github.com/cloudbase/juju-core@v0.0.0-20140504232958-a7271ac7912f/state/apiserver/common/watch.go (about)

     1  // Copyright 2013 Canonical Ltd.
     2  // Licensed under the AGPLv3, see LICENCE file for details.
     3  
     4  package common
     5  
     6  import (
     7  	"launchpad.net/juju-core/state"
     8  	"launchpad.net/juju-core/state/api/params"
     9  	"launchpad.net/juju-core/state/watcher"
    10  )
    11  
    12  // AgentEntityWatcher implements a common Watch method for use by
    13  // various facades.
    14  type AgentEntityWatcher struct {
    15  	st          state.EntityFinder
    16  	resources   *Resources
    17  	getCanWatch GetAuthFunc
    18  }
    19  
    20  // NewAgentEntityWatcher returns a new AgentEntityWatcher. The
    21  // GetAuthFunc will be used on each invocation of Watch to determine
    22  // current permissions.
    23  func NewAgentEntityWatcher(st state.EntityFinder, resources *Resources, getCanWatch GetAuthFunc) *AgentEntityWatcher {
    24  	return &AgentEntityWatcher{
    25  		st:          st,
    26  		resources:   resources,
    27  		getCanWatch: getCanWatch,
    28  	}
    29  }
    30  
    31  func (a *AgentEntityWatcher) watchEntity(tag string) (string, error) {
    32  	entity0, err := a.st.FindEntity(tag)
    33  	if err != nil {
    34  		return "", err
    35  	}
    36  	entity, ok := entity0.(state.NotifyWatcherFactory)
    37  	if !ok {
    38  		return "", NotSupportedError(tag, "watching")
    39  	}
    40  	watch := entity.Watch()
    41  	// Consume the initial event. Technically, API
    42  	// calls to Watch 'transmit' the initial event
    43  	// in the Watch response. But NotifyWatchers
    44  	// have no state to transmit.
    45  	if _, ok := <-watch.Changes(); ok {
    46  		return a.resources.Register(watch), nil
    47  	}
    48  	return "", watcher.MustErr(watch)
    49  }
    50  
    51  // Watch starts an NotifyWatcher for each given entity.
    52  func (a *AgentEntityWatcher) Watch(args params.Entities) (params.NotifyWatchResults, error) {
    53  	result := params.NotifyWatchResults{
    54  		Results: make([]params.NotifyWatchResult, len(args.Entities)),
    55  	}
    56  	if len(args.Entities) == 0 {
    57  		return result, nil
    58  	}
    59  	canWatch, err := a.getCanWatch()
    60  	if err != nil {
    61  		return params.NotifyWatchResults{}, err
    62  	}
    63  	for i, entity := range args.Entities {
    64  		err := ErrPerm
    65  		watcherId := ""
    66  		if canWatch(entity.Tag) {
    67  			watcherId, err = a.watchEntity(entity.Tag)
    68  		}
    69  		result.Results[i].NotifyWatcherId = watcherId
    70  		result.Results[i].Error = ServerError(err)
    71  	}
    72  	return result, nil
    73  }