feat(task): add task lifecycle doc and testcases to explain task state transition. (#651)

* doc(task): add task document for lifecycle explaination

* test(task): add testcases for task lifecycle

Author: JiaLiPassion

doc/error.png

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+@startuml
+scheduling --> unknown: zoneSpec.onScheduleTask\nor task.scheduleFn\nthrow error
+running --> scheduled: error in \ntask.callback\nand task is\nperiodical\ntask
+running --> notScheduled: error in\ntask.callback\nand\ntask is not\nperiodical
+running: zoneSpec.onHandleError
+running --> throw: error in\n task.callback\n and \nzoneSpec.onHandleError\n return true
+canceling --> unknown: zoneSpec.onCancelTask\n or task.cancelFn\n throw error
+unknown --> throw
+@enduml

doc/error.puml

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+@startuml
+[*] --> notScheduled: initialize
+notScheduled --> scheduling: addEventListener
+
+scheduling: zoneSpec.onScheduleTask
+scheduling: zoneSpec.onHasTask
+
+scheduling --> scheduled
+scheduled --> running: event\n triggered
+running: zoneSpec:onInvokeTask
+
+scheduled --> canceling: removeEventListener
+canceling: zoneSpec.onCancelTask
+canceling --> notScheduled
+canceling: zoneSpec.onHasTask
+
+running --> scheduled: callback\n finished
+running: zoneSpec.onHasTask
+running --> canceling: removeEventListener
+
+@enduml

doc/eventtask.png

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+@startuml
+[*] --> notScheduled: initialize
+notScheduled --> scheduling: promise.then/\nprocess.nextTick\nand so on
+
+scheduling: zoneSpec.onScheduleTask
+scheduling: zoneSpec.onHasTask
+
+scheduling --> scheduled
+scheduled --> running: callback
+running: zoneSpec:onInvokeTask
+
+running --> notScheduled
+running: zoneSpec.onHasTask
+@enduml

doc/eventtask.puml

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+@startuml
+[*] --> notScheduled: initialize
+notScheduled --> scheduling: setTimeout/\nXMLHttpRequest.send\nand so on
+
+scheduling: zoneSpec.onScheduleTask
+scheduling: zoneSpec.onHasTask
+
+scheduling --> scheduled
+scheduled --> running: timeout callback\nreadystatechange\ncallback
+running: zoneSpec:onInvokeTask
+
+scheduled --> canceling: clearTimeout\n/abort request
+canceling: zoneSpec.onCancelTask
+canceling --> notScheduled
+canceling: zoneSpec.onHasTask
+running --> notScheduled
+running: zoneSpec.onHasTask
+@enduml

doc/microtask.png

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+@startuml
+[*] --> notScheduled: initialize
+notScheduled --> scheduling: scheduleTask
+
+scheduling: zoneSpec.onScheduleTask
+scheduling: zoneSpec.onHasTask
+
+scheduling --> scheduled: override with\n anotherZone
+scheduled --> running: timeout callback\nreadystatechange\ncallback
+running: anotherZoneSpec:onInvokeTask
+
+scheduled --> canceling: clearTimeout\n/abort request
+canceling: anotherZoneSpec.onCancelTask
+canceling --> notScheduled
+canceling: zneSpec.onHasTask
+running --> notScheduled
+running: zoneSpec.onHasTask
+@enduml