WKWebView NSInternalInconsistencyException: Fix DecisionHandler Error

Hey guys! Ever wrestled with the infamous NSInternalInconsistencyException while working with WKWebView in your iOS apps? Specifically, the one that screams "webView:decidePolicyForNavigationAction:decisionHandler was not called"? If you're nodding your head, you're in the right place. This error, while seemingly cryptic, often stems from a straightforward issue in how we handle navigation policies within our web views. Let's dive deep into understanding this error, dissecting its causes, and, most importantly, exploring practical solutions to squash it for good.

Understanding the Culprit: NSInternalInconsistencyException

At its core, NSInternalInconsistencyException is a runtime exception in Objective-C and Swift, signaling that your code has reached an inconsistent state – a state where things aren't quite as they should be. In the context of WKWebView, this exception, particularly the "webView:decidePolicyForNavigationAction:decisionHandler was not called" variant, arises when the decisionHandler block within the webView(_:decidePolicyForNavigationAction:decisionHandler:) delegate method isn't executed. Think of it this way: the web view is asking you, "Hey, what should I do with this navigation request?" and it's expecting a clear answer – either to allow the navigation or to cancel it. If you don't provide that answer through the decisionHandler, the web view gets confused and throws this exception.

This error often surfaces when you're trying to control navigation within your WKWebView, perhaps to prevent external links from opening directly in the web view or to perform some custom action before a page loads. The webView(_:decidePolicyForNavigationAction:decisionHandler:) delegate method is your primary tool for this, giving you the power to inspect the navigation action and decide whether to proceed or not. However, with great power comes great responsibility – you absolutely must call the decisionHandler to signal your decision back to the web view. Failing to do so leaves the web view hanging, leading to the dreaded exception. The key takeaway here is that the decisionHandler is not optional; it's a critical part of the navigation policy process, and your code needs to ensure it's always called, regardless of your decision (allowing or canceling navigation). Let's break down some common scenarios where this exception might pop up and how to handle them effectively.

Common Scenarios and Solutions

So, where does this error usually rear its ugly head? Let's explore some typical scenarios and how to tackle them:

1. The Forgotten decisionHandler

This is the most common culprit. You've implemented webView(_:decidePolicyForNavigationAction:decisionHandler:), inspected the navigationAction, made a decision in your head, but... you forgot to actually tell the web view what to do! It's like ordering a pizza and then forgetting to tell the delivery guy your address.

Solution: Ensure that you always call decisionHandler within your webView(_:decidePolicyForNavigationAction:decisionHandler:) method. This is non-negotiable. Whether you're allowing the navigation or canceling it, the decisionHandler must be invoked. The fix is usually as simple as adding decisionHandler(.allow) or decisionHandler(.cancel) at the end of your conditional logic.

func webView(_ webView: WKWebView, decidePolicyFor navigationAction: WKNavigationAction, decisionHandler: @escaping (WKNavigationActionPolicy) -> Void) {
 if let url = navigationAction.request.url, url.host != "yourdomain.com" {
 // We want to prevent navigation to external links
 decisionHandler(.cancel) // Tell the web view to cancel navigation
 } else {
 // Allow navigation to links within your domain
 decisionHandler(.allow) // Tell the web view to allow navigation
 }
}

In this example, we're checking if the URL's host is different from "yourdomain.com". If it is, we cancel the navigation. If not, we allow it. The crucial part is that decisionHandler(.cancel) and decisionHandler(.allow) are always called, ensuring the web view gets a definitive answer.

2. Missing Delegate Assignment

Another frequent blunder is forgetting to set the navigationDelegate of your WKWebView. If the delegate isn't assigned, the webView(_:decidePolicyForNavigationAction:decisionHandler:) method will never be called in the first place, and you won't even get a chance to handle the navigation policy. It's like trying to send a letter without putting it in the mailbox – it's not going anywhere.

Solution: Make sure you've properly assigned the delegate. Typically, this is done in your view controller's viewDidLoad() method.

webView.navigationDelegate = self

This line of code tells the WKWebView that your view controller (represented by self) is responsible for handling navigation events. Without this, your delegate methods will remain silent observers, and the web view will proceed without your input, eventually leading to the dreaded exception if a navigation action requires a decision.

3. Asynchronous Operations and the decisionHandler

Things get trickier when you introduce asynchronous operations into your navigation policy decisions. Imagine you need to perform some network request or database lookup before deciding whether to allow or cancel a navigation. If you're not careful, the decisionHandler might get lost in the asynchronous shuffle.

Solution: Ensure the decisionHandler is called within the completion handler of your asynchronous operation. This guarantees that the decision is made only after the operation completes.

func webView(_ webView: WKWebView, decidePolicyFor navigationAction: WKNavigationAction, decisionHandler: @escaping (WKNavigationActionPolicy) -> Void) {
 if let url = navigationAction.request.url, url.scheme == "customscheme" {
 // Perform an asynchronous operation
 someAsynchronousFunction(url) { result in
 if result == .success {
 decisionHandler(.allow) // Allow navigation if successful
 } else {
 decisionHandler(.cancel) // Cancel navigation if unsuccessful
 }
 }
 } else {
 decisionHandler(.allow) // Allow navigation for non-custom schemes
 }
}

In this scenario, we're checking for a custom URL scheme and then performing an asynchronous operation (someAsynchronousFunction). The crucial part is that the decisionHandler is called inside the completion handler of this function, ensuring that the decision is made based on the result of the asynchronous operation. If you were to call decisionHandler outside the completion handler, it might be invoked before the asynchronous operation finishes, leading to unexpected behavior and potentially the NSInternalInconsistencyException.

4. Multiple Calls to decisionHandler

This might sound obvious, but it's worth mentioning: you should only call decisionHandler once for each navigation action. Calling it multiple times is a recipe for disaster, as the web view will get confused by the conflicting decisions. It's like trying to steer a car in two directions at once – it's not going to end well.

Solution: Double-check your logic to ensure that decisionHandler is called exactly once for each navigation action. Carefully examine any conditional statements or asynchronous operations that might lead to multiple calls.

A common mistake is calling decisionHandler in both the if and else branches of a conditional statement when one of the branches should have been the final decision. Another potential pitfall is calling decisionHandler multiple times within an asynchronous operation's completion handler due to some logical error. Thoroughly review your code to eliminate any such scenarios.

5. Edge Cases and Unexpected Scenarios

Sometimes, the NSInternalInconsistencyException might appear in less obvious situations. For example, you might encounter it if you're programmatically navigating the web view while also handling navigation policies in the delegate.

Solution: Carefully analyze the call stack and the sequence of events leading up to the exception. Use breakpoints and logging to trace the execution flow and identify any unexpected interactions between your code and the web view's navigation process. It's often helpful to simplify your code and incrementally add complexity back in, testing at each step to pinpoint the exact cause of the issue. This methodical approach can help you uncover hidden assumptions or race conditions that might be triggering the exception.

Debugging Tips and Tricks

Okay, so you've got the NSInternalInconsistencyException staring you in the face. What now? Here are some debugging strategies to help you track down the culprit:

• Breakpoints: Set breakpoints within your webView(_:decidePolicyForNavigationAction:decisionHandler:) method, especially at the beginning and before each call to decisionHandler. This will allow you to step through the code and inspect the navigationAction and the state of your application. Pay close attention to the URL, the navigation type (link click, form submission, etc.), and any other relevant information in the navigationAction.
• Logging: Add print statements to log when webView(_:decidePolicyForNavigationAction:decisionHandler:) is called and what decision you're making (allow or cancel). Log the URL and other relevant information from the navigationAction as well. This will give you a timeline of navigation events and decisions, helping you identify any inconsistencies or unexpected behavior.
• Call Stack Analysis: Examine the call stack in Xcode when the exception occurs. This will show you the sequence of method calls that led to the exception, giving you valuable clues about the source of the problem. Look for any unexpected method calls or patterns in the call stack that might indicate an issue.
• Simplify and Isolate: If the problem is complex, try simplifying your code and isolating the relevant parts. Comment out sections of code that are not directly related to navigation policy and see if the exception still occurs. This will help you narrow down the source of the problem and focus your debugging efforts.

Preventing Future Headaches

Prevention is always better than cure, right? Here are some best practices to help you avoid the NSInternalInconsistencyException in the first place:

• Always Call decisionHandler: Make it a habit to always call decisionHandler within your webView(_:decidePolicyForNavigationAction:decisionHandler:) method, even if you think the decision is obvious. This simple rule will eliminate the most common cause of this exception.
• Use Guard Statements: Use guard statements to handle edge cases and ensure that decisionHandler is called in all scenarios. This can help you avoid accidentally falling through without making a decision.
• Test Thoroughly: Test your navigation policy logic with a variety of URLs and navigation scenarios. This will help you identify any potential issues before they make it into production.
• Code Reviews: Have a colleague review your code to catch any potential errors. A fresh pair of eyes can often spot mistakes that you might have missed.

Conclusion: Mastering WKWebView Navigation Policies

The NSInternalInconsistencyException: webView:decidePolicyForNavigationAction:decisionHandler was not called error can be a real pain, but understanding its root cause and applying the right solutions can turn it into a minor speed bump. Remember, the key is to always ensure that the decisionHandler is called exactly once for each navigation action. By following the guidelines and debugging tips outlined in this article, you'll be well-equipped to conquer this exception and build robust, well-behaved WKWebView integrations in your iOS apps. Now go forth and tame those web views, guys!