Preface

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Discussion

If a large block of memory is allocated but never released, that’s a memory leak — it isn’t holding caused by memory contention; the root cause is simply not releasing it properly. In normal flows (proper use of alloc/release, malloc/free, or new/delete), the main cause of memory leaks is mutual holding caused by resource contention. That’s the trigger for resources not being released normally. This tool is good at solving problems where objects hold each other and never release memory; it pinpoints the instance type and memory address, and lets you visually find the related objects.

Memory Detection Tool — Introduction

• FBMemoryProfiler

  FBMemoryProfiler is a combination of several components, including FBAllocationTracker and FBRetainCycleDetector. A visualization tool that’s embedded directly into the app. It lets you view memory usage directly inside the app and filter for potentially leaking objects.

• FBAllocationTracker

  Mainly used to quickly detect potentially leaking objects and hand them to FBRetainCycleDetector for detection. This is a tool that proactively tracks allocation and deallocation of all NSObject subclasses.

FBAllocationTracker detects the allocation of all instances during app runtime. Its principle is to use method swizzling to replace the original alloc method, so all instance allocations can be recorded.

When needed, call the currentAllocationSummary method to get the current overall instance allocation summary (provided it was initialized in main, as described below):

• FBRetainCycleDetector

  FBRetainCycleDetector takes a runtime instance, then traverses all of its properties recursively, level by level. If it encounters a repeated instance during traversal, a retain cycle exists and a report is produced.

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  FBRetainCycleDetector *detector = [FBRetainCycleDetector new];
 [detector addCandidate:myObject];
 NSSet *retainCycles = [detector findRetainCycles];
 NSLog(@"%@", retainCycles);

Principle of Detecting Retain Cycles in Code

Detecting an object’s memory layout and instance address at runtime.

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const char *class_getIvarLayout(Class cls);
const char *class_getWeakIvarLayout(Class cls);

support for Objective-C++

Used in code like this:

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FBRetainCycleDetector *detector = [[FBRetainCycleDetector alloc] initWithConfiguration:nil];
[detector addCandiate:myObject];
NSSet<NSArray<FBObjectiveCGraphElement *> *> *retainCycles = [detector findRetainCycles];
NSLog(@"%@", retainCycles);

Here, myObject is the instance variable we suspect.

FBObjectiveCGraphElement is the base class for all object-finding types. All finders are implemented based on it. This class doesn’t need to be called externally; it’s mainly used internally for queries. Its main features are:

• Provides an initializer that wraps an object (i.e., the object passed to addCandiate).
• Gets all objects held by this object: - (NSSet *)allRetainedObjects;. The object types obtained by the base class FBObjectiveCGraphElement are objects held via associated objects. Associated objects are tracked for holding by Facebook’s own fishhook, which hooks the original objc_setAssociatedObject and objc_removeAssociatedObjects.
• Provides a filter interface - (NSSet *)filterObjects:(nullable NSArray *)objects;, which is used in combination with FBObjectGraphConfiguration (described below).

FBObjectGraphConfiguration is a class that provides the whitelist-related filtering configuration.

I won’t go into the rest in detail here.

How findRetainCycles works — DFS, depth-first search

The search here uses depth-first traversal.

How to Use

Example scenario analysis

Example code

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@property (nonatomic, strong) NSTimer *timer;
@property(copy,nonatomic)NSString *name;

 self.timer = [NSTimer scheduledTimerWithTimeInterval:0.1
                                              target:self
                                            selector:@selector(handleTimer)
                                            userInfo:nil
                                             repeats:YES];

- (void)handleTimer
{
     self.name = @"123";
}

References

FBRetainCycleDetector Analysis
automatic-memory-leak-detection-on-ios