Objective-C中的异常/错误处理（iPhone应用程序）

Question

I actually have two questions regarding exception/error handling in the iPhone app that I am making:

1. The app uses Internet, but when there's no connection, the app just dies (during launch). How can I handle this to print some infomsg to the user, instead of just getting thrown back to the springboard?

2. Can someone show me an example of how to handle for instance a "page not found" or "no contact with server" error, so I can give some sort of info to the user in the same way as above?

Answer 1

For crashes, the first step is to use error messages and the debugger to figure out what call is causing the problem. If the problem is caused by an uncaught exception, read this Apple article on exception handling . The specific answer really depends on your code and exactly what is causing the crash, so I won't speculate about a particular solution.

As far as detecting server error response codes (such as 404), that's more specific to WebKit. I assume you're using UIWebView on iPhone, and you've probably noticed that none of the primary methods return errors. This is because it uses a delegate model to report progress or errors asynchronously. (It makes sense because you don't want your UI code to be at the mercy of a slow-loading (or non-existent) webpage. To be notified of such errors, there are a few steps.

1. Adopt the UIWebViewDelegate protocol, usually in the same class that will start the webpage load for convenience.
2. Set that object as the delegate of the UIWebView instance. (It has a delegate property, so you can use something like either uiView.delegate = self or [uiView setDelegate:self] based on what you prefer.)
3. Implement the webView:didFailLoadWithError: method in that class. (You can be notified when the load finishing by implementing webViewDidFinishLoad: as well.) This is where you include the logic of what should happen when an error occurs.

I didn't see any detailed documentation on the content of any particular errors handed back via this delegate method, but it's a standard NSError object, and I recommend checking out the contents by calling its methods, such as -localizedDescription and -userInfo .

Here is some sample code with #import statements excluded for brevity.

MyClass.h

@interface MyClass : NSObject <UIWebViewDelegate> {
  IBOutlet UIWebView* myWebView;
}
-(void)webView:(UIWebView*)webView didFailLoadWithError:(NSError *)error;
@end

MyClass.m

@implementation MyClass
- (id) init {
  if ((self = [super init]) == nil)
    return nil;
  // initialize myWebView
  myWebView.delegate = self;
  return self;
}

- (void) webView:(UIWebView*)webView didFailLoadWithError:(NSError*)error {
  ...
}
@end

Answer 2

Testing for a connection is pretty easy...

NSString * test = [NSString stringWithContentsOfURL:[NSURL URLWithString:@"http://www.stackoverflow.com"]];
if (test == nil) {
  //display an alertview saying you don't have an internet connection
}

Answer 3

Using a URL to test for a connection is not a good idea, it is not robust enough to determine if the internet connection is down, the website is down or some other network issue etc and above all it adds an overhead to the call as far as network traffic.

Look at the Reachability demo on the Apple site, it uses the correct way to determine connectivity, including whether you are on wifi etc.