Swift stringByEvaluatingJavaScriptFromString

Question

I try to use some javascript on my WebView with the new

stringByEvaluatingJavaScriptFromString function

I m not quiet familiar with the syntax so i tried

 func stringByEvaluatingJavaScriptFromString( "document.documentElement.style.webkitUserSelect='none'": String) -> String? 

as shown here https://developer.apple.com/library/ios/documentation/UIKit/Reference/UIWebView_Class/index.html#//apple_ref/occ/instm/UIWebView/stringByEvaluatingJavaScriptFromString: but i get a error that "Expected '.' separator"

Answer 1

The method you are trying to call is prototyped as the following:

func stringByEvaluatingJavaScriptFromString(_ script: String) -> String?

This means :

• It takes a String as single parameter
• It returns an optional String (String?)

You need to have an instance of UIWebView to use it:

let result = webView.stringByEvaluatingJavaScriptFromString("document.documentElement.style.webkitUserSelect='none'")

Because the return type is optional, it needs to be unwrapped before you can use it. But be careful, it may not have a value (i.e. it may be equal to nil) and unwrapping nil values leads to runtime crashes.

So you need to check for that before you can use the returned string:

if let returnedString = result {
    println("the result is \(returnedString)")
}

This means: If result is not nil then unwrap it and assign it to a new constant called returnedString.

Additionally, you can wrap it together with:

let script = "document.documentElement.style.webkitUserSelect='none'"
if let returnedString = webView.stringByEvaluatingJavaScriptFromString(script) {
    println("the result is \(returnedString)")
}

Hope this makes sense to you.

Answer 2

This method is used to call the javascript script directly from uiwebview

let htmlTitle = myWebView.stringByEvaluatingJavaScriptFromString("document.title");
println(htmlTitle) 

http://sourcefreeze.com/uiwebview-example-using-swift-in-ios/

Answer 3

1) Create JavaScript constant:

let jsString: String = "document.documentElement.style.webkitUserSelect='none'"

2) Using evaluateJavaScript function for WKWebView (iOS 8.0+)

self?.webView?.evaluateJavaScript("", completionHandler: { result, error in
       print(result)
})

or stringByEvaluatingJavaScriptFromString for UIWebView (iOS 2.0–12.0)

let result = webView.stringByEvaluatingJavaScriptFromString(jsString)
print(result)

Also from Apple doc:

The stringByEvaluatingJavaScriptFromString: method waits synchronously for JavaScript evaluation to complete. If you load web content whose JavaScript code you have not vetted, invoking this method could hang your app. Best practice is to adopt the WKWebView class and use its evaluateJavaScript:completionHandler: method instead.