如何在NSInvocation中存储一个初始化调用？

Question

I'm trying to set up an app that works with UIPageViewController. I technically could instantiate all the view at once and put them in an array, but that would be expensive, and it seems better to initialize the views as needed. I read that an NSInvocation is a message rendered static-- so I was thinking I could have an array that basically contains: [SubclassA alloc]init], [SubclassB alloc]init], etc... with those messages wrapped in an NSInvocation. I could then return the result of that message in pageViewController:ViewControllerAfter/BeforeViewController:.

I'm still pretty new at this, so it is very possible I am fundamentally misunderstanding NSInvocation, but either way, an answer would still be helpful.

Answer 1

NSInvocation is for manipulating calls where you don't know the method or number/types of arguments until runtime. Here, the method called is fixed at compile time. You don't need NSInvocation .

If you want to store an action to perform as an object that you can put into an array, you should use blocks.

Answer 2

Sounds like you want some kind of lazy instantiation.

You can use this helper class

@interface LazyObject : NSObject

@property (copy) id (^block)(void));
@property (readonly) id object;

+ (instancetype)create:(id (^)(void))block;

@end

@implementation LazyObject {
    id _object;
}

+ (instancetype)create:(id (^)(void))block
{
    LazyObject *obj = [[self alloc] init];
    obj.block = block;
    return obj;
}

- (id)object
{
    if (!_object) _object = self.block();
    return _object;
}

@end

---

NSArray *array = @[[LazyObject create:^id{ return [[SomeClassA alloc] init];}],
                   [LazyObject create:^id{ return [[SomeClassB alloc] init];}]];

SomeClassA *a = [array[0] object];
SomeClassB *b = [array[1] object];

---

As others said, NSInvocation is not a good solution for your original problem. It is designed to invoke method dynamically. You don't need it for most of the cases.