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171 lines
4.5 KiB
Markdown
171 lines
4.5 KiB
Markdown
# NSTimer 中的内存泄露
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- GCD 的 timer
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- NSProxy
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- 采用 Block 的形式为 NSTimer 增加分类
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```objective-c
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@interface ViewController()
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@property (nonatomic, strong) NSTimer *timer;
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@end
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@implementation ViewController
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- (void)viewDidLoad {
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[super viewDidLoad];
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self.timer = [NSTimer scheduledTimerWithTimeInterval:0.1
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target:self
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selector:@selector(p_doSomeThing)
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userInfo:nil
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repeats:YES];
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}
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- (void)p_doSomeThing {
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// doSomeThing
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}
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- (void)p_stopDoSomeThing {
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[self.timer invalidate];
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self.timer = nil;
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}
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- (void)dealloc {
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[self.timer invalidate];
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}
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@end
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```
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上面的代码主要是利用定时器重复执行 p_doSomeThing 方法，在合适的时候调用 p_stopDoSomeThing 方法使定时器失效。
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能看出问题吗？在开始讨论上面代码问题之前，需要对 NSTimer 做一点说明。NSTimer 的 `scheduledTimerWithTimeInterval:target:selector:userInfo:repeats:` 方法的最后一个参数为 YES 时，NSTimer 会保留目标对象，等到自身失效才释放目标对象。执行完任务后，一次性的定时器会自动失效；重复性的定时器，需要主动调用 invalidate 方法才会失效。
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当前的 VC 和 定时器互相引用，造成循环引用。
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如果能在合适的时机打破循环引用就不会有问题了
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1. 控制器不再强引用定时器
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2. 定时器不再保留当前的控制器
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```objective-c
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//.h文件
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#import <Foundation/Foundation.h>
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@interface NSTimer (UnRetain)
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+ (NSTimer *)lbp_scheduledTimerWithTimeInterval:(NSTimeInterval)inerval
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repeats:(BOOL)repeats
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block:(void(^)(NSTimer *timer))block;
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@end
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//.m文件
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#import "NSTimer+SGLUnRetain.h"
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@implementation NSTimer (SGLUnRetain)
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+ (NSTimer *)lbp_scheduledTimerWithTimeInterval:(NSTimeInterval)inerval repeats:(BOOL)repeats block:(void (^)(NSTimer *timer))block{
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return [NSTimer scheduledTimerWithTimeInterval:inerval target:self selector:@selector(lbp_blcokInvoke:) userInfo:[block copy] repeats:repeats];
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}
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+ (void)lbp_blcokInvoke:(NSTimer *)timer {
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void (^block)(NSTimer *timer) = timer.userInfo;
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if (block) {
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block(timer);
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}
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}
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@end
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//控制器.m
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#import "ViewController.h"
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#import "NSTimer+UnRetain.h"
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//定义了一个__weak的self_weak_变量
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#define weakifySelf \
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__weak __typeof(&*self)weakSelf = self;
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//局域定义了一个__strong的self指针指向self_weak
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#define strongifySelf \
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__strong __typeof(&*weakSelf)self = weakSelf;
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@interface ViewController ()
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@property(nonatomic， strong) NSTimer *timer;
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@end
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@implementation ViewController
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- (void)viewDidLoad {
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[super viewDidLoad];
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__block NSInteger i = 0;
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weakifySelf
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self.timer = [NSTimer lbp_scheduledTimerWithTimeInterval:0.1 repeats:YES block:^(NSTimer *timer) {
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strongifySelf
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[self p_doSomething];
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NSLog(@"----------------");
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if (i++ > 10) {
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[timer invalidate];
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}
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}];
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}
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- (void)p_doSomething {
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}
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- (void)dealloc {
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// 务必在当前线程调用invalidate方法，使得Runloop释放对timer的强引用(具体请参阅官方文档)
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[self.timer invalidate];
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}
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@end
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```
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上面的方法之所以能解决内存泄漏的问题，关键在于把保留转移到了定时器的类对象身上，这样就避免了实例对象被保留。
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当我们谈到循环引用时，其实是指实例对象间的引用关系。类对象在 App 杀死时才会释放，在实际开发中几乎不用关注类对象的内存管理。下面的代码摘自苹果开源的 NSObject.mm 文件，从中可以看出，对于类对象，并不需要像实例对象那样进行内存管理。
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```objective-c
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+ (id)retain {
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return (id)self;
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}
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// Replaced by ObjectAlloc
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- (id)retain {
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return ((id)self)->rootRetain();
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}
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+ (oneway void)release {
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}
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// Replaced by ObjectAlloc
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- (oneway void)release {
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((id)self)->rootRelease();
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}
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+ (id)autorelease {
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return (id)self;
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}
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// Replaced by ObjectAlloc
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- (id)autorelease {
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return ((id)self)->rootAutorelease();
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}
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+ (NSUInteger)retainCount {
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return ULONG_MAX;
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}
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- (NSUInteger)retainCount {
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return ((id)self)->rootRetainCount();
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}
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```
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iOS 10 中，定时器 api 增加了 block 方法，实现原理与此类似，这里采用分类为 NSTimer 增加 block 参数的方法，最终的行为一致 |