虚拟滚动(未完成)

    let ind = this.limitIndex;
            let size = this.limitSize;

            const top = this.$refs.parentScroller.scrollTop;
            if (top < 0) return;
            let n = parseInt(top / 42);
            let count = 0;
            if (n > ind) {
                let s = n - ind;
                for (var i = 1; i <= s; i++) {
                    let num = ind + size + i - 1;
                    if (num > -1 && num < countries.length) {
                        count++;
                        if (countryCode.indexOf(num) < -1) continue;
                        console.dir(num);
                        this.targetCountries.push(countries[num]);
                        countryCode.push(num);
                    }
                }
                this.targetCountries.sort((k, m) => k.id - m.id);
                this.targetCountries.splice(0, count);
                this.limitIndex = n;
            } else if (n < ind) {
                let s = ind - n;
                for (var i = 0; i < s; i++) {
                    let num = n + i;
                    console.dir(num);
                    if (num > -1 && num < countries.length) {
                        count++;
                        this.targetCountries.push(countries[num]);
                    }
                }
                this.targetCountries.sort((k, m) => k.id - m.id);
                this.targetCountries.splice(this.targetCountries.length - count, count);
                this.limitIndex = n;
            }
        },