How to safely read properties of a WebScriptObject?

Question

When communicating between JavaScript in a WebView instance and a WebViewDelegate, JavaScript types and Objective-C types are converted back and forth. For instance, when calling an Objective-C function from JavaScript, a string becomes an NSString, a number becomes an NSNumber, and an Object becomes a WebScriptObject.

The others are pretty simple to deal with, but WebScriptObject seems weird.

When passing a dictionary like {"foo": 1, "bar": 2}, most of the code I see extracts the properties using valueForKey, such as in [[arg valueForKey:@"foo"] intValue] == 1

But what about if you're not sure if the property exists? What if the keys are optional? [arg valueForKey:@"baz"] throws an exception.

One thing I can do is something like

@try {
  foo = [[arg valueForKey:@"baz"] intValue];
}
@catch (NSException* e) {
  foo = 0;
}

but I've heard that exceptions in Objective-C are unsafe and should not be used for flow control.

The only other way I can think of is some variation of the method used here: http://edotprintstacktrace.blogspot.com/2011/10/sample-webscriptobject-javascript.html

In other words: 1. use evaluateWebScript to define a JavaScript function that implements Object.keys 2. call that function on your WebScriptObject 3. iterate through the returned array of keys, and only call valueForKey if we find a match.

This seems incredibly inefficient, to me. There must be a better way... is there?

Answer 1

Since OS X 10.9 and iOS 7 there is a ways more easy way to do this. Apple introduced a class named JSValue wich can be used to bridge JavaScript objects to regular ObjC objects:

// WebScriptObject *options; obtained from a callback in ObjC
id objCObject = [[options JSValue] toObject];

if ([objCObject isKindOfClass:[NSArray class]]) {
    for (id object in objCObject) {
        NSLog(@"object: %@", object);
    }
}
else if ([objCObject isKindOfClass:[NSDictionary class]]) {
    for (id<NSCopying> key in [objCObj allKeys]) {
       NSLog(@"object for key %@: %@", key, [objCObject objectForKey:key]);
    }
}

Answer 2

I think I found something that works - the trick is that you can convert WebScriptObject to JSObjectRef with -JSObject, and there's a whole bunch of C methods that work on JSObjectRefs, though the docs are a bit lacking so it's hard to figure out what to do exactly.

Here's how you can check if a property exists:

id getProperty(WebScriptObject *obj, NSString *prop) {
    JSStringRef jsProp = JSStringCreateWithCFString((__bridge CFStringRef) prop);

    if (JSObjectHasProperty(self.frame.globalContext, [obj JSObject], jsProp)) {
       return [options valueForKey:prop];
    } else {
       return nil;
    }

    JSStringRelease(jsString);
}

If you want to list all properties (whose keys aren't known in advance), you'll need to use a few more functions:

JSPropertyNameArrayRef properties =
    JSObjectCopyPropertyNames(self.frame.context, [obj JSObject]);

size_t count = JSPropertyNameArrayGetCount(properties);

for (NSInteger i = 0; i < count; i++) {
    JSStringRef property = JSPropertyNameArrayGetNameAtIndex(properties, i);
    // ... etc. as above
}