iOS底層系列32 -- performSelector方法的探索

performSelector方法

  • performSelector在運行時,調用方去找目標方法selector,在編譯時不做校驗;

延遲執行 -- 與RunLoop有關

  • 調用performSelector:withObject:afterDelay方法實現延遲執行,底層的本質是會創建NSTimer定時器去執行目標方法selector;
- (void)viewDidLoad {
    [super viewDidLoad];
    [self performSelector:@selector(test) withObject:nil afterDelay:3];
}

- (void)test {
    NSLog(@"%s",__func__);
    NSLog(@"%@",[NSThread currentThread]);
}
@end
  • 在主線程中,延遲3秒后執行test方法,可以執行成功;
  • 若將performSelector:withObject:afterDelay方法 放在子線程中調用,如下:
@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
   
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        [self performSelector:@selector(test) withObject:nil afterDelay:3];
    });
}

- (void)test {
    NSLog(@"%s",__func__);
    NSLog(@"%@",[NSThread currentThread]);
}
@end
  • 在子線程中調用performSelector:withObject:afterDelay方法 是不會執行test方法的,因為NSTimer定時器依賴于RunLoop才能執行,必須開啟子線程的RunLoop,做如下修改:
@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
 
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        [self performSelector:@selector(test) withObject:nil afterDelay:3];
        [[NSRunLoop currentRunLoop] run];
    });
}

- (void)test {
    NSLog(@"%s",__func__);
    NSLog(@"%@",[NSThread currentThread]);
}
@end

開啟子線程執行任務 -- 與多線程有關

  • performSelector: onThread:withObject: waitUntilDone: 可指定線程執行目標方法任務;
@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"%@",[NSThread currentThread]);
        NSLog(@"11111");
        [self performSelector:@selector(test) onThread:[NSThread currentThread] withObject:nil waitUntilDone:YES];
        NSLog(@"22222");
    });
}

- (void)test {
    NSLog(@"%s",__func__);
    NSLog(@"%@",[NSThread currentThread]);
}
@end
  • 控制臺的調試結果如下:
image.png
  • performSelector發送消息與消息的執行是處于同一個線程的;
  • waitUntilDone參數為Yes,表示test方法必須執行完成,才會執行之后的打印2222,即會阻塞當前線程的繼續執行;

performSelector:方法傳遞多參數的實現方案

  • 第一種方案:將所有參數放到字典或者數組中,再傳遞集合即可;
@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    NSDictionary *params = @{
        @"name":@"yanzi",
        @"age":@"30"
    };
    [self performSelector:@selector(test:) withObject:params];

}

- (void)test:(NSDictionary *)params {
    NSLog(@"%@--%@",params[@"name"],params[@"age"]);
}
@end
  • 第二種方案:利用objc_msgSend()進行傳遞,其可以傳遞多個參數;
#import "ViewController.h"
#import <objc/message.h>

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    ((void (*)(id,SEL,NSString *, NSString *, NSString *))objc_msgSend)(self, @selector(testWithParam:param2:param3:),@"111",@"222",@"333");
}

//有三個參數的方法
- (void)testWithParam:(NSString *)param1 param2:(NSString *)param2 param3:(NSString *)param3 {
    NSLog(@"param1:%@, param2:%@, param3:%@",param1, param2, param3);
}
@end
  • 第三種方案:利用NSInvocation進行傳遞
@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    //調用方法
    NSArray *paramArray = @[@"112",@[@2,@13],@12];
    [self performSelector:@selector(testFunctionWithParam:param2:param3:) withObjects:paramArray];
}

//可以傳多個參數的方法
- (id)performSelector:(SEL)selector withObjects:(NSArray *)objects{
    // 方法簽名(方法的描述)
    NSMethodSignature *signature = [[self class] instanceMethodSignatureForSelector:selector];
    if (signature == nil) {
        //可以拋出異常也可以不操作。
    }
    
    //NSInvocation: 利用一個NSInvocation對象包裝一次方法調用(方法調用者、方法名、方法參數、方法返回值)
    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
    invocation.target = self;
    invocation.selector = selector;
    
    //設置參數
    NSInteger paramsCount = signature.numberOfArguments - 2; // 除self、_cmd以外的參數個數
    paramsCount = MIN(paramsCount, objects.count);
    for (NSInteger i = 0; i < paramsCount; i++) {
        id object = objects[i];
        if ([object isKindOfClass:[NSNull class]]) continue;
        [invocation setArgument:&object atIndex:i + 2];
    }
    
    //調用方法
    [invocation invoke];
    
    //獲取返回值
    id returnValue = nil;
    if (signature.methodReturnLength) { // 有返回值類型,才去獲得返回值
        [invocation getReturnValue:&returnValue];
    }
    return returnValue;
}

//要調用的方法
- (void)testFunctionWithParam:(NSString *)param1 param2:(NSArray *)param2 param3:(NSInteger)param3 {
    NSLog(@"param1:%@, param2:%@, param3:%ld",param1, param2, param3);
}
@end
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