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Chapter1 - iOS/1.45.md
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@@ -1,10 +1,6 @@
# NSTimer 中的内存泄露
- GCD 的 timer
- NSProxy
- 采用 Block 的形式为 NSTimer 增加分类
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
```objective-c
@interface ViewController()
@@ -50,7 +46,332 @@
1. 控制器不再强引用定时器
2. 定时器不再保留当前的控制器
```objective-c
## 解决方案：
### 替换 NSTimer API
```objectivec
__weak typeof(self) weakSelf = self;
self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
[weakSelf timerTest];
}];
```
### GCD Timer
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。
GCD 的定时器会更加准时，底层依赖系统内核。
```objectivec
@property (nonatomic, strong) dispatch_source_t timer;
// 创建队列
dispatch_queue_t queue = dispatch_get_main_queue();
// 创建 GCD 定时器
dispatch_source_t timerSource = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
uint64_t start = 2.0;
uint64_t interval = 1.0;
// 设置定时器周期
dispatch_source_set_timer(timerSource, dispatch_time(DISPATCH_TIME_NOW, start * NSEC_PER_SEC), interval * NSEC_PER_SEC, 0);
// 设置定时器任务
dispatch_source_set_event_handler(timerSource, ^{
NSLog(@"tick tock");
});
// 启动定时器
dispatch_resume(timerSource);
self.timer = timerSource;
```
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。
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。
### 打破循环引用，NSTimer target 自定义
```objectivec
@interface LBPProxy : NSObject
+ (instancetype)proxyWithObject:(id)target;
@property (nonatomic, weak) id target;
@end
@implementation LBPProxy
+ (instancetype)proxyWithObject:(id)target{
LBPProxy *proxy = [[LBPProxy alloc] init];
proxy.target = target;
return proxy;
}
- (id)forwardingTargetForSelector:(SEL)aSelector{
return self.target;
}
@end
self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:[LBPProxy proxyWithObject:self] selector:@selector(timerTest) userInfo:nil repeats:YES];
```
### 高精度定时器封装
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
```objectivec
#import <Foundation/Foundation.h>
@interface PreciousTimer : NSObject
+ (NSString *)execTask:(void(^)(void))task
start:(NSTimeInterval)start
interval:(NSTimeInterval)interval
repeats:(BOOL)repeats
async:(BOOL)async;
+ (NSString *)execTask:(id)target
selector:(SEL)selector
start:(NSTimeInterval)start
interval:(NSTimeInterval)interval
repeats:(BOOL)repeats
async:(BOOL)async;
+ (void)cancelTask:(NSString *)name;
@end
#import "PreciousTimer.h"
@implementation PreciousTimer
static NSMutableDictionary *timers_;
dispatch_semaphore_t semaphore_;
+ (void)initialize
{
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
timers_ = [NSMutableDictionary dictionary];
semaphore_ = dispatch_semaphore_create(1);
});
}
+ (NSString *)execTask:(void (^)(void))task start:(NSTimeInterval)start interval:(NSTimeInterval)interval repeats:(BOOL)repeats async:(BOOL)async
{
if (!task || start < 0 || (interval <= 0 && repeats)) return nil;
// 队列
dispatch_queue_t queue = async ? dispatch_get_global_queue(0, 0) : dispatch_get_main_queue();
// 创建定时器
dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
// 设置时间
dispatch_source_set_timer(timer,
dispatch_time(DISPATCH_TIME_NOW, start * NSEC_PER_SEC),
interval * NSEC_PER_SEC, 0);
dispatch_semaphore_wait(semaphore_, DISPATCH_TIME_FOREVER);
// 定时器的唯一标识
NSString *name = [NSString stringWithFormat:@"%zd", timers_.count];
// 存放到字典中
timers_[name] = timer;
dispatch_semaphore_signal(semaphore_);
// 设置回调
dispatch_source_set_event_handler(timer, ^{
task();
if (!repeats) { // 不重复的任务
[self cancelTask:name];
}
});
// 启动定时器
dispatch_resume(timer);
return name;
}
+ (NSString *)execTask:(id)target selector:(SEL)selector start:(NSTimeInterval)start interval:(NSTimeInterval)interval repeats:(BOOL)repeats async:(BOOL)async
{
if (!target || !selector) return nil;
return [self execTask:^{
if ([target respondsToSelector:selector]) {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
[target performSelector:selector];
#pragma clang diagnostic pop
}
} start:start interval:interval repeats:repeats async:async];
}
+ (void)cancelTask:(NSString *)name
{
if (name.length == 0) return;
dispatch_semaphore_wait(semaphore_, DISPATCH_TIME_FOREVER);
dispatch_source_t timer = timers_[name];
if (timer) {
dispatch_source_cancel(timer);
[timers_ removeObjectForKey:name];
}
dispatch_semaphore_signal(semaphore_);
}
@end
```
使用 Demo
```objectivec
- (void)viewDidLoad{
[super viewDidLoad];
NSLog(@"now");
self.timerId = [PreciousTimer execTask:^{
NSLog(@"tick tock %@", [NSThread currentThread]);
} start:2 interval:1 repeats:YES async:YES];
}
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
[PreciousTimer cancelTask:self.timerId];
}
```
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
![](/Users/lbp/Desktop/GitHub/knowledge-kit/assets/ignoreXcodewarning.png)
### NSProxy
```objectivec
#import "LBPProxy.h"
@implementation LBPProxy
+ (instancetype)proxyWithObject:(id)target{
LBPProxy *proxy = [LBPProxy alloc];
proxy.target = target;
return proxy;
}
- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel{
return [self.target methodSignatureForSelector:sel];
}
- (void)forwardInvocation:(NSInvocation *)invocation{
// 方法1
invocation.target = self.target;
[invocation invoke];
// 方法2
[invocation invokeWithTarget:self.target];
}
@end
self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:[LBPProxy proxyWithObject:self] selector:@selector(timerTest) userInfo:nil repeats:YES];
```
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？
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。
看一段神奇的代码
`LBPProxy`
```objectivec
@interface LBPProxy : NSObject
+ (instancetype)proxyWithObject:(id)target;
@property (nonatomic, weak) id target;
@end
@implementation LBPProxy
+ (instancetype)proxyWithObject:(id)target{
LBPProxy *proxy = [LBPProxy alloc];
proxy.target = target;
return proxy;
}
- (id)forwardingTargetForSelector:(SEL)aSelector{
return self.target;
}
@end
```
`LBPProxy2`
```objectivec
@interface LBPProxy2 : NSProxy
+ (instancetype)proxyWithObject:(id)target;
@property (nonatomic, weak) id target;
@end
@implementation LBPProxy2
+ (instancetype)proxyWithObject:(id)target{
LBPProxy2 *proxy = [LBPProxy2 alloc];
proxy.target = target;
return proxy;
}
- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel{
return [self.target methodSignatureForSelector:sel];
}
- (void)forwardInvocation:(NSInvocation *)invocation{
// 方法1
invocation.target = self.target;
[invocation invoke];
// 方法2
[invocation invokeWithTarget:self.target];
}
@end
```
main.m
```objectivec
ViewController *vc = [[ViewController alloc] init];
LBPProxy *p1 = [LBPProxy proxyWithObject:vc];
LBPProxy2 *p2 = [LBPProxy2 proxyWithObject:vc];
NSLog(@"%d %d",
[p1 isKindOfClass:[UIViewController class]],
[p2 isKindOfClass:[UIViewController class]]);
appDelegateClassName = NSStringFromClass([AppDelegate class]);
// 0 1
```
为什么打印出 `0 1`。
分析：
- p1 是 LBPProxy 类，继承于 NSObject 所以就不是 UIViewController 类型。
- 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。
这一点可以查看 GUN 查看下源码印证。`NSProxy.m`
```objectivec
- (BOOL) isKindOfClass: (Class)aClass
{
NSMethodSignature *sig;
NSInvocation *inv;
BOOL ret;
sig = [self methodSignatureForSelector: _cmd];
inv = [NSInvocation invocationWithMethodSignature: sig];
[inv setSelector: _cmd];
[inv setArgument: &aClass atIndex: 2];
[self forwardInvocation: inv];
[inv getReturnValue: &ret];
return ret;
}
```
可以看到内部直接调用了消息转发。
### 采用 Block 的形式为 NSTimer 增加分类
```objectivec
//.h文件
#import <Foundation/Foundation.h>
@@ -166,6 +487,4 @@ __strong __typeof(&*weakSelf)self = weakSelf;
}
```
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