ios嵌套的非同步並行任務情境

標籤：dispatch_group   嵌套   非同步   並行任務   

情境1：

從資料來源讀到N個值，然後需要遍曆這N個值，分別發起http請求。處理完成之後，調用一個最終匯總的方法

這個情境如果用js的async架構，很容易就能實現：

async.series([task1, task2, task3], function(err){    // 匯總代碼});function task1(callback){    // http請求    callback(null);}function task2(callback){    // http請求    callback(null);}function task3(callback){    // http請求    callback(null);}

但是在ios裡，就比較麻煩，最後是用dispatch_group實現的：

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(), ^{    // 匯總代碼});

情境2：

後來需求變化了，情境更複雜了點。針對每個值，都要發起多個http請求（情境1隻需要請求1次）。所有請求都結束以後，再調用匯總方法

所以一開始我寫成類似這樣：

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(), ^{    // 匯總代碼});

寫完之後發現，其實在這個情境下，不需要嵌套group，共用一個group也沒問題

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(), ^{    // 匯總代碼});

因為這裡並不需要針對每個組做匯總處理，如果每個組還有自己的匯總邏輯需要執行的話，就需要嵌套group了