markdown Objective-C备忘录

Posted 2021-05-06

## Structure
- You have .h and .m file
- .h files are "Header" files, where you declare functions and variables
- .m files are "Implementation" files, where you implement functions
- You have to import .h files in your .m file
- `@property` is the keyword for declaring a variable in a .h
- A property is public
- A instance variable is private

## Allocation
- To allocate a new objet, you have to follow this syntax: `Person *person1 = [[Person alloc] init];`

## Pointer
- You have to add an `*` when declaring a new variable, just before the name (for all non-number objects)
(i.e: `NSString *myStr = @"Hello"`)

## Getter
- You can override the getter of a property in your .m like following: `-(type)myproperty{...}`
- `_myproperty` will be the property value

## Setter
- You can override the setter of a property in your .m like following: `-(void)setMyproperty:(type)myproperty{...}`
- `myproperty` will be the new value
- `_myproperty`will be the old value

## String
- You have to use the `@`sign before the string (i.e: `@"Test"`)
- You have to use the format function to construct string with variables
(i.e: `[NSString stringWithFormat: @"My name is %@!", firstName];`)

## Number
- You can use `int`, `float` or `double` but if you need to store numbers in an array you have to use `NSNumber`
(i.e: `NSNumber *myNum = [NSNumbre numberWithInt:8];)`

## Dot VS Brackets
- Use dot just for properties (when possible)
- 

## Boolean
- Use `BOOL` type
- Use `YES` and `NO` as value
- Always use `if (myBool) {}` instead of `if (myBool == YES) {}`
- Always use `if (!myBool) {}` instead of `if (myBool == NO) {}`

## Methods
- You can declare a function as static with `+` attribute instead of `-`

## Arrays
- Use `NSArray` for array initialized with data (you can't modify data)
- Use `NSMutableArray` for array that should be modified

## Atomic vs Nonatomic
- Atomic is thread safe but slower
- Nonatomic is not thread safe but fast

## Strong vs Weak
- Strong ensure that the compiler does not destroy the object (until the property receive a `nil` value)
- Weak is used for delegates, outlets, subviews, controls... It specifie a reference that is strongly referenced by an object. Once the object is destroyed, the weak property is destroyed as well. It avoid *retain cycle* (memory leak)

## Nullability
- `_Nullable` is the equivalent of `?` in Swift
- `_Nonnull` is the equivalent of `!` in Swift

## Category and Extension
- A category in Objective-C allows to add features to a Class
- An extension allows is like a category but also allows to modify properties of a class