WKWebView evaluate JavaScript return value

Question

I need to change a function to evaluate JavaScript from UIWebView to WKWebView. I need to return result of evaluating in this function.

Now, I am calling:

[wkWebView evaluateJavaScript:call completionHandler:^(NSString *result, NSError *error)
{
    NSLog(@"Error %@",error);
    NSLog(@"Result %@",result);
}];

But I need get result like return value, like in UIWebView. Can you suggest a solution?

NSString *returnVal = [self.webView stringByEvaluatingJavaScriptFromString:@"func(\"arg\")"]; doesn't this work? — l0gg3r
– l0gg3r, Commented Nov 6, 2014 at 12:20
No this function is in UIWebView and is working, I need to change it to WKWebView. I can solve it with some callback, but it is too complicated in my project. — redak105
– redak105, Commented Nov 6, 2014 at 12:25

Cody Gray · Accepted Answer · 2019-03-27 21:30:42Z

51

Update: This is not working on iOS 12+ anymore.

I solved this problem by waiting for result until result value is returned.

I used NSRunLoop for waiting, but I'm not sure it's best way or not...

Here is the category extension source code that I'm using now:

@interface WKWebView(SynchronousEvaluateJavaScript)
- (NSString *)stringByEvaluatingJavaScriptFromString:(NSString *)script;
@end

@implementation WKWebView(SynchronousEvaluateJavaScript)

- (NSString *)stringByEvaluatingJavaScriptFromString:(NSString *)script
{
    __block NSString *resultString = nil;
    __block BOOL finished = NO;

    [self evaluateJavaScript:script completionHandler:^(id result, NSError *error) {
        if (error == nil) {
            if (result != nil) {
                resultString = [NSString stringWithFormat:@"%@", result];
            }
        } else {
            NSLog(@"evaluateJavaScript error : %@", error.localizedDescription);
        }
        finished = YES;
    }];

    while (!finished)
    {
        [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
    }

    return resultString;
}
@end

Example code:

NSString *userAgent = [_webView stringByEvaluatingJavaScriptFromString:@"navigator.userAgent"];

NSLog(@"userAgent: %@", userAgent);

edited Mar 27, 2019 at 21:30

Cody Gray♦

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answered Jan 16, 2015 at 10:25

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14 Comments

Brams Over a year ago

The issue with this code is that if a JS error is raised, your native function will run infinitely ! The solution would be to check a block safe boolean instead of resultString. Look below

Nikita Over a year ago

will cause crash while multiple wkwebview are used;

Mike Keskinov Over a year ago

This solution IS NOT WORKING anymore. The callback now called from the main Thread and if you lock the main thread in the while loop (like in this solution), the callback handler will be never called. evaluateJavaScript waits for main thread released, but it never happens because it locked in the loop.

Dmitry Over a year ago

Any ideas for iOS 12 or iOS 13?

Keerthiraj Over a year ago

This solution doesn't work in iOS 13. Does anyone has the working solution for iOS 13 ?

|

Slavcho · Accepted Answer · 2015-10-26 12:43:15Z

29

This solution also works if the javascript's code raise NSError:

- (NSString *)stringByEvaluatingJavaScriptFromString:(NSString *)script {
    __block NSString *resultString = nil;
    __block BOOL finished = NO;

    [self evaluateJavaScript:script completionHandler:^(id result, NSError *error) {
        if (error == nil) {
            if (result != nil) {
                resultString = [NSString stringWithFormat:@"%@", result];
            }
        } else {
            NSLog(@"evaluateJavaScript error : %@", error.localizedDescription);
        }
        finished = YES;
    }];

    while (!finished)
    {
        [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
    }

    return resultString;
}

edited Oct 26, 2015 at 12:43

Slavcho

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answered Jun 17, 2015 at 14:38

Brams

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Comments

mort3m · Accepted Answer · 2016-06-17 01:06:35Z

14

I just stumbled about the same problem and wrote a little Swift (3.0) WKWebView extension for it, thought I might share it:

extension WKWebView {
    func evaluate(script: String, completion: (result: AnyObject?, error: NSError?) -> Void) {
        var finished = false

        evaluateJavaScript(script) { (result, error) in
            if error == nil {
                if result != nil {
                    completion(result: result, error: nil)
                }
            } else {
                completion(result: nil, error: error)
            }
            finished = true
        }

        while !finished {
            RunLoop.current().run(mode: .defaultRunLoopMode, before: Date.distantFuture)
        }
    }
}

answered Jun 17, 2016 at 1:06

mort3m

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3 Comments

Wesley Lalieu Over a year ago

I can't use your code because of the RunLoop. My compiler can't find the RunLoop variable. I did import WKWebKit and Foundation... When I use the NSRunLoop.currentRunLoop().run() it keeps running, and I can't say for how long to run. What am I doing wrong?

Wesley Lalieu Over a year ago

I used: NSRunLoop.currentRunLoop().runMode("NSDefaultRunLoopMode", beforeDate: NSDate.distantFuture()) this worked! Thanks for your answer!!!

Alexandre G Over a year ago

Sorry but what's the point of this if you are using a callback anyway?

Nicolas Mandica · Accepted Answer · 2020-04-16 14:22:43Z

5

Base on @mort3m's answer, here is a WKWebView extension working with Swift 5.

extension WKWebView {
    func evaluate(script: String, completion: @escaping (Any?, Error?) -> Void) {
        var finished = false

        evaluateJavaScript(script, completionHandler: { (result, error) in
            if error == nil {
                if result != nil {
                    completion(result, nil)
                }
            } else {
                completion(nil, error)
            }
            finished = true
        })

        while !finished {
            RunLoop.current.run(mode: RunLoop.Mode(rawValue: "NSDefaultRunLoopMode"), before: NSDate.distantFuture)
        }
    }
}

answered Apr 16, 2020 at 14:22

Nicolas Mandica

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5 Comments

JDev Over a year ago

How does one use this code within a typical webView.evaluateJavaScript(jsCode) { (value, error) in statement?

Nicolas Mandica Over a year ago

You have to use this new function evaluate instead of evaluateJavaScript.

JDev Over a year ago

I totally missed that it was just evaluate. Late night I suppose. Code is working great, btw!

JDev Over a year ago

Hey @Nicolas, I'm getting new crashing errors with this code (introduced with Swift 5 I believe).

Nicolas Mandica Over a year ago

@JustinBush It's working well with Swift 5 for me. What is your error ?

Steve Yost · Accepted Answer · 2017-03-21 18:50:26Z

4

I've found that the value of final statement in your injected javascript is the return value passed as the id argument to the completion function, if there are no exceptions. So, for example:

[self.webview evaluateJavaScript:@"var foo = 1; foo + 1;" completionHandler:^(id result, NSError *error) {
    if (error == nil)
    {
        if (result != nil)
        {
            NSInteger integerResult = [result integerValue]; // 2
            NSLog(@"result: %d", integerResult);
        }
    }
    else
    {
        NSLog(@"evaluateJavaScript error : %@", error.localizedDescription);
    }
}];

answered Mar 21, 2017 at 18:50

Steve Yost

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Comments

Tri Do · Accepted Answer · 2022-06-07 04:03:37Z

Base on @mort3m's comment. Here is working Objective-C version.

@implementation WKWebView(SynchronousEvaluateJavaScript)

- (void)stringByEvaluatingJavaScriptFromString:(NSString *)script completionHandler:(void (^ _Nullable)(_Nullable id, NSError * _Nullable error))completionHandler
{
    __block BOOL finished = FALSE;
    [self evaluateJavaScript:script completionHandler:^(id result, NSError *error) {
        if (error == nil) {
            if (result != nil) {
                completionHandler(result, error);
            }
        } else {
            completionHandler(NULL, error);
        }
        finished = TRUE;
    }];

    while(!finished) {
        [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
    }
}

@end

boraseoksoon · Accepted Answer · 2023-01-21 08:55:58Z

Swift 5.7

All you need should be

webView.evaluateJavaScript("your js code") { res, err in 
    // TODO
}

a full code snippet example is

let js = """
function add(x, y) {
    return x + y
}

add(1, 2)
"""

webView.evaluateJavaScript(js) { res, err in
    print("res : \(res)") 
}

if you prefer async/await:

let js = """
function add(x, y) {
    return x + y
}

add(1, 2)
"""

Task {
    let res = try? await webView.evaluate(javascript: js)
    print("res : \(res)")
}

extension WKWebView {
    
    @discardableResult
    func evaluate(javascript: String) async throws -> Any {
        return try await withCheckedThrowingContinuation({ continuation in
            evaluateJavaScript(javascript, in: nil, in: .page) { result in
                switch result {
                case .success(let output):
                    continuation.resume(returning: output)
                case .failure(let error):
                    continuation.resume(throwing: error)
                }
            }
        })
    }
}

ma sa · Accepted Answer · 2021-06-29 09:51:42Z

Only this works,the answers above not work for me.

- (NSString *)stringByEvaluatingJavaScriptFromString:(NSString *)script
{
    __block NSString *resultString = nil;
    dispatch_semaphore_t sem = dispatch_semaphore_create(0);
    [self evaluateJavaScript:script completionHandler:^(id result, NSError *error) {
        if (error == nil) {
            if (result != nil) {
                resultString = [NSString stringWithFormat:@"%@", result];
            }
        } else {
            NSLog(@"evaluateJavaScript error : %@", error.localizedDescription);
        }
        dispatch_semaphore_signal(sem);
    }];
    dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
    
    return resultString;
}

void · Accepted Answer · 2019-11-25 19:50:00Z

-2

It's possible to use dispatch semaphore. It works on iOS12+

Example:

    __block NSString *resultString = nil;
    dispatch_semaphore_t sem = dispatch_semaphore_create(0);
    [self evaluateJavaScript:script completionHandler:^(id result, NSError *error) {
        if (error == nil) {
            if (result != nil) {
                resultString = [NSString stringWithFormat:@"%@", result];
                dispatch_semaphore_signal(sem);
            }
        } else {
            NSLog(@"evaluateJavaScript error : %@", error.localizedDescription);
        }
        finished = YES;
    }];

    dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);

    //process resultString here.

answered Nov 25, 2019 at 19:50

void

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2 Comments

void Over a year ago

@Shizam you maybe willing to ensure that your WKWebview is loaded first. Such approach doesnt work indeed when you are trying to check if document is ready, meanwhile it works fine for me just for extracting some innerHTML.

Ivan Over a year ago

The documentation says: "The completion handler always runs on the main thread." So if this code is called on main thread it will block forever.

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