任务依赖关系如下：
#mermaid-svg-MWovzER0YYs6lSFt .label{font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family);fill:#333;color:#333}#mermaid-svg-MWovzER0YYs6lSFt .label text{fill:#333}#mermaid-svg-MWovzER0YYs6lSFt .node rect,#mermaid-svg-MWovzER0YYs6lSFt .node circle,#mermaid-svg-MWovzER0YYs6lSFt .node ellipse,#mermaid-svg-MWovzER0YYs6lSFt .node polygon,#mermaid-svg-MWovzER0YYs6lSFt .node path{fill:#ECECFF;stroke:#9370db;stroke-width:1px}#mermaid-svg-MWovzER0YYs6lSFt .node .label{text-align:center;fill:#333}#mermaid-svg-MWovzER0YYs6lSFt .node.clickable{cursor:pointer}#mermaid-svg-MWovzER0YYs6lSFt .arrowheadPath{fill:#333}#mermaid-svg-MWovzER0YYs6lSFt .edgePath .path{stroke:#333;stroke-width:1.5px}#mermaid-svg-MWovzER0YYs6lSFt .flowchart-link{stroke:#333;fill:none}#mermaid-svg-MWovzER0YYs6lSFt .edgeLabel{background-color:#e8e8e8;text-align:center}#mermaid-svg-MWovzER0YYs6lSFt .edgeLabel rect{opacity:0.9}#mermaid-svg-MWovzER0YYs6lSFt .edgeLabel span{color:#333}#mermaid-svg-MWovzER0YYs6lSFt .cluster rect{fill:#ffffde;stroke:#aa3;stroke-width:1px}#mermaid-svg-MWovzER0YYs6lSFt .cluster text{fill:#333}#mermaid-svg-MWovzER0YYs6lSFt div.mermaidTooltip{position:absolute;text-align:center;max-width:200px;padding:2px;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family);font-size:12px;background:#ffffde;border:1px solid #aa3;border-radius:2px;pointer-events:none;z-index:100}#mermaid-svg-MWovzER0YYs6lSFt .actor{stroke:#ccf;fill:#ECECFF}#mermaid-svg-MWovzER0YYs6lSFt text.actor>tspan{fill:#000;stroke:none}#mermaid-svg-MWovzER0YYs6lSFt .actor-line{stroke:grey}#mermaid-svg-MWovzER0YYs6lSFt .messageLine0{stroke-width:1.5;stroke-dasharray:none;stroke:#333}#mermaid-svg-MWovzER0YYs6lSFt .messageLine1{stroke-width:1.5;stroke-dasharray:2, 2;stroke:#333}#mermaid-svg-MWovzER0YYs6lSFt #arrowhead path{fill:#333;stroke:#333}#mermaid-svg-MWovzER0YYs6lSFt .sequenceNumber{fill:#fff}#mermaid-svg-MWovzER0YYs6lSFt #sequencenumber{fill:#333}#mermaid-svg-MWovzER0YYs6lSFt #crosshead path{fill:#333;stroke:#333}#mermaid-svg-MWovzER0YYs6lSFt .messageText{fill:#333;stroke:#333}#mermaid-svg-MWovzER0YYs6lSFt .labelBox{stroke:#ccf;fill:#ECECFF}#mermaid-svg-MWovzER0YYs6lSFt .labelText,#mermaid-svg-MWovzER0YYs6lSFt .labelText>tspan{fill:#000;stroke:none}#mermaid-svg-MWovzER0YYs6lSFt .loopText,#mermaid-svg-MWovzER0YYs6lSFt .loopText>tspan{fill:#000;stroke:none}#mermaid-svg-MWovzER0YYs6lSFt .loopLine{stroke-width:2px;stroke-dasharray:2, 2;stroke:#ccf;fill:#ccf}#mermaid-svg-MWovzER0YYs6lSFt .note{stroke:#aa3;fill:#fff5ad}#mermaid-svg-MWovzER0YYs6lSFt .noteText,#mermaid-svg-MWovzER0YYs6lSFt .noteText>tspan{fill:#000;stroke:none}#mermaid-svg-MWovzER0YYs6lSFt .activation0{fill:#f4f4f4;stroke:#666}#mermaid-svg-MWovzER0YYs6lSFt .activation1{fill:#f4f4f4;stroke:#666}#mermaid-svg-MWovzER0YYs6lSFt .activation2{fill:#f4f4f4;stroke:#666}#mermaid-svg-MWovzER0YYs6lSFt .mermaid-main-font{font-family:"trebuchet ms", verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-MWovzER0YYs6lSFt .section{stroke:none;opacity:0.2}#mermaid-svg-MWovzER0YYs6lSFt .section0{fill:rgba(102,102,255,0.49)}#mermaid-svg-MWovzER0YYs6lSFt .section2{fill:#fff400}#mermaid-svg-MWovzER0YYs6lSFt .section1,#mermaid-svg-MWovzER0YYs6lSFt .section3{fill:#fff;opacity:0.2}#mermaid-svg-MWovzER0YYs6lSFt .sectionTitle0{fill:#333}#mermaid-svg-MWovzER0YYs6lSFt .sectionTitle1{fill:#333}#mermaid-svg-MWovzER0YYs6lSFt .sectionTitle2{fill:#333}#mermaid-svg-MWovzER0YYs6lSFt .sectionTitle3{fill:#333}#mermaid-svg-MWovzER0YYs6lSFt .sectionTitle{text-anchor:start;font-size:11px;text-height:14px;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-MWovzER0YYs6lSFt .grid .tick{stroke:#d3d3d3;opacity:0.8;shape-rendering:crispEdges}#mermaid-svg-MWovzER0YYs6lSFt .grid .tick text{font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-MWovzER0YYs6lSFt .grid path{stroke-width:0}#mermaid-svg-MWovzER0YYs6lSFt .today{fill:none;stroke:red;stroke-width:2px}#mermaid-svg-MWovzER0YYs6lSFt .task{stroke-width:2}#mermaid-svg-MWovzER0YYs6lSFt .taskText{text-anchor:middle;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-MWovzER0YYs6lSFt .taskText:not([font-size]){font-size:11px}#mermaid-svg-MWovzER0YYs6lSFt .taskTextOutsideRight{fill:#000;text-anchor:start;font-size:11px;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-MWovzER0YYs6lSFt .taskTextOutsideLeft{fill:#000;text-anchor:end;font-size:11px}#mermaid-svg-MWovzER0YYs6lSFt .task.clickable{cursor:pointer}#mermaid-svg-MWovzER0YYs6lSFt .taskText.clickable{cursor:pointer;fill:#003163 !important;font-weight:bold}#mermaid-svg-MWovzER0YYs6lSFt .taskTextOutsideLeft.clickable{cursor:pointer;fill:#003163 !important;font-weight:bold}#mermaid-svg-MWovzER0YYs6lSFt .taskTextOutsideRight.clickable{cursor:pointer;fill:#003163 !important;font-weight:bold}#mermaid-svg-MWovzER0YYs6lSFt .taskText0,#mermaid-svg-MWovzER0YYs6lSFt .taskText1,#mermaid-svg-MWovzER0YYs6lSFt .taskText2,#mermaid-svg-MWovzER0YYs6lSFt .taskText3{fill:#fff}#mermaid-svg-MWovzER0YYs6lSFt .task0,#mermaid-svg-MWovzER0YYs6lSFt .task1,#mermaid-svg-MWovzER0YYs6lSFt .task2,#mermaid-svg-MWovzER0YYs6lSFt .task3{fill:#8a90dd;stroke:#534fbc}#mermaid-svg-MWovzER0YYs6lSFt .taskTextOutside0,#mermaid-svg-MWovzER0YYs6lSFt .taskTextOutside2{fill:#000}#mermaid-svg-MWovzER0YYs6lSFt .taskTextOutside1,#mermaid-svg-MWovzER0YYs6lSFt .taskTextOutside3{fill:#000}#mermaid-svg-MWovzER0YYs6lSFt .active0,#mermaid-svg-MWovzER0YYs6lSFt .active1,#mermaid-svg-MWovzER0YYs6lSFt .active2,#mermaid-svg-MWovzER0YYs6lSFt .active3{fill:#bfc7ff;stroke:#534fbc}#mermaid-svg-MWovzER0YYs6lSFt .activeText0,#mermaid-svg-MWovzER0YYs6lSFt .activeText1,#mermaid-svg-MWovzER0YYs6lSFt .activeText2,#mermaid-svg-MWovzER0YYs6lSFt .activeText3{fill:#000 !important}#mermaid-svg-MWovzER0YYs6lSFt .done0,#mermaid-svg-MWovzER0YYs6lSFt .done1,#mermaid-svg-MWovzER0YYs6lSFt .done2,#mermaid-svg-MWovzER0YYs6lSFt .done3{stroke:grey;fill:#d3d3d3;stroke-width:2}#mermaid-svg-MWovzER0YYs6lSFt .doneText0,#mermaid-svg-MWovzER0YYs6lSFt .doneText1,#mermaid-svg-MWovzER0YYs6lSFt .doneText2,#mermaid-svg-MWovzER0YYs6lSFt .doneText3{fill:#000 !important}#mermaid-svg-MWovzER0YYs6lSFt .crit0,#mermaid-svg-MWovzER0YYs6lSFt .crit1,#mermaid-svg-MWovzER0YYs6lSFt .crit2,#mermaid-svg-MWovzER0YYs6lSFt .crit3{stroke:#f88;fill:red;stroke-width:2}#mermaid-svg-MWovzER0YYs6lSFt .activeCrit0,#mermaid-svg-MWovzER0YYs6lSFt .activeCrit1,#mermaid-svg-MWovzER0YYs6lSFt .activeCrit2,#mermaid-svg-MWovzER0YYs6lSFt .activeCrit3{stroke:#f88;fill:#bfc7ff;stroke-width:2}#mermaid-svg-MWovzER0YYs6lSFt .doneCrit0,#mermaid-svg-MWovzER0YYs6lSFt .doneCrit1,#mermaid-svg-MWovzER0YYs6lSFt .doneCrit2,#mermaid-svg-MWovzER0YYs6lSFt .doneCrit3{stroke:#f88;fill:#d3d3d3;stroke-width:2;cursor:pointer;shape-rendering:crispEdges}#mermaid-svg-MWovzER0YYs6lSFt .milestone{transform:rotate(45deg) scale(0.8, 0.8)}#mermaid-svg-MWovzER0YYs6lSFt .milestoneText{font-style:italic}#mermaid-svg-MWovzER0YYs6lSFt .doneCritText0,#mermaid-svg-MWovzER0YYs6lSFt .doneCritText1,#mermaid-svg-MWovzER0YYs6lSFt .doneCritText2,#mermaid-svg-MWovzER0YYs6lSFt .doneCritText3{fill:#000 !important}#mermaid-svg-MWovzER0YYs6lSFt .activeCritText0,#mermaid-svg-MWovzER0YYs6lSFt .activeCritText1,#mermaid-svg-MWovzER0YYs6lSFt .activeCritText2,#mermaid-svg-MWovzER0YYs6lSFt .activeCritText3{fill:#000 !important}#mermaid-svg-MWovzER0YYs6lSFt .titleText{text-anchor:middle;font-size:18px;fill:#000;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-MWovzER0YYs6lSFt g.classGroup text{fill:#9370db;stroke:none;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family);font-size:10px}#mermaid-svg-MWovzER0YYs6lSFt g.classGroup text .title{font-weight:bolder}#mermaid-svg-MWovzER0YYs6lSFt g.clickable{cursor:pointer}#mermaid-svg-MWovzER0YYs6lSFt g.classGroup rect{fill:#ECECFF;stroke:#9370db}#mermaid-svg-MWovzER0YYs6lSFt g.classGroup line{stroke:#9370db;stroke-width:1}#mermaid-svg-MWovzER0YYs6lSFt .classLabel .box{stroke:none;stroke-width:0;fill:#ECECFF;opacity:0.5}#mermaid-svg-MWovzER0YYs6lSFt .classLabel .label{fill:#9370db;font-size:10px}#mermaid-svg-MWovzER0YYs6lSFt .relation{stroke:#9370db;stroke-width:1;fill:none}#mermaid-svg-MWovzER0YYs6lSFt .dashed-line{stroke-dasharray:3}#mermaid-svg-MWovzER0YYs6lSFt #compositionStart{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-MWovzER0YYs6lSFt #compositionEnd{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-MWovzER0YYs6lSFt #aggregationStart{fill:#ECECFF;stroke:#9370db;stroke-width:1}#mermaid-svg-MWovzER0YYs6lSFt #aggregationEnd{fill:#ECECFF;stroke:#9370db;stroke-width:1}#mermaid-svg-MWovzER0YYs6lSFt #dependencyStart{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-MWovzER0YYs6lSFt #dependencyEnd{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-MWovzER0YYs6lSFt #extensionStart{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-MWovzER0YYs6lSFt #extensionEnd{fill:#9370db;stroke:#9370db;stroke-width:1}#mermaid-svg-MWovzER0YYs6lSFt .commit-id,#mermaid-svg-MWovzER0YYs6lSFt .commit-msg,#mermaid-svg-MWovzER0YYs6lSFt .branch-label{fill:lightgrey;color:lightgrey;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-MWovzER0YYs6lSFt .pieTitleText{text-anchor:middle;font-size:25px;fill:#000;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-MWovzER0YYs6lSFt .slice{font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-MWovzER0YYs6lSFt g.stateGroup text{fill:#9370db;stroke:none;font-size:10px;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-MWovzER0YYs6lSFt g.stateGroup text{fill:#9370db;fill:#333;stroke:none;font-size:10px}#mermaid-svg-MWovzER0YYs6lSFt g.statediagram-cluster .cluster-label text{fill:#333}#mermaid-svg-MWovzER0YYs6lSFt g.stateGroup .state-title{font-weight:bolder;fill:#000}#mermaid-svg-MWovzER0YYs6lSFt g.stateGroup rect{fill:#ECECFF;stroke:#9370db}#mermaid-svg-MWovzER0YYs6lSFt g.stateGroup line{stroke:#9370db;stroke-width:1}#mermaid-svg-MWovzER0YYs6lSFt .transition{stroke:#9370db;stroke-width:1;fill:none}#mermaid-svg-MWovzER0YYs6lSFt .stateGroup .composit{fill:white;border-bottom:1px}#mermaid-svg-MWovzER0YYs6lSFt .stateGroup .alt-composit{fill:#e0e0e0;border-bottom:1px}#mermaid-svg-MWovzER0YYs6lSFt .state-note{stroke:#aa3;fill:#fff5ad}#mermaid-svg-MWovzER0YYs6lSFt .state-note text{fill:black;stroke:none;font-size:10px}#mermaid-svg-MWovzER0YYs6lSFt .stateLabel .box{stroke:none;stroke-width:0;fill:#ECECFF;opacity:0.7}#mermaid-svg-MWovzER0YYs6lSFt .edgeLabel text{fill:#333}#mermaid-svg-MWovzER0YYs6lSFt .stateLabel text{fill:#000;font-size:10px;font-weight:bold;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-MWovzER0YYs6lSFt .node circle.state-start{fill:black;stroke:black}#mermaid-svg-MWovzER0YYs6lSFt .node circle.state-end{fill:black;stroke:white;stroke-width:1.5}#mermaid-svg-MWovzER0YYs6lSFt #statediagram-barbEnd{fill:#9370db}#mermaid-svg-MWovzER0YYs6lSFt .statediagram-cluster rect{fill:#ECECFF;stroke:#9370db;stroke-width:1px}#mermaid-svg-MWovzER0YYs6lSFt .statediagram-cluster rect.outer{rx:5px;ry:5px}#mermaid-svg-MWovzER0YYs6lSFt .statediagram-state .divider{stroke:#9370db}#mermaid-svg-MWovzER0YYs6lSFt .statediagram-state .title-state{rx:5px;ry:5px}#mermaid-svg-MWovzER0YYs6lSFt .statediagram-cluster.statediagram-cluster .inner{fill:white}#mermaid-svg-MWovzER0YYs6lSFt .statediagram-cluster.statediagram-cluster-alt .inner{fill:#e0e0e0}#mermaid-svg-MWovzER0YYs6lSFt .statediagram-cluster .inner{rx:0;ry:0}#mermaid-svg-MWovzER0YYs6lSFt .statediagram-state rect.basic{rx:5px;ry:5px}#mermaid-svg-MWovzER0YYs6lSFt .statediagram-state rect.divider{stroke-dasharray:10,10;fill:#efefef}#mermaid-svg-MWovzER0YYs6lSFt .note-edge{stroke-dasharray:5}#mermaid-svg-MWovzER0YYs6lSFt .statediagram-note rect{fill:#fff5ad;stroke:#aa3;stroke-width:1px;rx:0;ry:0}:root{--mermaid-font-family: '"trebuchet ms", verdana, arial';--mermaid-font-family: "Comic Sans MS", "Comic Sans", cursive}#mermaid-svg-MWovzER0YYs6lSFt .error-icon{fill:#522}#mermaid-svg-MWovzER0YYs6lSFt .error-text{fill:#522;stroke:#522}#mermaid-svg-MWovzER0YYs6lSFt .edge-thickness-normal{stroke-width:2px}#mermaid-svg-MWovzER0YYs6lSFt .edge-thickness-thick{stroke-width:3.5px}#mermaid-svg-MWovzER0YYs6lSFt .edge-pattern-solid{stroke-dasharray:0}#mermaid-svg-MWovzER0YYs6lSFt .edge-pattern-dashed{stroke-dasharray:3}#mermaid-svg-MWovzER0YYs6lSFt .edge-pattern-dotted{stroke-dasharray:2}#mermaid-svg-MWovzER0YYs6lSFt .marker{fill:#333}#mermaid-svg-MWovzER0YYs6lSFt .marker.cross{stroke:#333} :root { --mermaid-font-family: "trebuchet ms", verdana, arial;} #mermaid-svg-MWovzER0YYs6lSFt { color: rgba(0, 0, 0, 0.75); font: ; } A D B E C F即A、B执行完毕才执行D，以此类推。
先用dispatch_group_create()函数获取3个group；并行的任务分到一个组，A和B加入g1，B和C加入g2，D和E加入g3；依赖项(D)利用函数dispatch_group_notify函数等待被依赖项(A, B)所在的group执行完毕。 - (void)viewDidLoad { [super viewDidLoad]; dispatch_group_t g1 = dispatch_group_create(); dispatch_group_t g2 = dispatch_group_create(); dispatch_group_t g3 = dispatch_group_create(); dispatch_group_enter(g1); dispatch_async(dispatch_get_global_queue(0, 0), ^{ for(int i = 0; i < 3; i++){ NSLog(@"----A----"); } dispatch_group_leave(g1); }); dispatch_group_enter(g1); dispatch_group_enter(g2); dispatch_async(dispatch_get_global_queue(0, 0), ^{ for(int i = 0; i < 3; i++){ NSLog(@"----B----"); } dispatch_group_leave(g1); dispatch_group_leave(g2); }); dispatch_group_enter(g2); dispatch_async(dispatch_get_global_queue(0, 0), ^{ for(int i = 0; i < 3; i++){ NSLog(@"----C----"); } dispatch_group_leave(g2); }); dispatch_group_enter(g3); dispatch_group_notify(g1, dispatch_get_global_queue(0, 0), ^{ for(int i = 0; i < 3; i++){ NSLog(@"----D----"); } dispatch_group_leave(g3); });; dispatch_group_enter(g3); dispatch_group_notify(g2, dispatch_get_global_queue(0, 0), ^{ for(int i = 0; i < 3; i++){ NSLog(@"----E----"); } dispatch_group_leave(g3); }); dispatch_group_notify(g3, dispatch_get_global_queue(0, 0), ^{ for(int i = 0; i < 3; i++){ NSLog(@"----F----"); } }); }执行结果如下：
