#dbc #契约式设计

dbc

// 示例来源：https://qiita.com/Jxck_/items/defd80843a4beb5fcfc8

# 定义公共的契约调用规则
function contract(rule) {
  return function(target, name, descriptor) {
    let rule = new Rule(target, name, descriptor);
    let original = descriptor.value;
    descriptor.value = function() {
      if (rule.before) rule.before.apply(rule, arguments);
      if (rule.invaliant) rule.invaliant(this);
      let result = original.apply(this, arguments);
      if (rule.invaliant) rule.invaliant(this);
      if (rule.after) rule.after(result);
      return result;
    }
  }
}

# 具体规则
class Rule {
  constructor(target, name, descriptor) {
    this.target = target;
    this.name = name;
    this.descriptor = descriptor;
  }
  // 前置条件
  before(i) {
    console.log('before', i);
    console.assert(i > 0, `arg for ${this.name} should be positive value`);
  }
  // 后置条件
  after(result) {
    console.log('after', result);
    console.assert(result > 0, `result of ${this.name} should be positive value`);
  }
  // 不可变式
  invaliant(_this) {
    console.log('invaliant', _this);
    console.assert(_this.current > 0, '`current` should be positive value');
  }
}

function main() {

  class Counter {
    constructor(initial = 0) {
      this.current = initial;
    }

    @contract(Rule)
    up(i) {
      this.current += i;
      return this.current;
    }
  }

  let counter = new Counter(10);
  console.log(counter.up(2));
}

main();