UIWebView沒有提供設(shè)置UserAgent的接口，但是有一個辦法可以間接的設(shè)置。

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

通過設(shè)置NSUserDefaults中UserAgent的值來修改，但是這種設(shè)置方法有一個限制，需要在UIWebView的loadRequest之前調(diào)用才能生效(加載PDF比較特殊)。這是Cordova源碼中關(guān)于這個問題的描述

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的情況下，如果每個WebView都有不同的UserAgent，就會產(chǎn)生數(shù)據(jù)競爭的問題，大家都要修改NSUserDefaults中UserAgent的值，于是需要對資源加鎖來保證每個WebView都設(shè)置預(yù)期的UserAgent。在Cordova中，專門有一個類CDVUserAgentUtil來實現(xiàn)這個功能。

CDVUserAgentUtil.h文件中定義了四個方法

// 獲取UIWebView默認(rèn)的UserAgent
+ (NSString*)originalUserAgent;
// 獲取鎖
+ (void)acquireLock:(void (^)(NSInteger lockToken))block;
// 釋放鎖
+ (void)releaseLock:(NSInteger*)lockToken;
// 設(shè)置UIWebView的UserAgent
+ (void)setUserAgent:(NSString*)value lockToken:(NSInteger)lockToken;

加鎖

每次加鎖成功會返回一個NSInteger類型的token，在釋放鎖的時候需要把token傳入。token會不斷遞增，保證每次加鎖返回的token都不回重復(fù)。加鎖的實現(xiàn)代碼如下：

// 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];
    }
}

調(diào)用acquireLock:，首先會判斷gCurrentLockToken是否等于0

• 如果是0說明沒有模塊正在修改UserAgent，能夠成功獲取到鎖，gCurrentLockToken遞增，標(biāo)致當(dāng)前有模塊正在修改UserAgent，并回調(diào)block，返回gCurrentLockToken
• 如果不為0說明當(dāng)前有模塊正在修改UserAgent，將block回調(diào)存在一個隊列gPendingSetUserAgentBlocks中

釋放鎖

釋放鎖需要傳入token，釋放鎖代碼如下：

+ (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;
}

• 如果要釋放的lockToken為0，說明還沒加過鎖，就調(diào)用釋放了，直接返回
• 從隊列gPendingSetUserAgentBlocks中取出最早加入的block，從隊列中移除
• gCurrentLockToken遞增生成新token，回調(diào)block
• 如果隊列gPendingSetUserAgentBlocks釋放完成，說明釋放鎖的調(diào)用次數(shù)>加鎖的次數(shù)，不做操作，然后把gCurrentLockToken置為0

設(shè)置UserAgent

在Cordova實際運(yùn)用中，操作鎖的時機(jī)：
加鎖時機(jī)：CDVViewController加載完畢，在viewDidLoad里調(diào)用
釋放鎖時機(jī)：

• UIWebView的webViewDidFinishLoad:回調(diào)
• UIWebView的webView:didFailLoadWithError:回調(diào)
• CDVViewController的dealloc
• CDVViewController的viewDidUnload

加鎖代碼，省略了不相關(guān)代碼

// 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];
    }];
}

釋放鎖代碼，這里只看正常邏輯，在網(wǎng)頁加載完成回調(diào)webViewDidFinishLoad:中釋放邏輯。不考慮異常情況，省略了不相關(guān)代碼。

// 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:回調(diào)時，UserAgent已經(jīng)設(shè)置成功，所以可以釋放鎖，讓其它WebView操作UserDefault了

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