如何在iPhone的webview中实现双击缩放?

我在web视图中加载了一个HTML页面。但在双击网页视图时,它不会缩放。我还将“scalesPageToFit”属性设置为“YES”,但在web视图中看不到缩放效果。

CGRect webFrame = CGRectMake(0, 0, 320, 480);
webView = [[UIWebView alloc] initWithFrame:webFrame];
webView.backgroundColor = [UIColor clearColor];
webView.scalesPageToFit = YES;
webView.autoresizingMask = (UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight);
webView.delegate = self;
webView.multipleTouchEnabled = YES;
[self.view addSubview:webView];
[self loadWebView];