枚举对象的 BOOL * stop 参数是什么？

The stop argument to the Block allows you to stop the enumeration prematurely. It's the equivalent of break from a normal for loop. You can ignore it if you want to go through every object in the array.

for( id obj in arr ){
if( [obj isContagious] ){
break;    // Stop enumerating
}


if( ![obj isKindOfClass:[Perefrigia class]] ){
continue;    // Skip this object
}


[obj immanetizeTheEschaton];
}

[arr enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
if( [obj isContagious] ){
*stop = YES;    // Stop enumerating
return;
}


if( ![obj isKindOfClass:[Perefrigia class]] ){
return;    // Skip this object
}


[obj immanentizeTheEschaton];
}];

That is an out parameter because it is a reference to a variable from the calling scope. It needs to be set inside your Block, but read inside of enumerateObjectsUsingBlock:, the same way NSErrors are commonly passed back to your code from framework calls.

- (void)enumerateObjectsUsingBlock:(void (^)(id obj, NSUInteger idx, BOOL *stop))block {
// N.B: This is probably not how this method is actually implemented!
// It is just to demonstrate how the out parameter operates!


NSUInteger idx = 0;
for( id obj in self ){


BOOL stop = NO;


block(obj, idx++, &stop);


if( stop ){
break;
}
}
}