這個課程可以在iTune U找到且免費。其包含了所有課程 slides & homeworks，我也將會全部完成。
Let. 1 notes:
- Let & Var
The
let keyword defines a constant:let theAnswer = 42
The
theAnswer cannot be changed afterwards. <-- This is why anything optional or weak can't be written using let. They need to change during runtime and must be wrote using var.
The
var defines an ordinary variable:
What is interesting:
The value of a constant doesn’t need to be known at compile time, but you must assign it a value exactly once.
Another strange feature:
You can use almost any character you like for constant and variable names, including Unicode characters:
let 🐶🐮 = "dog cow"credit: community wiki @stackoverflow
- ! & ? Optional (Wrapped & unwrapped)
In a type declaration the
! is similar to the ?. Both are an optional, but the ! is an "implicitly unwrapped" optional, meaning that you do not have to unwrap it to access the value (but it can still be nil).
This is basically the behavior we already had in objective-c. A value can be nil, and you have to check for it, but you can also just access the value directly as if it wasn't an optional (with the important difference that if you don't check for nil you'll get a runtime error)
// Cannot be nil
var x: Int = 1
// The type here is not "Int", it's "Optional Int"
var y: Int? = 2
// The type here is "Implicitly Unwrapped Optional Int"
var z: Int! = 3Usage:
// you can add x and z
x + z == 4
// ...but not x and y, because y needs to be unwrapped
x + y // error
// to add x and y you need to do:
x + y!
// but you *should* do this:
if let y_val = y {
x + y_val
}
credit: Jiaaro
- Computed properties
我自己程式中的應用：
var displayValue: Double {
get {
return Double(display.text!)!
}
set {
display.text = String(newValue)
}
}
supplement:
useful materials
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