mirror of
https://github.com/NohamR/knowledge-kit.git
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490 lines
14 KiB
Markdown
490 lines
14 KiB
Markdown
# NSTimer 中的内存泄露
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NSTimer、CADisplayLink 的 基础 API `[NSTimer scheduledTimersWithTimeInterval:1 repeat:YES block:nil]` 和当前的 VC 都会互相持有，造成环，会存在内存泄漏问题
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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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## 解决方案：
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### 替换 NSTimer API
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```objectivec
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__weak typeof(self) weakSelf = self;
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self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
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[weakSelf timerTest];
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}];
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```
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### GCD Timer
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CADisplayLink、NSTimer 都是依靠 RunLoop 实现的，所以当 RunLoop 任务繁重的时候，定时器可能不准。
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GCD 的定时器会更加准时，底层依赖系统内核。
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```objectivec
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@property (nonatomic, strong) dispatch_source_t timer;
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// 创建队列
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dispatch_queue_t queue = dispatch_get_main_queue();
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// 创建 GCD 定时器
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dispatch_source_t timerSource = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
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uint64_t start = 2.0;
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uint64_t interval = 1.0;
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// 设置定时器周期
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dispatch_source_set_timer(timerSource, dispatch_time(DISPATCH_TIME_NOW, start * NSEC_PER_SEC), interval * NSEC_PER_SEC, 0);
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// 设置定时器任务
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dispatch_source_set_event_handler(timerSource, ^{
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NSLog(@"tick tock");
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});
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// 启动定时器
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dispatch_resume(timerSource);
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self.timer = timerSource;
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```
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为什么 GCD timer 会更准确？因为普通定时器运行依赖 RunLoop，RunLoop 一个运行周期内的任务繁忙程度是不确定的。当某次任务繁重，那么定时器调度就不准时。
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GCD timer 不依赖 RunLoop，系统底层驱动，所以会更加准确。因为和 RunLoop 无关，所以和 UI 滚动，RunLoop mode 切换到 UITrackingMode 也不影响 GCD timer。
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### 打破循环引用，NSTimer target 自定义
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```objectivec
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@interface LBPProxy : NSObject
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+ (instancetype)proxyWithObject:(id)target;
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@property (nonatomic, weak) id target;
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@end
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@implementation LBPProxy
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+ (instancetype)proxyWithObject:(id)target{
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LBPProxy *proxy = [[LBPProxy alloc] init];
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proxy.target = target;
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return proxy;
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}
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- (id)forwardingTargetForSelector:(SEL)aSelector{
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return self.target;
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}
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@end
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self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:[LBPProxy proxyWithObject:self] selector:@selector(timerTest) userInfo:nil repeats:YES];
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```
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### 高精度定时器封装
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项目中经常使用定时器，普通定时器存在精度丢失的问题、循环引用的问题，为了使用方法我们封装一个定时器
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```objectivec
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#import <Foundation/Foundation.h>
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@interface PreciousTimer : NSObject
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+ (NSString *)execTask:(void(^)(void))task
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start:(NSTimeInterval)start
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interval:(NSTimeInterval)interval
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repeats:(BOOL)repeats
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async:(BOOL)async;
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+ (NSString *)execTask:(id)target
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selector:(SEL)selector
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start:(NSTimeInterval)start
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interval:(NSTimeInterval)interval
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repeats:(BOOL)repeats
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async:(BOOL)async;
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+ (void)cancelTask:(NSString *)name;
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@end
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#import "PreciousTimer.h"
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@implementation PreciousTimer
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static NSMutableDictionary *timers_;
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dispatch_semaphore_t semaphore_;
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+ (void)initialize
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{
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static dispatch_once_t onceToken;
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dispatch_once(&onceToken, ^{
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timers_ = [NSMutableDictionary dictionary];
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semaphore_ = dispatch_semaphore_create(1);
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});
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}
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+ (NSString *)execTask:(void (^)(void))task start:(NSTimeInterval)start interval:(NSTimeInterval)interval repeats:(BOOL)repeats async:(BOOL)async
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{
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if (!task || start < 0 || (interval <= 0 && repeats)) return nil;
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// 队列
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dispatch_queue_t queue = async ? dispatch_get_global_queue(0, 0) : dispatch_get_main_queue();
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// 创建定时器
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dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
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// 设置时间
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dispatch_source_set_timer(timer,
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dispatch_time(DISPATCH_TIME_NOW, start * NSEC_PER_SEC),
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interval * NSEC_PER_SEC, 0);
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dispatch_semaphore_wait(semaphore_, DISPATCH_TIME_FOREVER);
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// 定时器的唯一标识
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NSString *name = [NSString stringWithFormat:@"%zd", timers_.count];
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// 存放到字典中
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timers_[name] = timer;
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dispatch_semaphore_signal(semaphore_);
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// 设置回调
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dispatch_source_set_event_handler(timer, ^{
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task();
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if (!repeats) { // 不重复的任务
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[self cancelTask:name];
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}
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});
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// 启动定时器
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dispatch_resume(timer);
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return name;
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}
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+ (NSString *)execTask:(id)target selector:(SEL)selector start:(NSTimeInterval)start interval:(NSTimeInterval)interval repeats:(BOOL)repeats async:(BOOL)async
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{
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if (!target || !selector) return nil;
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return [self execTask:^{
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if ([target respondsToSelector:selector]) {
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#pragma clang diagnostic push
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#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
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[target performSelector:selector];
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#pragma clang diagnostic pop
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}
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} start:start interval:interval repeats:repeats async:async];
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}
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+ (void)cancelTask:(NSString *)name
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{
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if (name.length == 0) return;
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dispatch_semaphore_wait(semaphore_, DISPATCH_TIME_FOREVER);
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dispatch_source_t timer = timers_[name];
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if (timer) {
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dispatch_source_cancel(timer);
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[timers_ removeObjectForKey:name];
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}
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dispatch_semaphore_signal(semaphore_);
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}
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@end
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```
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使用 Demo
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```objectivec
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- (void)viewDidLoad{
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[super viewDidLoad];
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NSLog(@"now");
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self.timerId = [PreciousTimer execTask:^{
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NSLog(@"tick tock %@", [NSThread currentThread]);
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} start:2 interval:1 repeats:YES async:YES];
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}
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- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
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{
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[PreciousTimer cancelTask:self.timerId];
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}
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```
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说明：直接 `performSelector` 存在警告，可以告诉编译器忽略警告。可以在 Xcode 点开警告，查看详情，复制 `[]` 里面的字符串去忽略警告
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### NSProxy
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```objectivec
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#import "LBPProxy.h"
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@implementation LBPProxy
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+ (instancetype)proxyWithObject:(id)target{
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LBPProxy *proxy = [LBPProxy alloc];
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proxy.target = target;
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return proxy;
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}
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- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel{
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return [self.target methodSignatureForSelector:sel];
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}
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- (void)forwardInvocation:(NSInvocation *)invocation{
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// 方法1
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invocation.target = self.target;
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[invocation invoke];
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// 方法2
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[invocation invokeWithTarget:self.target];
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}
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@end
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self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:[LBPProxy proxyWithObject:self] selector:@selector(timerTest) userInfo:nil repeats:YES];
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```
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QA: 自己写的继承自 NSObject 的代理对象和继承自 NSProxy 的代理有何区别？
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NSProxy 效率更高。继承自 NSObject 的代理，内部运行的时候还是存在方法查找（isa、superclass、cache、methods）流程。
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看一段神奇的代码
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`LBPProxy`
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```objectivec
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@interface LBPProxy : NSObject
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+ (instancetype)proxyWithObject:(id)target;
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@property (nonatomic, weak) id target;
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@end
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@implementation LBPProxy
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+ (instancetype)proxyWithObject:(id)target{
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LBPProxy *proxy = [LBPProxy alloc];
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proxy.target = target;
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return proxy;
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}
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- (id)forwardingTargetForSelector:(SEL)aSelector{
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return self.target;
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}
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@end
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```
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`LBPProxy2`
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```objectivec
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@interface LBPProxy2 : NSProxy
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+ (instancetype)proxyWithObject:(id)target;
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@property (nonatomic, weak) id target;
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@end
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@implementation LBPProxy2
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+ (instancetype)proxyWithObject:(id)target{
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LBPProxy2 *proxy = [LBPProxy2 alloc];
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proxy.target = target;
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return proxy;
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}
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- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel{
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return [self.target methodSignatureForSelector:sel];
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}
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- (void)forwardInvocation:(NSInvocation *)invocation{
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// 方法1
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invocation.target = self.target;
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[invocation invoke];
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// 方法2
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[invocation invokeWithTarget:self.target];
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}
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@end
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```
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main.m
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```objectivec
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ViewController *vc = [[ViewController alloc] init];
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LBPProxy *p1 = [LBPProxy proxyWithObject:vc];
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LBPProxy2 *p2 = [LBPProxy2 proxyWithObject:vc];
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NSLog(@"%d %d",
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[p1 isKindOfClass:[UIViewController class]],
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[p2 isKindOfClass:[UIViewController class]]);
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appDelegateClassName = NSStringFromClass([AppDelegate class]);
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// 0 1
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```
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为什么打印出 `0 1`。
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分析：
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- p1 是 LBPProxy 类，继承于 NSObject 所以就不是 UIViewController 类型。
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- p2 是 LBPProxy2 类，继承自 NSProxy，当调用 isKindOfClass 这个方法的时候，也会进行消息转发，即调用 `forwardInvocation` 方法，其内部实现 `[invocation invokeWithTarget:self.target];` 则触发 self.target 的逻辑。此时 self.target 就是 VC，所以为 1。
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这一点可以查看 GUN 查看下源码印证。`NSProxy.m`
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```objectivec
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- (BOOL) isKindOfClass: (Class)aClass
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{
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NSMethodSignature *sig;
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NSInvocation *inv;
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BOOL ret;
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sig = [self methodSignatureForSelector: _cmd];
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inv = [NSInvocation invocationWithMethodSignature: sig];
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[inv setSelector: _cmd];
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[inv setArgument: &aClass atIndex: 2];
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[self forwardInvocation: inv];
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[inv getReturnValue: &ret];
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return ret;
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}
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```
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可以看到内部直接调用了消息转发。
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### 采用 Block 的形式为 NSTimer 增加分类
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```objectivec
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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 |