传递给不同类中的方法时，无法更改Objective-C NSArray

Question

I have an NSArray and I need to change the order of the items within it. I have written a method that will determine the new order:

+(NSArray*)sortProxyForms:(NSArray*)arrayOfForms
{
    NSArray* sortedForms = [arrayOfForms sortedArrayUsingComparator:^(HPSModelFormProxy* a, HPSModelFormProxy* b) {

        return [@(a.ordinal) compare:@(b.ordinal)]; // @(a.ordinal) aka Boxing turns int into NSNumber

    }];

    arrayOfForms = [sortedForms copy]; // DOES NOT WORK

    return sortedOfForms; // WORKS IF ASSIGNED IN THE CALLER

}

So, I can pass the NSArray to be sorted into the method. I call the method like this:

[HPSModelUtilities sortProxyForms:_formProxies];

If I actually try setting arrayOfForms (a reference to _formProxies ) within the method then once I have returned from the method then the array is unchanged.

However, if I return the sorted array from the method and assign it to the NSArray in the calling method then the assignment works:

_formProxies = [HPSModelUtilities sortProxyForms:_formProxies]; // _formProxies NSArray is changed

_formProxies is declared in the calling class, and "HPSModelUtilities" is a different class.

How come the NSArray can be changed in the caller, but not changed in the called method, even though it is passed by reference?

Answer 1

When you pass a value into a method it is copied. This is called "pass by value". The arrayOfForms you are passing in is a pointer to an NSArray . This means that the pointer is copied when passed in. Redirecting this pointer to another instance of an NSArray does not change where the original pointer is pointing.

I would rename your method to (NSArray*)sortedArrayFromProxyForms:(NSArray*)proxyForms

If you really want to change where your NSArray reference is pointing in the method. Do it like this.

+ (void)sortProxyForms:(NSArray**)proxyForms {
    *proxyForms = sortedForms;
}

Answer 2

You are passing a copy of the array reference (subtly different than passing by reference), but then you are changing where that reference points with this line:

arrayOfForms = [sortedForms copy];

arrayOfForms no longer points to the array instance you passed, but to a different array. You could pass a pointer of pointer, and change where the caller's pointer is pointing, but for what you are doing, I think the reassignment is fine.

If you'd really like here's what your function would look like with pointer of pointer:

+(void)sortProxyForms:(NSArray**)arrayOfForms {
    NSArray* sortedForms = [arrayOfForms sortedArrayUsingComparator:^(HPSModelFormProxy* a, HPSModelFormProxy* b) {

        return [@(a.ordinal) compare:@(b.ordinal)]; // @(a.ordinal) aka Boxing turns int into NSNumber

    }];

    *arrayOfForms = [sortedForms copy]; 
}

but I'll add the caveat that this isn't a pattern you see often in objective-c, so I'd avoid it when there are other alternatives available.

Also note when calling this function you need to add the & to get the extra level of indirection:

[HPSModelUtilities sortProxyForms:&_formProxies];