Parse.com：保存时获取唯一号码

Question

I use the Parse.com Cloud service in my Android app to sync data between devices.

I use the app mainly offline and use the local data store.

There is one class called Point that has a unique number as identifier I want to display. So when working offline I want to create a Point as draft (with a draft text as number) and when synchronizing I want it to get the real number that is unique over all the devices.

How would I set the number when saving? I was thinking about adding a WebHook in the cloud when saving the Point and giving it a unique number and then in my app use

newPoint.saveEventually(new SaveCallback() {            
        public void done(ParseException e) {
            //query for the number
        }
    });

to query the point from the cloud to get the number after it has been saved. But this seems kind of too complicated for such a simple requirement. And I am not sure if the SaveCallback() is always triggered when saving it.

Answer 1

I would recommend using an afterSave trigger on the Point class to set the unique identifier when the object is newly created. Then, as you've mentioned, you'll need to fetch the value before displaying it.

Here's what the cloud code could look like:

// Assign the Point a unique identifier on creation
Parse.Cloud.afterSave("Point", function(request) {

    // Check if the Point is new
    if (!(request.object.existed())) {

        // Get the unique identifier
        var uniqueIdentifier = ...

        // Set the unique identifier
        request.object.set('uniqueIdentifier', uniqueIdentifier);
    }
});

One important bit of information to keep in mind about using saveEventually with SaveCallback() is:

The callback will only be called if the operation completes within the current app lifecycle. If the app is closed, the callback can't be called, even if the save eventually is completed.

Source: Hector Ramos

If the unique identifier should be immediately displayed in the app or if the callback needs to be handled consistently, it would probably be best to use saveInBackground rather than saveEventually .

Another option would be to dynamically change the callback depending on network availability and/or offline settings. For example, if the offline mode is used anytime when the cell signal or wifi is unavailable, then network reachability could be used to check for a network and then use saveInBackground or saveEventually as appropriate.

Update

OP ended up using this code:

Parse.Cloud.beforeSave("Point", function(request, response) {
    if (!(request.object.existed())) {
        var query = new Parse.Query("Point");
        query.addDescending("uniqueIdentifier");
        query.first({
            success: function(result) {             
                var maxId = result.get("uniqueIdentifier");             
                request.object.set("uniqueIdentifier", maxId + 1);                  
            },
            error: function() {                     
            }
        });            
    }
    response.success();
});