Autoreleasepool problem summary

AutoreleasepoolPage

call autorelease The object eventually passes through AutoreleasepoolPage Objects to manage .AutoreleasepoolPage The structure of the object is as follows

Next： Point to the latest autorelease The next position of the object , That is, the first available location .

thread： Current thread .

parent： Point to parent node , Of the first node parent yes nil.

Child： Point to the child node , Of the last node child Point to nil.

Depth： Expressing depth .

magic： Used to verify AutoreleasepoolPage Whether the structure is complete .

Hiwat： Upper limit of data container

 

Every autoreleasepoolpage Object occupancy 4096 Byte memory , In addition to storing internal member variables , The rest is stored autorelease Address of the object .autoreleasepoolpage The object is through Double linked list Connected together in the form of .

push and pop Method

When an object calls [object autorelease] Function will add the object to autoreleasepool in , That is to say, it will go on push operation .

push operation , Will be called to autoreleaseFast function . This is its initialization function . By calling hotpage() function , Judge that there are three situations ：

1、 If hotpage There is , And the capacity is not full , Then call directly add Function to add an object .

2、 If hotpage There is , But it's full , call autoreleaseFullPage function , And initialize a AutoreleasePage Marked as hotpage.

3、 If hotpage non-existent , call autoreleaseNoPage function , Create a AutoreleasePage Marked as hotpage, Add sentinel object .

 

Autoreleasepool Of pop The operation is mainly

1、 Get the location through the sentinel object autoreleasepoolpage

2、 call page->releaseUntil(stop) function , To the top of the stack stop Objects call objc_release() Operation reference count -1

3、 Last call child Of kill Method .

 

autoreleasepool Creation and release time of

establish

App After starting , Main thread runloop Two will be added in observer, The first is surveillance loop Of entry event , Within the callback event autoreleaspoolpush Create an automatic release pool . And set this observer Of order The highest priority （ Negative 2 Of 32 Power -1）, Ensure that the automatic release pool is created before other callbacks .

Release

the second observer Monitoring is to prepare for hibernation and about to exit runloop, It calls autoreleasepoolpop Release the automatic release pool , Its observer Of order Set the lowest priority (2 Of 32 Power -1), It ensures that the automatic release pool is released after other callbacks . It prevents the automatic release pool from releasing objects prematurely and causing crashes .

 

Object release timing

mrc Next

Manual call autorelease Method , The release timing is not determined by the scope of the object , But according to runloop Different states of , When runloop The system will be cleaned up at the end autorelease Objects to be processed . and runloop The end is not fixed duration.

arc Next

Local objects are destroyed after they are out of scope , When the system is out of scope , Call it automatically release Method .

 

autoreleasepool Use scenarios

Official documents indicate that there are three scenarios ：

1、 When writing a command line program , No, UI Framework procedures . because autorelease The mechanism is based on UI framework, So if it's not based on UI framework Your program needs to manage the life cycle of the object itself .

2、 Write a cycle , It contains a large number of temporarily created objects . because autorelease The trigger time is next runloop When , So if there are a lot of autorelease object , Then next time runloop There is no chance to be released before coming back , Will cause memory explosion , May cause crash problems .

3、 Create your own thread .

 

What objects will be added to Autoreleasepool in

1、 Use alloc、copy、new Etc , System management object memory .

2、 Use __weak Decorated object , To ensure that references are not discarded , Will register to Autoreleasepool in .

3、id Pointer to or pointer to an object , It will be registered to Autoreleasepool in .