Memory leak when using WKScriptMessageHandler

What you're seeing is a WebKit bug: https://bugs.webkit.org/show_bug.cgi?id=136140. It was fixed in WebKit trunk a while ago, but does not appear to have been merged into any WebKit updates.

You can work around this by adding a -dealloc to WKScriptMessage that compensates for the over-retain. It could look something like this:

//
//  WKScriptMessage+WKScriptMessageLeakFix.m
//  TestWebkitMessages
//
//  Created by Mark Rowe on 6/27/15.
//  Copyright © Mark Rowe.
//
//  Permission is hereby granted, free of charge, to any person obtaining a copy of this software and
//  associated documentation files (the "Software"), to deal in the Software without restriction,
//  including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense,
//  and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so,
//  subject to the following conditions:
//
//  The above copyright notice and this permission notice shall be included in all copies or substantial
//  portions of the Software.
//
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT
//  LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
//  IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
//  WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
//  SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

#import <mach-o/dyld.h>
#import <objc/runtime.h>
#import <WebKit/WebKit.h>

// Work around <https://webkit.org/b/136140> WKScriptMessage leaks its body

@interface WKScriptMessage (WKScriptMessageLeakFix)
@end

@implementation WKScriptMessage (WKScriptMessageLeakFix)

+ (void)load
{
    // <https://webkit.org/b/136140> was fixed in WebKit trunk prior to the first v601 build being released.
    // Enable the workaround in WebKit versions < 601. In the unlikely event that the fix is backported, this
    // version check will need to be updated.
    int32_t version = NSVersionOfRunTimeLibrary("WebKit");
    int32_t majorVersion = version >> 16;
    if (majorVersion > 600)
        return;

    // Add our -dealloc to WKScriptMessage. If -[WKScriptMessage dealloc] already existed
    // we'd need to swap implementations instead.
    Method fixedDealloc = class_getInstanceMethod(self, @selector(fixedDealloc));
    IMP fixedDeallocIMP = method_getImplementation(fixedDealloc);
    class_addMethod(self, @selector(dealloc), fixedDeallocIMP, method_getTypeEncoding(fixedDealloc));
}

- (void)fixedDealloc
{
    // Compensate for the over-retain in -[WKScriptMessage _initWithBody:webView:frameInfo:name:].
    [self.body release];

    // Call our WKScriptMessage's superclass -dealloc implementation.
    [super dealloc];
}

@end

Drop this in an Objective-C file in your project, set the compiler flags for this file to contain -fno-objc-arc, and it should take care of the leak for you.

I've encountered same problem on iOS 9 SDK.

I noticed userContentController.addScriptMessageHandler(self, name: "handler") will keep the reference of the handler. To prevent leaks, simply remove the message handler when you no longer need it. e.g. when you dismiss the said controller, call a clean up method that will call removeScriptMessageHandlerForName().

You might consider move the addScriptMessageHandler() to viewWillAppear and add a corresponding removeScriptMessageHandlerForName() calls in viewWillDisappear.