从C调用Objective C

Question

I have a callback function in C which looks like this:

 void cfunc(char* labelUpdateText)

This callback function is supposed to update the text of a label in my app's UI by changing the label's current text to whatever labelUpdateText is.

So in cfunc I'm trying to call this Objective C function:

-(void)updateLabel:(char*) text
{
    [_labelProp setStringValue:[NSString stringWithFormat:@"%@", [NSString stringWithUTF8String:text]]];
}

However, I can't call the objective C function from my C function.

I tried like this:

updateLabel:labelUpdateText

But when I build the code, I get the warning 'Expression result unused'

And the 'updateLabel:labelUpdateText' doesn't execute.

What am I doing wrong?

PS: I am new to Objective C!

Answer 1

You need to provide the callback function with a pointer to the view containing the label and some means of updating it. All callback functions should have a context pointer of some sort which can hold information allowing them to perform context-specific actions. If you cannot change the callback function semantics then you are stuck I'm afraid, however if you can, do it something like this:

void cfunc(const char *labelUpdateText, void *contextInfo) {
    YourView *view = (YourView *)contextInfo;
    [view updateLabelWithText:[NSString stringWithUTF8String:labelUpdateText]];
}

You will need to provide the updateLabelWithText method in YourView and if you are using ARC there will be some bridging to do as well during the casting of contextInfo (the compiler will help you there though).

Answer 2

为什么不在Objective C类中创建一个包装器C函数来调用ObjC语法并从需要进行回调的C程序中调用C函数。