UIView.init（）在哪里记录？

Question

In the folowing example:

import UIKit

class MyView: UIView
{
    override init(frame: CGRect) {
       super.init(frame: frame)
       print ("...init with frame")
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        print("...init with coder")
    }
}

.......

 let var = MyView() //  "...init with frame" gets printed

So it seems MyView() calls UIView.init() which in turn calls init(frame: CGRect) . However according to the UIView documentation there is only a init(frame:) (and (init:coder)).

According to the documenation for NSObject. init()

does no initialization; it simply returns self.

So there must be a UIView.init() that overrides NSObject.init() and in turn calls init(frame:) . But this UIView.init() isn't documented? Or am I missing this documentation?

Answer 1

I have found no documentation for UIView.init , but UIView.init does exist. If you set a breakpoint at your print ("...init with frame") , you will see the call to [UIView init] in the Debug Navigator . The assembly code clearly shows it calling objc_msgSend to initWithFrame: using CGRectZero as the frame:

UIKit`-[UIView init]:
    0x10b3ba160 <+0>:  pushq  %rbp
    0x10b3ba161 <+1>:  movq   %rsp, %rbp
    0x10b3ba164 <+4>:  subq   $0x20, %rsp
    0x10b3ba168 <+8>:  movq   0xc52f71(%rip), %rsi      ; "initWithFrame:"
    0x10b3ba16f <+15>: movq   0xcd5f72(%rip), %rax      ; (void *)0x000000010f732690: CGRectZero
    0x10b3ba176 <+22>: movq   0x18(%rax), %rcx
    0x10b3ba17a <+26>: movq   %rcx, 0x18(%rsp)
    0x10b3ba17f <+31>: movq   0x10(%rax), %rcx
    0x10b3ba183 <+35>: movq   %rcx, 0x10(%rsp)
    0x10b3ba188 <+40>: movq   (%rax), %rcx
    0x10b3ba18b <+43>: movq   0x8(%rax), %rax
    0x10b3ba18f <+47>: movq   %rax, 0x8(%rsp)
    0x10b3ba194 <+52>: movq   %rcx, (%rsp)
    0x10b3ba198 <+56>: callq  *0xcd7042(%rip)           ; (void *)0x000000010c8b9800: objc_msgSend
    0x10b3ba19e <+62>: addq   $0x20, %rsp
    0x10b3ba1a2 <+66>: popq   %rbp
    0x10b3ba1a3 <+67>: retq   

Answer 2

UIView.init() exists — however, it's not publicly declared because that declaration is inherited from the NSObject class.

In neither Swift nor ObjC-bridged-to-Swift does a class have to declare as public API the methods/properties it inherits from a superclass or includes by conforming to a protocol. Inheritance or protocol adoption means that the class supports those methods/properties, but it doesn't tell you anything about what the class' implementation of them does . For that you have to read documentation... carefully.

Expanding a bit on the NSObject.init() doc you quoted (with some editing for emphasis):

Implemented by subclasses to initialize a new object (the receiver) immediately after memory for it has been allocated.

The init() method defined in the NSObject class does no initialization... (1)

In a custom implementation of this method, you must invoke super 's initializer then initialize and return the new object... (2)

This could probably be clearer ( I recommend filing a bug against the documentation so Apple knows to improve it ), so a quick translation:

1. In a "bare" NSObject instance, init() does nothing. That is, the method is there purely for subclassing.

2. Subclasses are expected to invoke super and perform their own initialization. Where you see "custom" and "you" here, read it as the implementor of a subclass — that, is, the "you" here applies equally to you, the third-party developer creating new classes in your app, and to the authors of other Apple classes that subclass NSObject . (You know, just about all of them.)

---

The way Apple's API documentation seems to work, they only have docs at the method level for methods that are declared by a class — there aren't docs on UIView for all the hundreds of methods that it inherits from all of its superclasses, nor on each of the many UIView subclasses for all of the methods they inherit from UIView .

As noted in other answers, the no-args initializer for UIView is equivalent to using init(frame:) and passing CGRect.zero . Even though Apple's docs don't have a place to attach descriptions to UIView.init() , it'd probably be helpful for them to discuss the effects of inherited initializers somewhere else. (Maybe in the UIView class overview ?) That's probably worth filing another documentation bug.

Answer 3

If you call MyView() in swift. It means it call default constructor ie init is called. If you want to call initWithFrame as in objective c then you have to call constructor like MyView(frame:CGRect). And for detail you must follow apple documentation.