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Implement garbage collector to speed up everything
Deallocating objc objects is costly, especially deallocating deeply nested MLXMLNodes and MLHandlers (with MLXMLNodes bound to it) can take several minutes in extreme situations, even if reading those objects from database only takes a few milliseconds. We don't want to block the receive queue that long, in fact we don't want to block any thread that long. This commit therefore moves the deallocation into a custom garbage collector. The garbage collector takes strong references to every object given to it by calling [MLDelayedDealloc delayFor:] and moves deallocation into a background QOS serial queue that periodically deallocates all objects only referenced by the garbage collector itself. We automatically add all MLXMLNodes and MLHandlers to the garbage collector and also add the list of stanzas and iq handlers when clearing the smacks queue or invalidating iq handlers (although this shouldn't be needed since we automatically add all MLXMLNodes and MLHandlers to th garbage collector inside their init methods). Other objects can be added to the garbage collector as needed or the NSObject init method swizzled to unconditionally add all objects to the garbage collector in the future.
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6 files changed

+314
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‎Monal/Classes/MLDelayedDealloc.h‎

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//
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// MLDelayedDealloc.h
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// monalxmpp
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//
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// Created by Thilo Molitor on 04.03.26.
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// Copyright © 2026 Monal.im. All rights reserved.
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//
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NS_ASSUME_NONNULL_BEGIN
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@interface MLDelayedDealloc : NSObject
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+(void) delayFor:(id) obj;
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@end
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NS_ASSUME_NONNULL_END

‎Monal/Classes/MLDelayedDealloc.m‎

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//
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// MLDelayedDealloc.m
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// monalxmpp
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//
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// Created by Thilo Molitor on 04.03.26.
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// Copyright © 2026 Monal.im. All rights reserved.
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//
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//#define DEBUG_DEALLOC_DEBUGGING
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#import <Foundation/Foundation.h>
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#import <objc/runtime.h>
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#import <monalxmpp/MLConstants.h>
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#import <monalxmpp/MLDelayedDealloc.h>
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static NSMutableArray* _deallocList;
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static dispatch_source_t _timer;
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static dispatch_queue_t _queue;
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#ifdef DEBUG
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static NSString* _oldDebugOutput = nil;
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static NSArray* ContainerChildren(id obj);
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static NSMutableSet* IvarChildren(id obj);
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static NSString* NodeName(id obj);
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static void DumpMemoryGraph(NSSet* objects);
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#endif
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@implementation MLDelayedDealloc
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+(void) initialize
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{
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_deallocList = [NSMutableArray new];
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//configure timer
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_queue = dispatch_queue_create("im.monal.dealloc.timer", dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_SERIAL, QOS_CLASS_BACKGROUND, 0));
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_timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, _queue);
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dispatch_source_set_timer(_timer,
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dispatch_time(DISPATCH_TIME_NOW, 0),
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500 * NSEC_PER_MSEC, //500ms period
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100 * NSEC_PER_MSEC); //100ms leeway
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//periodically dealloc objects
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dispatch_source_set_event_handler(_timer, ^{
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NSMutableArray* deallocCopy = nil;
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//cleanup _deallocList, make this section short to not block other threads
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@synchronized(_deallocList) {
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deallocCopy = [_deallocList mutableCopy];
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[_deallocList removeAllObjects]; //this won't deallocate anything, because we still reference everything in deallocCopy
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}
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NSUInteger count = [deallocCopy count];
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if(count > 0)
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{
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//requeue all entries that are still used by somebody else (retainCount == 1 being only us holding the object in deallocCopy)
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NSUInteger requeuedCount = 0;
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@autoreleasepool {
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NSMutableSet* retainedSet = [NSMutableSet new];
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for(id entry in deallocCopy)
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{
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NSUInteger retainCount = CFGetRetainCount((__bridge CFTypeRef)entry);
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if(retainCount > 1 && retainCount < 65536) //only requeue if still used but not due to some tagged pointer foo etc.
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{
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[self delayFor:entry];
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count--;
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requeuedCount++;
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#ifdef DEBUG
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[retainedSet addObject:entry];
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//DDLogDebug(@"Requeued deallocation with retain count %lu (%lu) of: %p %@", retainCount, CFGetRetainCount((__bridge CFTypeRef)entry), entry, entry);
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#endif
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}
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}
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#ifdef DEBUG
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DumpMemoryGraph(retainedSet);
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#endif
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retainedSet = nil;
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}
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//check if, after removing still retained entries, count is still > 0 and start deallocation, if so
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if(count > 0)
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{
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//for(id entry in deallocCopy)
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// DDLogVerbose(@"Deallocating: %@", NodeName(entry));
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DDLogVerbose(@"Deallocating %lu objects, %lu still used objects requeued for later deallocation...", count, requeuedCount);
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NSDate* start = [NSDate date];
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deallocCopy = nil;
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NSTimeInterval elapsed = [[NSDate date] timeIntervalSinceDate:start];
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if(elapsed > 1.0)
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DDLogWarn(@"Done deallocating %lu objects in %.3fs (%lu still used objects requeued for later deallocation)...", count, elapsed, requeuedCount);
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else
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DDLogVerbose(@"Done deallocating %lu objects in %.3fs (%lu still used objects requeued for later deallocation)...", count, elapsed, requeuedCount);
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}
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}
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});
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dispatch_resume(_timer);
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}
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+(void) delayFor:(id) obj
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{
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if(obj == nil)
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return;
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@synchronized(_deallocList) {
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[_deallocList addObject:obj];
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}
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}
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@end
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//****************************** FOR DEBUGGING ******************************
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#ifdef DEBUG
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static inline __attribute__((always_inline)) NSArray* ContainerChildren(id obj)
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{
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if([obj isKindOfClass:[NSArray class]])
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return obj;
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if([obj isKindOfClass:[NSSet class]])
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return [obj allObjects];
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if([obj isKindOfClass:[NSDictionary class]])
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return [[(NSDictionary*)obj allKeys] arrayByAddingObjectsFromArray:[(NSDictionary*)obj allValues]];
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return @[];
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}
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static inline __attribute__((always_inline)) BOOL slotMatchesLayout(const uint8_t* layout, size_t slotIndex)
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{
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if(!layout)
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return NO;
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size_t cursor = 0;
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while(*layout)
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{
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uint8_t skip = *layout++;
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uint8_t count = *layout++;
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// #ifdef DEBUG_DEALLOC_DEBUGGING
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// DDLogVerbose(@"slotIndex=%d, skip=%d, count=%d", (int)slotIndex, (int)skip, (int)count);
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// #endif
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cursor += skip;
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if(slotIndex >= cursor && slotIndex < cursor + count)
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return YES;
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cursor += count;
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}
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return NO;
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}
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static inline __attribute__((always_inline)) BOOL propertyIsWeak(objc_property_t prop)
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{
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if(!prop)
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return NO;
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const char* attrs = property_getAttributes(prop);
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if(!attrs)
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return NO;
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// #ifdef DEBUG_DEALLOC_DEBUGGING
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// DDLogVerbose(@"Property attributes: %s", attrs);
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// #endif
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BOOL retval = strstr(attrs, ",W") != NULL;
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return retval;
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}
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static inline __attribute__((always_inline)) NSMutableSet* IvarChildren(id obj)
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{
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NSMutableSet* children = [NSMutableSet new];
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[children addObjectsFromArray:ContainerChildren(obj)]; //if we are a container
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if([children count] == 0) //if we aren't a container
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{
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Class cls = object_getClass(obj);
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while(cls)
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{
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unsigned int count = 0;
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Ivar* ivars = class_copyIvarList(cls, &count);
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//const uint8_t* strongLayout = class_getIvarLayout(cls);
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const uint8_t* weakLayout = class_getWeakIvarLayout(cls);
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for(unsigned int i = 0; i < count; i++)
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{
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const char* ivarName = ivar_getName(ivars[i]);
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const char* ivarType = ivar_getTypeEncoding(ivars[i]);
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if(ivarType[0] == '@')
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{
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//check ivar slot for weakness
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ptrdiff_t offset = ivar_getOffset(ivars[i]);
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size_t slotIndex = offset / sizeof(void*);
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//DDLogVerbose(@"Checking strong for %d: %s", (int)slotIndex, ivarName);
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//BOOL isStrong = slotMatchesLayout(strongLayout, slotIndex);
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BOOL isWeak = slotMatchesLayout(weakLayout, slotIndex);
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//check property wrapping this ivar for weakness
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objc_property_t prop = class_getProperty(cls, ivarName);
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if(!prop)
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if(ivarName[0] == '_') //ivars usually start with '_' while properties don't
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prop = class_getProperty(cls, ivarName + 1);
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if(propertyIsWeak(prop))
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isWeak = YES;
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id ivarValue = object_getIvar(obj, ivars[i]);
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#ifdef DEBUG_DEALLOC_DEBUGGING
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if(ivarValue && isWeak)
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DDLogError(@"WEAK IVAR[%u](%s::%s): %@", i, ivarType, ivarName, ivarValue);
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#endif
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if(ivarValue && !isWeak)
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{
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#ifdef DEBUG_DEALLOC_DEBUGGING
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DDLogError(@"STRONG IVAR[%u](%s::%s): %@", i, ivarType, ivarName, ivarValue);
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#endif
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[children addObjectsFromArray:@[ivarValue]];
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[children addObjectsFromArray:ContainerChildren(ivarValue)];
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}
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}
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}
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free(ivars);
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cls = class_getSuperclass(cls);
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}
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}
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return children;
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}
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static inline NSString* NodeName(id obj)
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{
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return [NSString stringWithFormat:@"%s_%p_%lu --> %@", class_getName(object_getClass(obj)), obj, CFGetRetainCount((__bridge CFTypeRef)obj), obj];
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}
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static inline __attribute__((always_inline)) __unused void DumpMemoryGraph(NSMutableSet* objects)
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{
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@autoreleasepool {
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NSDate* start = [NSDate date];
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NSMutableSet* referenced = [NSMutableSet set];
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for(id obj in objects)
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{
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NSMutableSet* targets = [NSMutableSet set];
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NSMutableSet* printableTargets = [NSMutableSet set];
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for(id child in IvarChildren(obj))
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if(child != obj && [objects containsObject:child])
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{
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[targets addObject:child];
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[printableTargets addObject:NodeName(child)];
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}
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#ifdef DEBUG_DEALLOC_DEBUGGING
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DDLogDebug(@"TARGETS OF %@: %@", NodeName(obj), printableTargets);
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#endif
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[referenced unionSet:targets];
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}
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[objects minusSet:referenced];
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NSMutableString* output = [NSMutableString new];
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[output appendString:@"\n// Roots:\n"];
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for(id root in objects)
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[output appendFormat:@"// %@\n", NodeName(root)];
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if(![output isEqualToString:_oldDebugOutput])
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DDLogDebug(@"Still used root nodes (calculated in %.3fs): %@", [[NSDate date] timeIntervalSinceDate:start], output);
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_oldDebugOutput = output;
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}
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}
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#endif

‎Monal/Classes/MLHandler.m‎

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#import "MLConstants.h"
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#import "MLHandler.h"
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#import "HelperTools.h"
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#import "MLDelayedDealloc.h"
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#define HANDLER_VERSION 1
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-(instancetype) init
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{
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self = [super init];
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//always delay own deallocation to not block receive/send queues or any other queues
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[MLDelayedDealloc delayFor:self];
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return self;
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}
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-(instancetype) initWithCoder:(NSCoder*) coder
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{
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self = [super init];
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self = [self init];
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_internalData = [coder decodeObjectForKey:@"internalData"];
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_invalidated = [coder decodeBoolForKey:@"invalidated"];
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return self;

‎Monal/Classes/MLXMLNode.m‎

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#import "XMPPMessage.h"
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#import "XMPPPresence.h"
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#import "XMPPDataForm.h"
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#import "MLDelayedDealloc.h"
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@import UIKit.UIApplication;
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self.cache = [NSCache new];
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self.queryEntryCache = [NSCache new];
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[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(handleMemoryPressureNotification) name:UIApplicationDidReceiveMemoryWarningNotification object:nil];
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//always delay own deallocation to not block receive/send queues or any other queues
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[MLDelayedDealloc delayFor:self];
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}
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-(id) init

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