One 、 brief introduction
stay iOS Application development , Customizing a class generally needs to inherit from NSObject Class or NSObject Subclass , however ,NSProxy Class does not inherit from NSObject Class or NSObject Subclass , It's a realization NSObject Abstract base class of protocol .
/* NSProxy.h
Copyright (c) 1994-2019, Apple Inc. All rights reserved.
*/
#import <Foundation/NSObject.h>
@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
NS_ASSUME_NONNULL_END
NSProxy Its function is to act as a principal-agent , Forward the message to a real object or an object loaded by yourself .
To learn more about NSProxy The role of classes , Let's implement a colleague call NSMutableString and NSMutableArray Delegate classes of methods in two classes , Simulate multiple inheritance .
First create TargetProxy class , Let him inherit NSProxy. And implement the initialization method .
@interface TargetProxy : NSProxy
/// Initialization method , Save two real objects
/// @param object1 The first real object
/// @param object2 The second real object
- (instancetype)initWithObject1:(id)object1 object2:(id)object2;
@end
@implementation TargetProxy {
// Save the first real object to forward the message to
// The method call priority of the first real object will be higher than that of the second real object
id _realObject1;
// Save the second real object to forward the message to
id _realObject2;
}
- (instancetype)initWithObject1:(id)object1 object2:(id)object2 {
_realObject1 = object1;
_realObject2 = object2;
return self;
}
And then in TargetProxy.m In file , rewrite - methodSignatureForSelector: Get the real object method signature , And rewrite - forwardInvocation: Method , Call real object methods .
- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
// obtain _realObject1 in sel Method signature for
NSMethodSignature *signature = [_realObject1 methodSignatureForSelector:sel];
// If _realObject1 There is this method in , Then return the signature of the method
// without , return _realObject1 Method signature
if (signature) {
return signature;
}
// obtain _realObject1 Medium sel Method signature for
signature = [_realObject2 methodSignatureForSelector:sel];
return signature;
}
- (void)forwardInvocation:(NSInvocation *)invocation {
// Get the real object that owns the method
id target = [_realObject1 methodSignatureForSelector:[invocation selector]] ? _realObject1 : _realObject2;
// Execution method
[invocation invokeWithTarget:target];
}
Last , Conduct Demo test
- (void)testTargetProxy {
NSMutableString *string = [NSMutableString string];
NSMutableArray *array = [NSMutableArray array];
id proxy = [[TargetProxy alloc] initWithObject1:string object2:array];
[proxy appendString:@"This "];
[proxy appendString:@"is "];
[proxy addObject:string];
[proxy appendString:@"a "];
[proxy appendString:@"test!"];
NSLog(@"The string is length is: %@", [proxy valueForKey:@"length"]);
NSLog(@"count should be 1, it is %ld", [proxy count]);
if ([[proxy objectAtIndex:0] isEqualToString:@"This is a test!"]) {
NSLog(@"Appending successful.");
} else {
NSLog(@"Appending failed,, got: '%@'", proxy);
}
}
Run the above code , The input log is as follows ：
2022-04-02 11:30:35.957145+0800 Demo[19783:586710] SuccessFully create Delegere Proxy automatically.
2022-04-02 11:30:35.959722+0800 Demo[19783:586710] The string is length is: 15
2022-04-02 11:30:35.960175+0800 Demo[19783:586710] count should be 1, it is 1
2022-04-02 11:30:40.086227+0800 Demo[19783:586710] Appending successful.
Above description , We use TargetProxy Class successfully implements message forwarding .
Of course , In most cases , Use NSObject Class can also implement message forwarding , Implementation and NSProxy similar , But in most cases NSProxy More appropriate . because ：
- NSProxy Class implements the following NSObject The basic methods required by the protocol in the inner base class
- adopt NSObject The proxy class implemented by class will not automatically forward NSObject The method in the agreement
- adopt NSObject The proxy class implemented by class will not automatically forward NSObject Methods in the category
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