java - 如何在继续之前等待带有结果回调的方法完成(Android)？

Question

所以我是 Android 的菜鸟，我正在编写一个简单的应用程序，它使用 Google Fit 来存储用户的健身类(class)和步数，然后检索它们。

我有两种方法，一种从云端获取给定日期范围内的所有 session ，另一种方法遍历这些并累加步数。

问题是，虽然我首先调用了获取方法，但直到我添加了步骤之后结果才会返回，因此步骤计数始终为零。

这是我的代码:

private ArrayList<> results;

    @Override
    public ArrayList<IndividualSession> readAllSessions(Date dateFrom, Date dateTo) {

    /* I haven't included the following code in this question just to keep things clean, but here there was
        - The initialisation of the results ArrayList
        - Creating the calendar and date objects
        - Building the session read request
    */

    Fitness.SessionsApi.readSession(mGoogleApiClient, readRequest).setResultCallback(new ResultCallback<SessionReadResult>() {
        @Override
        public void onResult(SessionReadResult sessionReadResult) {
            for (Session session : sessionReadResult.getSessions()) {
                List<DataSet> dataSets = sessionReadResult.getDataSet(session);
                for (DataSet dataSet : dataSets) {
                    for (DataPoint dataPoint : dataSet.getDataPoints()) {
                    // Create new IndividualSession object, add data to it then add it to arraylist
                        IndividualSession individualSessionObject = new IndividualSession();
                        individualSessionObject.setFromDate(new Date(session.getStartTime(TimeUnit.SECONDS)));
                        individualSessionObject.setToDate(new Date(session.getEndTime(TimeUnit.SECONDS)));
                        individualSessionObject.setStepCount(dataPoint.getValue(Field.FIELD_STEPS).asInt());
                        results.add(individualSessionObject);
                    }
                }
            }
            Log.i(TAG, "Number of sessions found while reading:  "+results.size());
        }
    });
    return results;
}



@Override
public int getDaySteps(Date dateTo) {
    int stepCount = 0; // to be returned

    // Sort out the dates
    Calendar calFrom = Calendar.getInstance();
    calFrom.add(Calendar.HOUR, -24);

    // Get the sessions for appropriate date range
    ArrayList results =  readAllSessions(calFrom.getTime(), dateTo);
    Log.i(TAG, "Number of sessions found while trying to get total steps: "+results.size());

    // Iterate through sessions to get count steps
    Iterator<IndividualSession> it = results.iterator();
    while(it.hasNext())
    {
        IndividualSession obj = it.next();
        stepCount += obj.getStepCount();
    }
    return stepCount;
}

这输出

"Number of sessions found while trying to get total steps: 0"
"Number of sessions found while reading:  8"

Answer 1

有两种解决方法:

选项 1:使用阻塞集合

1. 更改 ArrayList<> results到ArrayBlockingQueue<> results .
2. 调用readAllSessions后getDaySteps 中的方法方法，调用 while(results.take()!=null) { //rest of the logic }
3. 你需要某种机制来退出 while当读取所有结果时，在步骤 2 中循环

选项 2:使用 PendingResult 中的 await 方法

查看 SessionsAPI 类的文档，readSessions 方法似乎返回一个 PendingResult :

public abstract PendingResult readSession (GoogleApiClient client, SessionReadRequest request)

Reads data from the user's Google Fit store of the specific type(s) and for the specific session(s) selected from the request parameters.

查看 PendingResult 类中 await 方法的文档:

public abstract R await ()

Blocks until the task is completed. This is not allowed on the UI thread. The returned result object can have an additional failure mode of INTERRUPTED.

这是你能做的。不要将整个调用链接到 setResultCallBack，而是先调用 readSessions :

results = Fitness.SessionsApi.readSession(mGoogleApiClient, readRequest);

然后在 getDaySteps 方法中等待结果:

SessionReadResults sessionResults = results.await();
for (Session session : sessionReadResult.getSessions()) {
        List<DataSet> dataSets = sessionReadResult.getDataSet(session);
        for (DataSet dataSet : dataSets) {
            for (DataPoint dataPoint : dataSet.getDataPoints()) {
            // Create new IndividualSession object, add data to it then add it to arraylist
               IndividualSession individualSessionObject = new IndividualSession();
               individualSessionObject.setFromDate(new Date(session.getStartTime(TimeUnit.SECONDS)));
               individualSessionObject.setToDate(new Date(session.getEndTime(TimeUnit.SECONDS)));
            individualSessionObject.setStepCount(dataPoint.getValue(Field.FIELD_STEPS).asInt());
                    //use the results
            }
        }
    }

*results 必须声明为实例/类级变量，以便在类中的所有方法中都可以访问。变量 result 的类型为 PendingResult<SessionReadResults> .另外，看起来你可以取消结果 ArrayList因为你想要的一切都可以从 SessionReadResults 中提取出来由 await 返回方法。最后请注意，此答案尚未使用您的代码进行测试，因为您的代码示例不完整。