dispatch_group_t - EXC_BAD_INSTRUCTION

Question

这是我第一次使用dispatch_group_t，我犯了一个简单的错误。dispatch_group_notify在任何异步调用返回之前被调用，因此甚至在第一个调用之前被调用dispatch_group_leave(group)，然后抛出一个EXC_BAD_INSTRUCTION.

但是，我不知道为什么会发生这种情况。

dispatch_queue_t queue =      dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();

dispatch_group_async(group, queue, ^{
    [self getCategoriesMainWithParameters:nil withSuccess:^(NSArray *objects) {
        [categoriesMain addObjectsFromArray:objects];
        dispatch_group_leave(group);
    } failure:^(NSError *error) {
        groupError = error;
        dispatch_group_leave(group);
    }];
});

dispatch_group_async(group, queue, ^{
    [self getCategoriesSubWithParameters:nil withSuccess:^(NSArray *objects) {
        [categoriesSub addObjectsFromArray:objects];
        dispatch_group_leave(group);
    } failure:^(NSError *error) {
        groupError = error;
        dispatch_group_leave(group);
    }];
});

dispatch_group_async(group, queue, ^{
    [self getCategoriesProductWithParameters:nil withSuccess:^(NSArray *objects) {
        [categoriesProduct addObjectsFromArray:objects];
        dispatch_group_leave(group);
    } failure:^(NSError *error) {
        groupError = error;
        dispatch_group_leave(group);
    }];
});

dispatch_group_notify(group,dispatch_get_main_queue(),^{
   ...
});

Answer 1

该函数dispatch_group_async在分派块时自动 \xe2\x80\x9centers\xe2\x80\x9d 组，并在分派块完成时自动 \xe2\x80\x9cleaves\xe2\x80\x9d 。但是您再次调用dispatch_group_leave，因此最终会产生太多 \xe2\x80\x9cleave\xe2\x80\x9d 调用，从而导致您描述的错误。

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人们应该调用dispatch_group_async（使用所有为您进行的 \xe2\x80\x9center\xe2\x80\x9d 和 \xe2\x80\x9cleave\xe2\x80\x9d 调用），或者在dispatch_group_enter异步调用之前手动调用并dispatch_group_leave在异步进程的完成处理程序。

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在这种情况下，因为您在分派块内调用异步任务，所以该dispatch_group_async模式将\xe2\x80\x99 无法工作。您需要dispatch_group_enter在调用异步方法之前手动执行此操作，然后让完成处理程序调用dispatch_group_leave，就像您所做的那样。当每个 \xe2\x80\x9center\xe2\x80\x9d 与 \xe2\x80\x9cleave\xe2\x80\x9d 匹配时，该组将收到通知。并且因为您调用的方法已经是异步的，所以您根本不需要使用dispatch_asyncor 。dispatch_group_async

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因此，它可能看起来像：

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dispatch_group_t group = dispatch_group_create();\n\ndispatch_group_enter(group);\n[self getCategoriesMainWithParameters:nil withSuccess:^(NSArray *objects) {\n    [categoriesMain addObjectsFromArray:objects];\n    dispatch_group_leave(group);\n} failure:^(NSError *error) {\n    groupError = error;\n    dispatch_group_leave(group);\n}];\n\ndispatch_group_enter(group);\n[self getCategoriesSubWithParameters:nil withSuccess:^(NSArray *objects) {\n    [categoriesSub addObjectsFromArray:objects];\n    dispatch_group_leave(group);\n} failure:^(NSError *error) {\n    groupError = error;\n    dispatch_group_leave(group);\n}];\n\ndispatch_group_enter(group);\n[self getCategoriesProductWithParameters:nil withSuccess:^(NSArray *objects) {\n    [categoriesProduct addObjectsFromArray:objects];\n    dispatch_group_leave(group);\n} failure:^(NSError *error) {\n    groupError = error;\n    dispatch_group_leave(group);\n}];\n\ndispatch_group_notify(group,dispatch_get_main_queue(),^{\n    ...\n});\n

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