由于 WebView 导致应用程序崩溃

Question

在我的应用程序中，我使用 UIwebview 来显示网站，但是每当我单击后退按钮并开始使用应用程序中的其他部分时，应用程序大多会无缘无故地崩溃。我怀疑是 webview 导致了这个问题，因为它只有在我尝试打开 webview 时才会崩溃。

我已经使用NSURLConnection来加载webview并制作了webview，视图中为nil的连接对象将Disappear方法。

@implementation NewsWebSiteViewController

@synthesize connection,rcvdData,spinner1,currentSite,webView,newsWebsite;

- (void)viewDidLoad {
    [super viewDidLoad];
    @try {
        self.webView.delegate=self;
        [UIApplication sharedApplication].networkActivityIndicatorVisible=YES;
        self.title= self.newsWebsite.title;
        self.webView.backgroundColor =[UIColor groupTableViewBackgroundColor];
        self.spinner1 = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
        CGRect center = [self.view bounds];
        CGSize winCenter = center.size;
        CGPoint pont = CGPointMake(winCenter.width/2,winCenter.height/2);
        [spinner1 setCenter:pont];
        [self.view addSubview:spinner1];
        [self.spinner1 startAnimating];
        NSString *url = newsWebsite.link;
        NSURLRequest *theReq =[NSURLRequest requestWithURL:[NSURL URLWithString:url] cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:20.0];
        self.connection = [[NSURLConnection alloc] initWithRequest:theReq delegate:self];
        if(self.connection) {
            self.rcvdData = [[NSMutableData data] retain];
        }
        else {
        UIAlertView *alert = [[[UIAlertView alloc] initWithTitle:@"Error!" message:@"We are having a problem connecting to the internet, why not try again or try sometime later!.." delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil] autorelease];
        [alert show];
        }
        webView.multipleTouchEnabled=YES;
        webView.scalesPageToFit=YES;
    }
    @catch (NSException * e) {
        }  
}

-(void) goBack {
    self.webView =nil;
    [self.navigationController popViewControllerAnimated:YES];
}

-(void) viewWillDisappear:(BOOL)animated {

    [self.connection cancel];
    self.connection=nil;
    [self.webView stopLoading];
    self.webView=nil;

    [UIApplication sharedApplication].networkActivityIndicatorVisible=NO;

    if (self.spinner1 ==nil) {

    }

    else {
        [self.spinner1 stopAnimating];
    }

}

-(void) webViewDidFinishLoad:(UIWebView *)webView {

    [UIApplication sharedApplication].networkActivityIndicatorVisible=NO;

    if (self.spinner1 ==nil) {

    }
    else {
        [self.spinner1 stopAnimating];
    }
}

-(void) viewDidAppear:(BOOL)animated  {
    [UIApplication sharedApplication].networkActivityIndicatorVisible=NO;

}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
}


- (void)viewDidUnload {
    [super viewDidUnload];
}


-(void) connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
    [self.rcvdData setLength:0];
}


-(void) connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    [self.rcvdData appendData:data];
}


-(void) connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
    if (self.spinner1 ==nil) {
        }
    else {
        [self.spinner1 stopAnimating];
        }
    [connection release];
    [rcvdData release];
    UIAlertView *alert = [[[UIAlertView alloc] initWithTitle:@"Error!" message:@"We are having a problem connecting to the internet, why not try again or try sometime later!.." delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil] autorelease];
    [alert show];
}


-(void)connectionDidFinishLoading:(NSURLConnection *)connection {
    [connection release];
    [rcvdData release];
    NSString *url = newsWebsite.link;
    NSURL *url1 =  [NSURL URLWithString:url];
    [self.webView loadData:self.rcvdData MIMEType:@"text/html" textEncodingName:@"UTF-8" baseURL:url1];
}

- (void)dealloc {
    [super dealloc];
    [self.spinner1 release];
}

@end

Answer 1

首先，这里是 C/Objective-C 中相等/不等式的小入门：

假设您有一个名为isEnabled 的BOOL 值（即，可以是“是”或“否”、“开”或“关”、“真”或“假”的值）。现在，如果我将这个 BOOL 值（有时称为“标志”）分配给“是”，我可以像这样有条件地测试它的值：

BOOL isEnabled = YES;

if (isEnabled)
{
   // value set to yes
}

除了上述之外，我还可以使用否定运算符（感叹号 - 也称为 NOT 运算符）来翻转isEnabled 的值，并测试它的相反值：

BOOL isEnabled = YES;

// the following reads as "if is *not* enabled"
if (!isEnabled)
{
   // value set to no
}

现在当然在上面的例子中isEnabled 设置为YES，所以条件会失败。但是，如果我们考虑下面的例子，if 的属性，即如果else if 在一系列if 和else if 中的任何位置都是“满足”（即，为真），它将执行里面的所有代码，然后忽略其他任何内容：

BOOL isEnabled = NO;

if (isEnabled)
{
   // any code here will not run, as the above if condition will be false
}
else if (!isEnabled)
{
   // this code will run, since the above condition (!isEnabled) will evaluate to true
}
else if (isEnabled)
{
   // this code could never run, since the previous condition was true and all following else if's are ignored
}
else if (!isEnabled)
{
   // this code could never run, since the previous condition was true and all following else if's are ignored
}

虽然上面有两个多余的else if，但这是演示条件代码如何工作的好方法。

因此，在您的 webViewDidFinishLoad: 方法中，您可以将其替换为更简单的条件，而不是使用空白 if 来评估 if/else 条件：

-(void) webViewDidFinishLoad:(UIWebView *)webView 
{
    [UIApplication sharedApplication].networkActivityIndicatorVisible=NO;

    // or, (self.spinner1 != nil)
    if (!self.spinner1) 
    {
        [self.spinner1 stopAnimating];
    }
}

当您对所有 if 和 else if 进行了上述更改后，发布一个堆栈跟踪，我会看看它可能是什么。