问题
I need a little help with an issue in App Inventor 2. The case is that I have a JSON result that is parsed, and then have it stored in a tinyDB tag as a list (storeparseData).
Problem is that, I have this function done as screen initiates, so to have the tinyDB tag populated with that JSON list and then searched for a specific value on user request.
As the app is running and I input a search criteria, I get the following error "The arguments [empty-string] are the wrong number of arguments for GetValue". I suppose that, it finds nothing yet to be stored in the tinyDB tag to search for, cause the JSON data take some time to be fetched and parsed.
Here is what I have done for now for checking if the stored list is in the tinyDB:
Here I check the stored list in tinyDB for an item:
So how can I check if the tinyDB is populated in this tag with the JSON results list, so I can then have it searched? Is this possible in App Inventor 2?
Please someone advise on this issue with a sample code blocks if possible, or a similar tutorial, if there is any. Thank you all in advance for your answers.
回答1:
This looks very strange... a complicated if then else structure, loads of empty sockets and your comparison if "storeparsedData" = <empty string> always will be false... obviously this does not make sense...
it looks like you are thinking too complicated? what about this:
As already recommended in my answer to your other question you should learn how to work with lists...
- how to work with lists
- how to work with list of lists (pdf) by appinventor.org
- see also An example of a complex List of Lists
Also doing some more tutorials might help to learn some more basics...
To answer your question "Can you explain why you've chosen to create an empty list there.": if you are working with lists as in the example and you are trying to read a list from TinyDB, then you also have to think about what should happen, if that tag is not available in TinyDB (for example after starting the app the first time). And for lists, in this case an empty list should be returned. Note: the is list empty? block always expects a list. If you set valueIfTagNotThere to an empty string, then you will get a runtime error...
回答2:
You can use Clock function to delay the time. Firstly, just set the global DelayCountdown to specific number in "when Screen1.Initialize" part. Then, by using "when Clock1.Timer" function, you just need to add another check whether the DelayCountdown is equal to zero before you do another function.
来源：https://stackoverflow.com/questions/34709320/how-to-check-if-a-tinydb-tag-has-any-attributes-in-app-inventor