`performSelector`

NSObject.h
- (id)performSelector:(SEL)aSelector;
- (id)performSelector:(SEL)aSelector withObject:(id)object;
- (id)performSelector:(SEL)aSelector withObject:(id)object1 withObject:(id)object2;

`performSelector`的使用

1.无参数调用

[self sendMessage];

- (void)sendMessageWithNumber:(NSString *)number {
    NSLog(@"%@",number);
}

2.单个参数调用

 [self sendMessageWithNumber:@"Number"];

- (void)sendMessageWithNumber:(NSString *)number {
    NSLog(@"%@",number);
}

3.两个参数调用

[self sendMessageWithNumber:@"Number" count:@"count"];

- (id)sendMessageWithNumber:(NSString *)number
                        count:(NSString *)count {
    NSLog(@"%@ %@",number,count);
    
    return nil;
}

performSelector 不能传递三个以及三个以上的参数。假如需要传的话，可以使用 NSInvocation 或者是 objc_msgSend

`performSelectorOnMainThread`

NSThread.h

`- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(nullable id)arg waitUntilDone:(BOOL)wait;`

基于默认模式调用主线程中接收器的方法,该方法不应该有明确的返回值并且最多只有一个id类型的参数，或者没有参数。wait指定是否阻塞当前线程直到指定选择器在主线程中执行完毕。选择YES会阻塞这个线程；选择NO，本方法会立刻返回。如果当前线程也是主线程，选择YES，消息会立即派发,处理。
``

例如

    NSLog(@"1");
    dispatch_async(dispatch_get_main_queue(), ^{
        NSLog(@"2");
        sleep(2);
        NSLog(@"3");
    });
    [self performSelectorOnMainThread:@selector(sendMessageWithNumber:) withObject:@"4" waitUntilDone:YES];

- (void)sendMessageWithNumber:(NSString *)number {
    NSLog(@"%@",number);
}

输出结果

1 4 2 3

`- (void)performSelector:(SEL)aSelector onThread:(NSThread *)thrwithObject:(nullable id)arg waitUntilDone:(BOOL)wait;`

调用指定线程中的某个方法。

`- (void)performSelector:(SEL)aSelector onThread:(NSThread *)thrwithObject:(nullable id)arg waitUntilDone:(BOOL)wait;`

开启子线程在后台运行

`performSelector afterDelay`

NSRunLoop.h

- (void)performSelector:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay inModes:(NSArray *)modes;
- (void)performSelector:(SEL)aSelector withObject:(id)anArgument afterDelay:(NSTimeInterval)delay;

这两个方法为异步执行，即使delay传参为0，仍为异步执行。只能在主线程中执行，在子线程中不会调到aSelector方法。可用于当点击UI中一个按钮会触发一个消耗系统性能的事件，在事件执行期间按钮会一直处于高亮状态，此时可以调用该方法去异步的处理该事件，就能避免上面的问题。

在方法未到执行时间之前，取消方法为：

+ (void)cancelPreviousPerformRequestsWithTarget:(id)aTarget selector:(SEL)aSelector object:(id)anArgument;
+ (void)cancelPreviousPerformRequestsWithTarget:(id)aTarget;
注意：调用该方法之前或在该方法所在的viewController生命周期结束的时候去调用取消函数，以确保不会引起内存泄露。

例如

NSLog(@"1");
    [self performSelector:@selector(sendMessageWithNumber:) withObject:@"2" afterDelay:3];
    NSLog(@"3");

打印出 1 3 2
而如下则打印出 1 3

dispatch_async(dispatch_get_global_queue(0, DISPATCH_QUEUE_PRIORITY_DEFAULT), ^{
        NSLog(@"1");
          [self performSelector:@selector(sendMessageWithNumber:) withObject:@"2" afterDelay:3];
          NSLog(@"3");
    });

 NSLog(@"1");
    [self performSelector:@selector(sendMessageWithNumber:) withObject:@"2" afterDelay:3];
    [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(sendMessageWithNumber:) object:@"2"];
    NSLog(@"3");

打印结果 1 3,因为打印2的事件已经在执行前被取消了

`NSInvocation`

SEL selector = @selector(sendMessageWithNumber:count:);
//    方法签名
    NSMethodSignature *singnature = [self methodSignatureForSelector:selector];
    if (singnature) {
        // NSInvocation : 利用一个NSInvocation对象包装一次方法调用（方法调用者、方法名、方法参数、方法返回值）
        NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:singnature];
        [invocation setTarget:self];
        [invocation setSelector:@selector(sendMessageWithNumber:count:)];
        
        NSString *number = @"1111";
        NSString *number2 = @"2ss";
        
         // 设置参数
        [invocation setArgument:&number atIndex:2];
        [invocation setArgument:&number2 atIndex:3];
        
        // 调用方法
        [invocation invoke];
        
        // 获取返回值
        NSString *result;
        if (singnature.methodReturnLength != 0) {// 有返回值类型，才去获得返回值
            [invocation getReturnValue:&result];
        }
        
        NSLog(@"%@",result);
    } else {
         //可以抛出异常也可以不操作。
        NSLog(@"singnature is nil");
    }

调用时可以传入多个参数；

但是要注意判断是否是签名成功；

因为前两个参数分别是方法调用者、方法名，因为自己传入的参数是从第三个开始；

objc_msgSend

`objc_msgSend`

SEL selector = @selector(sendMessageWithNumber:count:);
    NSString *result = ((NSString * (*) (id, SEL, NSString *, NSString *))objc_msgSend)(self, selector,@"2",@"3");
    NSLog(@"%@",result);

打印结果如下

2 3
3333

iOS 几种方法调用

`performSelector`