NSProxy

An abstract superclass defining an API for objects that act as stand-ins for other objects or for objects that don’t exist yet.

也就是說它是一個(gè)抽象超類，為子類對象定義API

@interface NSProxy <NSObject> {
    Class   isa;
}

通常，當(dāng)有消息發(fā)送給NSProxy，需要轉(zhuǎn)發(fā)給實(shí)際對象，NSproxy遵循NSObject協(xié)議，提供了消息轉(zhuǎn)發(fā)的通用接口，它只是實(shí)現(xiàn)了一些基礎(chǔ)方法，但是它并不提供初始化方法，當(dāng)收到一些它無法響應(yīng)的消息時(shí)，它會(huì)直接拋出異常。

使用NSProxy，需要?jiǎng)?chuàng)建一個(gè)子類，需要做這些事：

• 提供初始化方法

• 重寫methodSignatureForSelector:
  methodSignatureForSelector需要給消息參數(shù)提供類型，并構(gòu)造NSMethodSignature 對象

• respondsToSelector:
  是否響應(yīng)一個(gè)SEL

• 重寫forwardInvocation:
  forwardInvocation應(yīng)該處理整個(gè)調(diào)用流程所需要的任何操作，例如通過網(wǎng)絡(luò)轉(zhuǎn)發(fā)調(diào)用或加載實(shí)際對象并將調(diào)用傳遞給它

實(shí)際應(yīng)用

使用NSProxy來避免循環(huán)引用

使用WeakProxy來實(shí)現(xiàn)弱引用

創(chuàng)建NSProxy子類

WeakProxy.h

@interface WeakProxy : NSProxy

@property (weak,nonatomic,readonly)id target;

+ (instancetype)proxyWithTarget:(id)target;
- (instancetype)initWithTarget:(id)target;

@end

WeakProxy.m

#import "WeakProxy.h"

@implementation WeakProxy

- (instancetype)initWithTarget:(id)target{
    _target = target;
    return self;
}
+ (instancetype)proxyWithTarget:(id)target{
    return [[self alloc] initWithTarget:target];
}
- (void)forwardInvocation:(NSInvocation *)invocation{
    SEL sel = [invocation selector];
    if ([self.target respondsToSelector:sel]) {
        [invocation invokeWithTarget:self.target];
    }
}
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector{
    NSMethodSignature *s = [self.target methodSignatureForSelector:aSelector];
    return s;
}

- (BOOL)respondsToSelector:(SEL)aSelector{
    return [self.target respondsToSelector:aSelector];
}

@end

self.timer = [NSTimer timerWithTimeInterval:1
                                         target:[WeakProxy proxyWithTarget:self]
                                       selector:@selector(print:)
                                       userInfo:nil
                                        repeats:YES];
    [[NSRunLoop mainRunLoop] addTimer:self.timer forMode:NSRunLoopCommonModes];

實(shí)現(xiàn)多繼承

創(chuàng)建NSProxy子類TargetProxy

TargetProxy.h

@interface TargetProxy : NSProxy

- (id)initWithTarget1:(id)t1 target2:(id)t2;

@end

創(chuàng)建Study類

@interface Student : NSObject

- (void)study;

@end

TargetProxy.m

@interface TargetProxy ()

@property(nonatomic, strong) id realObject1;
@property(nonatomic, strong) id realObject2;

@end

@implementation TargetProxy

- (id)initWithTarget1:(id)t1 target2:(id)t2 {
    self.realObject1 = t1;
    self.realObject2 = t2;
    return self;
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    NSMethodSignature *sig;
    sig = [self.realObject1 methodSignatureForSelector:aSelector];
    if (sig) return sig;
    sig = [self.realObject2 methodSignatureForSelector:aSelector];
    return sig;
}

// Invoke the invocation on whichever real object had a signature for it.
- (void)forwardInvocation:(NSInvocation *)invocation {
    id target = [self.realObject1 methodSignatureForSelector:[invocation selector]] ? self.realObject1 : self.realObject2;
    [invocation invokeWithTarget:target];
}
 
// Override some of NSProxy's implementations to forward them...
- (BOOL)respondsToSelector:(SEL)aSelector {
    if ([self.realObject1 respondsToSelector:aSelector]) return YES;
    if ([self.realObject2 respondsToSelector:aSelector]) return YES;
    return NO;
}

NSMutableString *string = [[NSMutableString alloc] init];
    
    NSMutableArray *array = [[NSMutableArray alloc] init];
    
    id proxy = [[TargetProxy alloc] initWithTarget1:string target2:array];
    
    [proxy appendString:@"This "];
    [proxy appendString:@"is "];
    [proxy addObject:string];
    [proxy appendString:@"a "];
    [proxy appendString:@"test!"];
    
    NSLog(@"count should be 1, it is: %lu", (unsigned long)[proxy count]);
    
    if ([[proxy objectAtIndex:0] isEqualToString:@"This is a test!"]) {
        NSLog(@"Appending successful.");
    } else {
        NSLog(@"Appending failed, got: '%@'", proxy);
    }

這樣TargetProxy就同時(shí)繼承了NSMutableString類和NSMutableArray類