UIWebView没有提供设置UserAgent的接口，但是有一个办法可以间接的设置。

NSDictionary* dict = [[NSDictionary alloc] initWithObjectsAndKeys:value, @"UserAgent", nil];
[[NSUserDefaults standardUserDefaults] registerDefaults:dict];

通过设置NSUserDefaults中UserAgent的值来修改，但是这种设置方法有一个限制，需要在UIWebView的loadRequest之前调用才能生效(加载PDF比较特殊)。这是Cordova源码中关于这个问题的描述

Setting the UserAgent must occur before a UIWebView is instantiated.
It is read per instantiation, so it does not affect previously created views.
Except! When a PDF is loaded, all currently active UIWebViews reload their
User-Agent from the NSUserDefaults some time after the DidFinishLoad of the PDF bah!

CDVUserAgentUtil

在多WebView的情况下，如果每个WebView都有不同的UserAgent，就会产生数据竞争的问题，大家都要修改NSUserDefaults中UserAgent的值，于是需要对资源加锁来保证每个WebView都设置预期的UserAgent。在Cordova中，专门有一个类CDVUserAgentUtil来实现这个功能。

CDVUserAgentUtil.h文件中定义了四个方法

// 获取UIWebView默认的UserAgent
+ (NSString*)originalUserAgent;
// 获取锁
+ (void)acquireLock:(void (^)(NSInteger lockToken))block;
// 释放锁
+ (void)releaseLock:(NSInteger*)lockToken;
// 设置UIWebView的UserAgent
+ (void)setUserAgent:(NSString*)value lockToken:(NSInteger)lockToken;

加锁

每次加锁成功会返回一个NSInteger类型的token，在释放锁的时候需要把token传入。token会不断递增，保证每次加锁返回的token都不回重复。加锁的实现代码如下：

// CDVUserAgentUtil.m
+ (void)acquireLock:(void (^)(NSInteger lockToken))block
{
    if (gCurrentLockToken == 0) {
        gCurrentLockToken = ++gNextLockToken;
        VerboseLog(@"Gave lock %d", gCurrentLockToken);
        block(gCurrentLockToken);
    } else {
        if (gPendingSetUserAgentBlocks == nil) {
            gPendingSetUserAgentBlocks = [[NSMutableArray alloc] initWithCapacity:4];
        }
        VerboseLog(@"Waiting for lock");
        [gPendingSetUserAgentBlocks addObject:block];
    }
}

调用acquireLock:，首先会判断gCurrentLockToken是否等于0

• 如果是0说明没有模块正在修改UserAgent，能够成功获取到锁，gCurrentLockToken递增，标致当前有模块正在修改UserAgent，并回调block，返回gCurrentLockToken
• 如果不为0说明当前有模块正在修改UserAgent，将block回调存在一个队列gPendingSetUserAgentBlocks中

释放锁

释放锁需要传入token，释放锁代码如下：

+ (void)releaseLock:(NSInteger*)lockToken
{
    if (*lockToken == 0) {
        return;
    }
    NSAssert(gCurrentLockToken == *lockToken, @"Got token %ld, expected %ld", (long)*lockToken, (long)gCurrentLockToken);

    VerboseLog(@"Released lock %d", *lockToken);
    if ([gPendingSetUserAgentBlocks count] > 0) {
        void (^block)() = [gPendingSetUserAgentBlocks objectAtIndex:0];
        [gPendingSetUserAgentBlocks removeObjectAtIndex:0];
        gCurrentLockToken = ++gNextLockToken;
        NSLog(@"Gave lock %ld", (long)gCurrentLockToken);
        block(gCurrentLockToken);
    } else {
        gCurrentLockToken = 0;
    }
    *lockToken = 0;
}

• 如果要释放的lockToken为0，说明还没加过锁，就调用释放了，直接返回
• 从队列gPendingSetUserAgentBlocks中取出最早加入的block，从队列中移除
• gCurrentLockToken递增生成新token，回调block
• 如果队列gPendingSetUserAgentBlocks释放完成，说明释放锁的调用次数>加锁的次数，不做操作，然后把gCurrentLockToken置为0

设置UserAgent

在Cordova实际运用中，操作锁的时机：
加锁时机：CDVViewController加载完毕，在viewDidLoad里调用
释放锁时机：

• UIWebView的webViewDidFinishLoad:回调
• UIWebView的webView:didFailLoadWithError:回调
• CDVViewController的dealloc
• CDVViewController的viewDidUnload

加锁代码，省略了不相关代码

// CDVViewController.m
- (void)viewDidLoad
{
    [CDVUserAgentUtil acquireLock:^(NSInteger lockToken) {
        _userAgentLockToken = lockToken;
        [CDVUserAgentUtil setUserAgent:self.userAgent lockToken:lockToken];
        NSURLRequest* appReq = [NSURLRequest requestWithURL:appURL cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:20.0];
        [self.webViewEngine loadRequest:appReq];
    }];
}

释放锁代码，这里只看正常逻辑，在网页加载完成回调webViewDidFinishLoad:中释放逻辑。不考虑异常情况，省略了不相关代码。

// CDVUIWebViewNavigationDelegate.m
- (void)webViewDidFinishLoad:(UIWebView*)theWebView
{
    NSLog(@"Finished load of: %@", theWebView.request.URL);
    CDVViewController* vc = (CDVViewController*)self.enginePlugin.viewController;

    // It's safe to release the lock even if this is just a sub-frame that's finished loading.
    [CDVUserAgentUtil releaseLock:vc.userAgentLockToken];
}

在webViewDidFinishLoad:回调时，UserAgent已经设置成功，所以可以释放锁，让其它WebView操作UserDefault了