iOS nested asynchronous parallel task scenario

Scenario 1:

Reads from the data source to n values, and then needs to traverse these n values to initiate an HTTP request separately. After processing is complete, call a final summary method

This scenario can easily be achieved if you use the JS Async framework:

Async.series ([Task1, Task2, Task3], function (err) {    //Summary code}); function Task1 (callback) {    //HTTP request    callback (null);} function Task2 (callback) {    ///HTTP request    callback (NULL);} function Task3 (callback) {    ///HTTP request    callback (NULL);}

But in iOS, it's more troublesome, and finally it's implemented with Dispatch_group:

dispatch_group_t group = Dispatch_group_create ();            while ([Rs next]) {                    dispatch_group_enter (group);                   [SyncService Refreshmemberswithenterpriseid:enterpriseid latestsynctime:latestsyncdate Block:^ (BOOL flag) {        dispatch_group_leave (group);    }];}        Dispatch_group_notify (Group, Dispatch_get_main_queue (), ^{    //Summary Code});

Scenario 2:

Then the demand changed and the scene was a little more complicated. For each value, multiple HTTP requests are initiated (scenario 1 requires only 1 requests). After all requests are finished, call the Rollup method

So at first I wrote something like this:

dispatch_group_t group = Dispatch_group_create ();            while ([Rs next]) {                    dispatch_group_enter (group);        dispatch_group_t Group_inner = Dispatch_group_create ();                    Dispatch_group_enter (Group_inner);    [SyncService Refreshmemberswithenterpriseid:enterpriseid latestsynctime:latestsyncdate Block:^ (BOOL flag) {        dispatch_group_leave (group_inner);    }];                    Dispatch_group_enter (Group_inner);    [SyncService Refreshreportswithenterpriseid:enterpriseid latestsynctime:latestsyncdate Block:^ (BOOL flag) {        dispatch_group_leave (group_inner);    }];                    Dispatch_group_notify (Group_inner, dispatch_get_global_queue (0, 0), ^{        dispatch_group_leave (group);    });            Dispatch_group_notify (Group, Dispatch_get_main_queue (), ^{    //Summary Code});

After the completion of the discovery, in fact, in this scenario, do not need to nest group, sharing a group is no problem

dispatch_group_t group = Dispatch_group_create ();            while ([Rs next]) {                                    dispatch_group_enter (group);    [SyncService Refreshmemberswithenterpriseid:enterpriseid latestsynctime:contactlatestsync Block:^ (BOOL flag) {        dispatch_group_leave (group);    }];                    Dispatch_group_enter (group);    [SyncService Refreshreportswithenterpriseid:enterpriseid latestsynctime:reportlatestsync Block:^ (BOOL flag) {        dispatch_group_leave (group);    }];} Dispatch_group_notify (Group, Dispatch_get_main_queue (), ^{    //Summary Code});

Because there is no need for a rollup for each group, it is necessary to nest groups if each group has its own summary logic to perform.