Cordova SQLite插件承诺javascript链

考虑这个问题的一个好方法是，你的承诺链的构建与它们的执行无关。一旦你建立了这种心智模型，就很容易推理了。

让我们将其分解为多个步骤，并从那里构建我们的代码。

执行异步代码并允许操作在完成后发生：

function doSomeAsyncWork(){
   var promise1 = foo();
   var promise2 = bar();
   var promise3 = blah();
   return $q.all([promise1, promise2, promise3]);
}

重复此代码 N 次，但按顺序。运行第一次迭代后。

function doWorkNTimes(n) {
 var lastPromise;
 for (var i = 0; i < n; i++) {
     //After initial promise, keep
     // chaining them together
     if (lastPromise) {
       lastPromise = lastPromise.then(function(items) {
         return doStuffAsync(i);
       });
     } else {
       lastPromise = doStuffAsync(i);
     }
 }
 return lastPromise;
};

这是一个稍微复杂一些的工作示例，但应该说明这一点。

(function() {
  'use strict';
  function getRandomInt(min, max) {
    return Math.floor(Math.random() * (max - min)) + min;
  }
  function promiseController($q, $timeout) {
    var vm = this;
    vm.results = [];
    vm.numTimes = 3;
    vm.isBusy = false;
    vm.isDone = true;
    vm.doWork = function(times) {
      vm.results.splice(0);
      vm.isBusy = true;
      vm.isDone = false;
      doWorkNTimes(vm.numTimes || 1)
        .finally(function() {
          vm.isBusy = false;
          vm.isDone = true;
        });
    };
    function doWorkNTimes(n) {
      var lastPromise;
      for (var i = 0; i < n; i++) {
        (function(i) {
          if (lastPromise) {
            lastPromise = lastPromise.then(function(items) {
              return doStuffAsync(i);
            });
          } else {
            lastPromise = doStuffAsync(i);
          }
          lastPromise.then(function(items) {
            //console.log(items);
            vm.results.push(items);
          });
        }(i));
      }
      return lastPromise;
    };
    function doStuffAsync(i) {
      var prom1 = createPromise(i),
        prom2 = createPromise(i),
        prom3 = createPromise(i);
      return $q.all([prom1, prom2, prom3]);
    }
    function createPromise(i) {
      var timeout = getRandomInt(200, 2000);
      return $timeout(function() {
        var item = {
          iteration: i,
          executionTime: timeout
        };
        //console.log(item);
        return item;
      }, timeout);
    }
  }
  angular.module('promise-sample', [])
    .controller('promiseController', promiseController);
}());

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.2/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.4.0-beta.4/angular.min.js"></script>
<div class="container" ng-app="promise-sample" ng-controller="promiseController as ctrl">
  <div class="row">
    <div class="col-xs-6">
      <div class="form-group">
        <label>Number of Iterations</label>
        <input type="number" class="form-control" min="0" max="10" ng-model="ctrl.numTimes" />
      </div>
      <div class="form-group">
        <button type="button" class="btn btn-primary" ng-click="ctrl.doWork()">Run {{ctrl.numTimes}} Times</button>
      </div>
    </div>
    <div class="col-xs-6">
      <table class="table table-striped" ng-repeat="set in ctrl.results">
        <thead>
          <tr>
            <th>Run</th>
            <th>Time</th>
          </tr>
        </thead>
        <tbody>
          <tr ng-repeat="item in set">
            <td>{{::item.iteration}}</td>
            <td>{{::item.executionTime}}</td>
          </tr>
        </tbody>
      </table>
    </div>
  </div>
</div>