萌新在iOS开发时，使用block时候经常会遇到这个错误

Variable is not assignable (missing __block type specifier)

例如这样子的写法，block内部引用了一个外部对象

Person *aPerson = nil;
[participants ObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {   
    Person *participant = (Person*)obj;

    if ([participant.gender isEqualToString:@"M"]) {
        aPerson = participant;
        *stop = YES;
    }
}];

解决办法也很简单，就是在block外部这样声明就可以了。

__block Person *aPerson = nil;

即如下的样子

__block Person *aPerson = nil;
Person *aPerson = nil;
[participants ObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {   
    Person *participant = (Person*)obj;

    if ([participant.gender isEqualToString:@"M"]) {
        aPerson = participant;
        *stop = YES;
    }
}];