Question

我正在写一个Angular应用程序。我需要方法getData（）来始终返回一个promise。因此，如果从本地存储中检索数据并且它不为null，则应该将其作为promise返回，而不调用$ http.get part。

我该怎么写呢？

getData() {
      var data = localStoradge.getItem('data');
      if (data == null) {
        return $http.get('url').then(function(response){
          data = response;
          return data
          })
        }
    }

Answer 1

如果数据已经可用，则返回已解决的promise（使用ES6语法）：

function getData() {
    var data = localStorage.getItem('data');
    if (data == null) {
        return $http.get('url');
    } else {
        // return promise that is already resolved
        return Promise.resolve(data);
    }
}

然后，您可以始终使用该接口作为承诺，无论数据是否已经可用。

注意：由于您在.then()处理程序中所做的只是返回数据，因此您可以完全删除它。

或者，使用$ q Angular语法，它看起来像这样：

function getData() {
    var data = localStorage.getItem('data');
    if (data == null) {
        return $http.get('url');
    } else {
        // return promise that is already resolved
        return $q.resolve(data);
    }
}

Answer 2

$ q.when（）执行技巧angular $q API doc。同样有帮助：$http API doc。

function getData() {

  var data = localStoradge.getItem('data');
  if (!data) {
    // assigning the promise returned by $http.get to the data variable
    data = $http.get('url').then(function(response) {
      // setting the data to local storage so it will be fetched from there the next time
      // the response body of a $http response is in response.data
      // see $http API doc linked above..
      localStoradge.setItem('data', response.data);
      // returning this will make the promise resolve to the the content of the resource found at 'url'
      return response.data;
    });
  }
  // $q.when always returns a promise. 
  // If data was found in the local storage that data will be wrapped in a promise which will resolve automatically right away. 
  // If the local storage was not there the data variable will be the promise we get from $http and $q.when will return that promise.
  // In both cases your getData method returns a promise which resolves to your data
  return $q.when(data);
}

Answer 3

确保注入$q

function getData() {
  var data = localStoradge.getItem('data');
    if (data == null) {
      return $http.get('url')
    } else {
      return $q.resolve(data)
    }
}

Answer 4

使用$ q来处理Angular中的promise：

function getData() {
    let data = localStorage.getItem('data');
    let deferred = this.$q.defer();
    this.$http
            .get(data)
            .success(deferred.resolve)
            .error(deferred.reject);
    return deferred.promise;
}

如何回复承诺？

4 个答案: