Understanding Asynchronous Calls with SBJson Framework on iOS

In recent years, asynchronous programming has become an essential aspect of developing efficient and scalable applications. The SBJson framework is one such tool that simplifies the process of sending JSON data to a server using asynchronous calls.

However, in this article, we’ll delve into a specific issue that arises when making multiple requests with the same data, resulting in null values for response data.

Setting Up the Scene

To tackle this problem, let’s first understand how SBJson works. The framework converts JSON objects to and from native Objective-C objects. In our example, we’re using it to send a JSON representation of our request data to a server.

Here’s an excerpt of the code:

- (NSURLConnection *) GetHttpConnection:(NSString *)Path:(NSDictionary *)requestData:(UIView *)appView {
    // ...
}

In this snippet, requestData is converted to its JSON representation using the JSONRepresentation method. This string is then used to construct the request body.

ConnectionDidFinishLoading: Delegate Method

When the server responds, the connection delegate’s connectionDidFinishLoading: method is called. In our code, we use this method to process the response data and send it back to the delegate:

- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
    // ...
}

However, a crucial point to note is that there’s only one reference to responseData in our code. This means that if multiple requests are sent simultaneously, they’re all sharing the same responseData instance.

Connection:didReceiveData: Delegate Method

To address this issue, we’ve implemented the connection:didReceiveData: delegate method to accumulate response data:

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSMutableData*)data {
    [responseData appendData:data];
}

The Problem with Reentrancy

Now that we understand how SBJson and our code works, let’s examine the problem at hand. We’ve noticed that when making multiple requests with the same data, only one response is successfully received, resulting in null values for subsequent responses.

The issue lies in reentrancy. With a single reference to responseData, if two requests run simultaneously, it can lead to unexpected behavior and even crashes.

Solving the Problem

To overcome this problem, we need to ensure that each request has its own instance of responseData. One possible solution is to store the response data in a dictionary keyed by the NSURLConnection object itself:

NSMutableDictionary *requestDataDict = [[NSMutableDictionary alloc] init];
[requestDataDict setObject:responseData forKey:@"connection"];

However, this approach might not be suitable for our use case. Another viable solution is to create a new instance of our downloader for each request.

Implementing Non-Reentrant Behavior

To achieve non-reentrancy, we can modify the GetHttpConnection: method to return an instance of our downloader that has its own responseData:

- (NSURLConnection *) GetHttpConnection:(NSString *)Path:(NSDictionary *)requestData:(UIView *)appView {
    // ...
    NSURLConnection *connection = [[NSURLConnection alloc]initWithRequest:request delegate:self];
    if (connection) {
        responseData = [[NSMutableData data] retain]; // Create a new instance for each connection
    }
    return connection;
}

By doing so, each request will have its own responseData instance, ensuring that the responses are correctly processed and avoiding reentrancy issues.

Additional Considerations

While we’ve addressed the main issue at hand, there are additional considerations to keep in mind:

• Error Handling: It’s essential to handle potential errors that might occur during the request process. This can include checking for errors when setting up the request or handling invalid JSON responses.
• Response Validation: We should validate our response data to ensure it matches our expectations and adjust the processing accordingly.
• Request Cancellation: If needed, we may want to implement request cancellation logic to gracefully handle situations where a request is no longer required.

Conclusion

In this article, we explored a common issue with asynchronous calls using SBJson framework on iOS. We discovered that reentrancy was the root cause of the problem and provided solutions to address it. By implementing non-reentrant behavior in our code, we ensured that each request has its own responseData instance, guaranteeing correct response processing.

While there are additional considerations to keep in mind, such as error handling, response validation, and request cancellation, these changes can further enhance the robustness of our implementation.

Last modified on 2024-10-31