} = {}) =>
  Behavior({
    properties: {
      safeAreaInsetTop: {
        type: Boolean,
        value: safeAreaInsetTop
      },
      safeAreaInsetBottom: {
        type: Boolean,
        value: safeAreaInsetBottom
      }
    },

    created() {
      getSafeArea().then(({ isIPhoneX, statusBarHeight }) => {
        this.set({ isIPhoneX, statusBarHeight });
      });
    }
  });