Question

In the code below, I am calling Pouch.sync(), to which I chain deleteLocal(). Pouch.sync() executes localDb.sync(), which does not return a promise, but I can attach a .on() event handler to handle errors.

I've tried playing around with $q.when and $q.reject, but I can't seem to prevent deleteLocal() from firing if the sync fails, and onError() never executes (either as an error callback or in the catch statement).

// Controller
function _onSyncClick() {
    return Pouch.sync()
        .then(deleteLocal, onError); // I do not want deleteLocal() to execute if Pouch.sync() fails
        .catch(onError); 
}


// Pouch service
function sync() {
    return 
        localDb.sync(remoteDb) 
            .on('error', function (err) {
                return $q.reject('Sync error.');
            });
}

Answer 1

.synch() uses a callback API, you can use $q.when and $q.reject to promisify it.

return localDB.sync(remoteDB).on('complete', function () {
   return $q.when("done!");
}).on('error', function (err) {
   return $q.reject(err);
});

Also, when chaining promises, you need to make sure you either throw an error or return a rejected promise, otherwise you will trigger the resolve/success handler.

so either .on('error', errorHandler) didn't ran as you though it would or Pouch.sync() didn't invoke your function.

Answer 2

首先，您then()不应使用deleteLocal致电onError。

接下来，基于Angular文档：如果通过promise错误回调“捕获”错误，并且想要将错误转发到从当前promise中派生的promise，则必须“重新抛出”错误通过返回通过$q.reject

构建的拒绝

因此，您将回调事件视为承诺，也许这就是为什么不抛弃拒绝。

我认为正确的方法是使用sync()构造函数从$q Promisify回调事件。

// Controller
function _onSyncClick() {
  return Pouch.sync().then(deleteLocal).catch(onError);
}

// Pouch Service, method sync
function sync() {
  return $q(function (resolve, reject) {
    localDb.sync(remoteDb, {
      // options
    }).on('complete', function (info) {
      resolve(info);
    }).on('error', function (err) {
      reject(err);
    });
  });
}

提一下，$q.when包装一个可能是值的对象或（第三方）然后能够承诺进入$ q承诺，不要将它用作解析，用它来包装PouchDB承诺。

Using $q.reject to stop chain from executing

2 个答案: