How to Correctly handle Weak Self in Swift Blocks with Arguments

Question

In my TextViewTableViewCell  I have a variable to keep track of a block and a configure method where the block is passed in and assigned  Here is my TextViewTableViewCell class           TextViewTableViewCell swift     import UIKit  class TextViewTableViewCell  UITableViewCell  UITextViewDelegate         IBOutlet var textView   UITextView      var onTextViewEditClosure     text   String  - gt  Void        func configure  text  String   onTextEdit     text   String  - gt  Void             onTextViewEditClosure   onTextEdit         textView delegate   self         textView text   text                pragma mark - Text View Delegate      func textViewDidEndEditing textView  UITextView             if onTextViewEditClosure               onTextViewEditClosure  text  textView text                      When I use the configure method in my cellForRowAtIndexPath method  how do I properly use weak self in the block that I pass in  Here is what I have without the weak self    let myCell   tableView dequeueReusableCellWithIdentifier textViewCellIdenfitier  as TextViewTableViewCell myCell configure text  body  onTextEdit    text  String  in       THIS SELF NEEDS TO BE WEAK      self body   text    cell   bodyCell   UPDATE  I got the following to work using  weak self     let myCell   tableView dequeueReusableCellWithIdentifier textViewCellIdenfitier  as TextViewTableViewCell myCell configure text  body  onTextEdit    weak self   text  String  in         if let strongSelf   self                strongSelf body   text              cell   myCell   When I do  unowned self  instead of  weak self  and take out the if statement  the app crashes  Any ideas on how this should work with  unowned self

User · Answer

Put  unowned self  before  text  String     in your closure  This is called a capture list and places ownership instructions on symbols captured in the closure

User · Answer

If self could be nil in the closure use  weak self    If self will never be nil in the closure use  unowned self    If it s crashing when you use  unowned self  I would guess that self is nil at some point in that closure  which is why you had to go with  weak self  instead    I really liked the whole section from the manual on using strong  weak  and unowned in closures   https   developer apple com library content documentation Swift Conceptual Swift Programming Language AutomaticReferenceCounting html  Note  I used the term closure instead of block which is the newer Swift term   Difference between block  Objective C  and closure  Swift  in ios

User · Answer

You can use  weak self  or  unowned self  in the capture list prior to your parameters of the block   The capture list is optional syntax    unowned self  works good here because the cell will never be nil   Otherwise you can use  weak self

User · Answer

Use Capture list     Defining a Capture List      Each item in a capture list is a pairing of the weak or unowned   keyword with a reference to a class instance  such as self  or a   variable initialized with some value  such as delegate     self delegate    These pairings are written within a pair of square   braces  separated by commas       Place the capture list before a closure   s parameter list and return   type if they are provided    lazy var someClosure   Int  String  - gt  String          unowned self  weak delegate   self delegate    index  Int  stringToProcess  String  - gt  String in        closure body goes here          If a closure does not specify a parameter list or return type because   they can be inferred from   context  place the capture list at the very start of the closure    followed by the in keyword    lazy var someClosure  Void - gt  String          unowned self  weak delegate   self delegate   in        closure body goes here     additional explanations

User · Answer

Closure and strong reference cycles  As you know Swift s closure can capture the instance  It means that you are able to  use self inside a closure  Especially escaping closure About  can create a strong reference cycle About   By the way you have to explicitly use self inside escaping closure  Swift closure has Capture List feature which allows you to avoid such situation and break a reference cycle because do not have a strong reference to captured instance  Capture List element is a pair of weak unowned and a reference to class or variable  For example class A       private var completionHandler      - gt  Void       private var completionHandler2    String  - gt  Bool            func nonescapingClosure completionHandler     - gt  Void            print  quot Hello World quot                  func escapingClosure completionHandler   escaping    - gt  Void            self completionHandler   completionHandler                func escapingClosureWithPArameter completionHandler   escaping  String  - gt  Bool            self completionHandler2   completionHandler          class B       var variable    quot Var quot           func foo             let a   A                      nonescapingClosure         a nonescapingClosure               variable    quot nonescapingClosure quot                               escapingClosure           strong reference cycle         a escapingClosure               self variable    quot escapingClosure quot                               Capture List -  weak self          a escapingClosure   weak self  in             self  variable    quot escapingClosure quot                               Capture List -  unowned self          a escapingClosure   unowned self  in             self variable    quot escapingClosure quot                               escapingClosureWithPArameter         a escapingClosureWithPArameter    weak self   str  - gt  Bool in             self  variable    quot escapingClosureWithPArameter quot              return true                     weak - more preferable  use it when it is possible unowned - use it when you are sure that lifetime of instance owner is bigger than closure   weak vs unowned

User · Answer

From Swift 5 3  you do not have to unwrap self in closure if you pass  self  before in in closure  Refer someFunctionWithEscapingClosure    self  in x   100   in this swift doc

User · Answer

If you are crashing than you probably need  weak self   My guess is that the block you are creating is somehow still wired up   Create a prepareForReuse and try clearing the onTextViewEditClosure block inside that   func prepareForResuse        onTextViewEditClosure   nil    textView delegate   nil     See if that prevents the crash    It s just a guess

User · Answer

EDITED for Swift 4 2    As  Koen commented  swift 4 2 allows   guard let self   self else      return    Could not get a strong reference for self           Now self is a strong reference self doSomething     P S   Since I am having some up-votes  I would like to recommend the reading about escaping closures     EDITED  As  tim-vermeulen has commented  Chris Lattner said on Fri Jan 22 19 51 29 CST 2016  this trick should not be used on self  so please don t use it  Check the non escaping closures info and the capture list answer from  gbk     For those who use  weak self  in capture list  note that self could be nil  so the first thing I do is check that with a guard statement  guard let  self    self else      return   self doSomething     If you are wondering what the quote marks are around self is a pro trick to use self inside the closure without needing to change the name to this  weakSelf or whatever

User · Answer

EDIT  Reference to an updated solution by LightMan   See LightMan s solution  Until now I was using   input action      weak self  value in     guard let this   self else   return       this someCall value      this  isn t nil     Or   input action      weak self  value in     self  someCall value     call is done if self isn t nil     Usually you don t need to specify the parameter type if it s inferred   You can omit the parameter altogether if there is none or if you refer to it as  0 in the closure   input action      weak self  in     self  someCall  0     call is done if self isn t nil     Just for completeness  if you re passing the closure to a function and the parameter is not  escaping  you don t need a weak self    1 2 3 4 5  forEach   self someCall  0

User · Answer

Swift 4 2  let closure      weak self     parameter Int  in     guard let self   self else   return        self method parameter      https   github com apple swift-evolution blob master proposals 0079-upgrade-self-from-weak-to-strong md

User · Answer

As of swift 4 2  we can do          weak self  value in     guard let self   self else   return       print self     will never be nil

[ios] How to Correctly handle Weak Self in Swift Blocks with Arguments

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