These constants can be use in Swift code by calling Constant. ParseClientKey for example. But in my code, it also contains some Objective-C classes. So my question is how to use these constants in the Objective-C code? If this way to declare constants is not good then what is the best way to create global constants to be used in both Swift and Objective-C code?
Sad to say, you can not expose struct , nor global variables to Objective-C. If you use an Objective-C API that needs to process your data, or you need to fit your data model into an existing class hierarchy defined in an Objective-C framework, you might need to use classes and class inheritance to model your data.
For example, many Objective-C frameworks expose classes that you are expected to subclass. Why not create a file with both a struct and an objc class , something like this:. You should make the let statements private if you want to make other Swift types in your code to access these constants only via class:.
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Dinh Quan Dinh Quan 1, 2 2 gold badges 10 10 silver badges 9 9 bronze badges. If you send someone a table of expenses, you're not sending that person a label that represents that information — you're sending the information itself.
Without talking to anyone else, the listener could calculate a total, or write the expenses down to consult later. If the listener prints out the expenses and modifies them, that doesn't modify the table you still have. A value can be a number, perhaps representing a price, or a string — like a description. It could be a selection among options — an enum: was this expense for a dinner, for travel, or for materials? It could contain several other values in named positions, like the CLLocationCoordinate2D struct, which specifies a latitude and longitude.
Or it could be a list of other values Fido might run around and bark on his own accord. He might have special behavior that makes him different from every other dog. He might have relationships established with others. You can't just swap Fido out for another dog — your kids could tell the difference! But the table of expenses exists in isolation. Those strings and numbers don't do anything.
They aren't going to change out from under you. No matter how many different ways you write the "6" in the first column, it's still just a "6. Objective-C and C had value types, but Swift allows you to use them in previously impractical scenarios.
For instance, the generics system permits abstractions that handle value and reference types interchangeably: Array works equally well for Int s as for UIView s.
Enums are vastly more expressive in Swift, since they can now carry values and specify methods. Structs can conform to protocols and specify methods. Swift's enhanced support for value types affords a tremendous opportunity: value types are an incredibly flexible tool for making your code simpler.
You can use them to extract isolated, predictable components from fat classes. Value types enforce — or at least encourage — many properties that work together to create clarity by default. In this section, I'll describe some of the properties that value types encourage.
It's worth noting that you can make objects that have these properties, but the language provides no pressure to do that. If you see an object in some code, you have no reasonable expectation of these properties, whereas if you see a value type, you do. It's true that not all value types have these properties — we'll cover that shortly — but these are reasonable generalizations. A value type does not, in general, behave. It is typically inert. It stores data and exposes methods that perform computations using that data.
Some of those methods might cause the value type to mutate itself, but control flow is strictly controlled by the single owner of the instance.
And that's great! It's much easier to reason about code that will only execute when directly invoked by a single owner. By contrast, an object might register itself as a target of a timer. It might receive events from the system. These kinds of interactions require reference types' multiple-owner semantics. Because value types can only have a single owner and they don't have deinitializers, it's awkward to write value types that perform side effects on their own.
A typical value type has no implicit dependencies on the behavior of any external components. The struct statement defines a new data type, with more than one member for your program.
The structure tag is optional and each member definition is a normal variable definition, such as int i; or float f; or any other valid variable definition. At the end of the structure's definition, before the final semicolon, you can specify one or more structure variables but it is optional. To access any member of a structure, we use the member access operator. The member access operator is coded as a period between the structure variable name and the structure member that we wish to access.
You would use struct keyword to define variables of structure type. You can pass a structure as a function argument in very similar way as you pass any other variable or pointer. Now, you can store the address of a structure variable in the above-defined pointer variable.
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