NSProxy 实际应用

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.

也就是说它是一个抽象超类，为子类对象定义API

@interface NSProxy  {
    Class   isa;
}

通常，当有消息发送给NSProxy，需要转发给实际对象，NSproxy遵循NSObject协议，提供了消息转发的通用接口，它只是实现了一些基础方法，但是它并不提供初始化方法，当收到一些它无法响应的消息时，它会直接抛出异常。

使用NSProxy，需要创建一个子类，需要做这些事：

• 提供初始化方法

• 重写methodSignatureForSelector:
  methodSignatureForSelector需要给消息参数提供类型，并构造NSMethodSignature 对象

• respondsToSelector:
  是否响应一个SEL

• 重写forwardInvocation:
  forwardInvocation应该处理整个调用流程所需要的任何操作，例如通过网络转发调用或加载实际对象并将调用传递给它

实际应用

使用NSProxy来避免循环引用

使用WeakProxy来实现弱引用

创建NSProxy子类

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];

实现多继承

创建NSProxy子类TargetProxy

TargetProxy.h

@interface TargetProxy : NSProxy

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

@end

创建Study类

@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);
    }

这样TargetProxy就同时继承了NSMutableString类和NSMutableArray类