maybe check if [args[@"javascriptChannelNames"] is not nil as well?

if(![args[@"javascriptChannelNames"] && args[@"javascriptChannelNames"] isKindOfClass:[NSNull class]]) 

Or if we can be super careful here and add an NSArray type check as well, and then we can drop the NSNull check. Which i think is a better approach. I found some of the code in this classes that I did can be optimized this way as well.

Like below:

// initialize the object to be nil to avoid redundant NSNull check later.
NSArray * javaScriptChannelNames = nil;
// Check if the dictionary value is array class includes a nil check and a NSNull check as well as an array type check, which is cleaner.
if ([args[@"javascriptChannelNames"] isKindOfClass:[NSArray class]]) {
   javaScriptChannelNames = args[@"javascriptChannelNames"];
}
// Since we initialized `javaScriptChannelNames` to be nil, this value can either be nil or an array. hence the only nil check, dropping the NSNull check.
if (javaScriptChannelNames) {
   [_javaScriptChannelNames addObjectsFromArray:javaScriptChannelNames];
   [self registerJavaScriptChannels:_javaScriptChannelNames controller:userContentController];
}

can even make it better if logically we do not want to update the _javaScriptChannelNames and register at all when the [args[@"javascriptChannelNames"] is an empty array.

if (javaScriptChannelNames.count > 0) {
   [_javaScriptChannelNames addObjectsFromArray:javaScriptChannelNames];
   [self registerJavaScriptChannels:_javaScriptChannelNames controller:userContentController];
}

Thanks good point about checking for NSArray.
I believe [foo isKindOfClass:[NSArray class]] doesn't evaluate to true if foo is nil, so I guess this check covers all cases?

As for short-circuiting when the array is empty, I would think the performance gain is insignificant in this case(which doesn't happen many times) so I would choose a little simpler code in this case.

sounds good. and YES [foo isKindOfClass:[NSArray class]] covers all cases.

Can use a property and be lazily initialized.

What's the benefit over doing it this way?

lazy initializing the set can save memory (insignificant tho); In the case of this class, benefit is minimum since the set is used right after class initialization; I'm ok with not lazy initializing here.
Just a thought, if we do move the code [_javaScriptChannelNames addObjectsFromArray:javaScriptChannelNames]; out of the initiWithFrame method, then we can think about lazy initialize the _javaScriptChannelNames to save some memory.

We can't move it out of initWithFrame, as a set of JavaScript channels is passed with the creation params... we immediately need this field initialized...

Thanks good point about checking for NSArray.
I believe [foo isKindOfClass:[NSArray class]] doesn't evaluate to true if foo is nil, so I guess this check covers all cases?

As for short-circuiting when the array is empty, I would think the performance gain is insignificant in this case(which doesn't happen many times) so I would choose a little simpler code in this case.

What's the benefit over doing it this way?

if native send json string back flutter, it will got a string that hard to serialized like below.

{
    action = do_somethineg;
    data =     {
        key_1 = some_info;
        key_2 = "some_info";
    };
    timestamp = 1570005270866;
    token = "jwt_token";
}

is it possible to add code to serialize NSDictionary and NSArray before send message.body:

- (NSString *)messageFromBody:(NSString *)body {
    NSString *message;
    if ([body isKindOfClass:[NSDictionary class]] ||
        [body isKindOfClass:[NSArray class]]) {
        NSData *jsonData = [NSJSONSerialization dataWithJSONObject:body
                                                           options:NSJSONWritingPrettyPrinted
                                                             error:nil];
        message = [[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding];
    }
    if (message.length <= 0) {
        message = [NSString stringWithFormat:@"%@", body];
    }

    return message;
}

...

NSDictionary* arguments = @{
    @"channel" : _javaScriptChannelName,
    @"message" : [self messageFromBody: message.body]
  };

Thanks!

Do you mind filing this as an issue?

Ya, i'm created HERE