1.nstimer为什么回循环引用
2.NSObject如何解决NStimer循环引用
3.NSProxy如何解决NStimer循环引用
4.为什么要用NSProxy，优势在哪
围绕上面几个问题我们来思考一下
1.nstimer为什么回循环引用
self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerTest) userInfo:nil repeats:YES];
self强引用timer，timer强引用self.
那是否可以改为
__weak typeof(self)weakSelf = self;
self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:weakSelf selector:@selector(timerTest) userInfo:nil repeats:YES];
答案也是不行的。target其实里面做的是一个赋值操作。
我们可以从gnu里面的源码中发现这个问题。所以在外面设置weakSelf是不能解决这个问题，如果是block那是可以解决的。所以我们现在就要添加一个中间变量，来解开这个环。
2.NSObject如何解决NStimer循环引用
这里我们可以利用消息转发阶段的forwardingTargetForSelector函数来解决这个问题。
self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:[XHObjectProxy proxyWithTarget:self] selector:@selector(timerTest) userInfo:nil repeats:YES];
@interface XHObjectProxy : NSObject
+ (instancetype)proxyWithTarget:(id)target;
@property (weak, nonatomic) id target;
@end
@implementation XHObjectProxy
+ (instancetype)proxyWithTarget:(id)target
{
XHObjectProxy *proxy = [[XHObjectProxy alloc] init];
proxy.target = target;
return proxy;
}
- (id)forwardingTargetForSelector:(SEL)aSelector
{
return self.target;
}
@end
设置XHObjectProxy这个中间件，然后传入target，并且用一个弱引用的target来接受他。从而实现解开循环。
当timer调用timerTest方法的时候，很显然XHObjectProxy没有这个方法，所以就进入了消息转发阶段。
当到达forwardingTargetForSelector 这个阶段的时候，直接返回self.target也就是[XHObjectProxy proxyWithTarget:self] 中的self。所以可以解决这个问题。
3.NSProxy如何解决NStimer循环引用
NSProxy跟NSObject一样，也是一个基类,是一个实现<NSObject>协议的基类。
@class NSMethodSignature, NSInvocation;
NS_ASSUME_NONNULL_BEGIN
NS_ROOT_CLASS
@interface NSProxy <NSObject> {
__ptrauth_objc_isa_pointer Class isa;
}
+ (id)alloc;
+ (id)allocWithZone:(nullable NSZone *)zone NS_AUTOMATED_REFCOUNT_UNAVAILABLE;
+ (Class)class;
- (void)forwardInvocation:(NSInvocation *)invocation;
- (nullable NSMethodSignature *)methodSignatureForSelector:(SEL)sel NS_SWIFT_UNAVAILABLE("NSInvocation and related APIs not available");
- (void)dealloc;
- (void)finalize;
@property (readonly, copy) NSString *description;
@property (readonly, copy) NSString *debugDescription;
+ (BOOL)respondsToSelector:(SEL)aSelector;
- (BOOL)allowsWeakReference API_UNAVAILABLE(macos, ios, watchos, tvos);
- (BOOL)retainWeakReference API_UNAVAILABLE(macos, ios, watchos, tvos);
// - (id)forwardingTargetForSelector:(SEL)aSelector;
@end
里面有消息转发第三阶段的两个方法
- (void)forwardInvocation:(NSInvocation *)invocation;
- (nullable NSMethodSignature *)methodSignatureForSelector:(SEL)sel
所以可以这么写
@interface XHRealProxy : NSProxy
+ (instancetype)proxyWithTarget:(id)target;
@property (weak, nonatomic) id target;
@end
@implementation XHRealProxy
+ (instancetype)proxyWithTarget:(id)target
{
// NSProxy对象不需要调用init，因为它本来就没有init方法
XHRealProxy *proxy = [XHRealProxy alloc];
proxy.target = target;
return proxy;
}
- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel
{
return [self.target methodSignatureForSelector:sel];
}
- (void)forwardInvocation:(NSInvocation *)invocation
{
[invocation invokeWithTarget:self.target];
}
@end
经过测试我们发现在同时写forwardingTargetForSelector 和 methodSignatureForSelector 这两个方法的情况下，不会执行第二阶段的转发，直接进入methodSignatureForSelector。
4.为什么要用NSProxy，优势在哪
我们一般的方法调用都是通过objc_msgSend来进行的，然后经过消息转发，一步一步的进行。
这张图是NSObject的方法调用
ViewController *vc = [[ViewController alloc] init];
XHRealProxy *proxy1 = [XHRealProxy proxyWithTarget:vc];
XHObjectProxy *proxy2 = [XHObjectProxy proxyWithTarget:vc];
通过代码进行测试，发现继承NSProxy的类，直接执行了methodSignatureForSelector方法，而继承NSObject的类，直接执行了forwardingTargetForSelector方法。
然后又进了nstimer的方法进行测试。发现不管是NSProxy 还是 NSObject 都会先直接走forwardingTargetForSelector这个方法。
觉得很奇怪，然后去gnu上去看源码。发现里面重写了isKindOfClass这个方法，然后里面直接调用了methodSignatureForSelector
/**
* Calls the -forwardInvocation: method to determine if the 'real' object
* referred to by the proxy is an instance of the specified class.
* Returns the result.<br />
* NB. The default operation of -forwardInvocation: is to raise an exception.
*/
- (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;
}
/**
* Calls the -forwardInvocation: method to determine if the 'real' object
* referred to by the proxy is an instance of the specified class.
* Returns the result.<br />
* NB. The default operation of -forwardInvocation: is to raise an exception.
*/
- (BOOL) isMemberOfClass: (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;
}
- (BOOL) conformsToProtocol: (Protocol*)aProtocol
{
NSMethodSignature *sig;
NSInvocation *inv;
BOOL ret;
sig = [self methodSignatureForSelector: _cmd];
inv = [NSInvocation invocationWithMethodSignature: sig];
[inv setSelector: _cmd];
[inv setArgument: &aProtocol atIndex: 2];
[self forwardInvocation: inv];
[inv getReturnValue: &ret];
return ret;
}
为什么NSProxy的效率更高？因为他是直接走消息转发第三步methodSignatureForSelector，而nsobject需要走objc_msgSend整个流程，所以效率更高。
