Events Tracking

[alexanderbez]

Summary

This is a place holder to issue to update the way the SDK handles tags (events). With the advent of tendermint/tendermint#3643, we'll have to modify the way SDK tracks events to support more aggregate and complex events.

AC/Proposal

Instead of having handlers and ABCI methods explicitly return tags (now events), we can track events through the block and tx context lifecycle. At any point when we return an ABCI response, we simply aggregate all the events by type.

pseudo example:

func BeginBlock(ctx sdk.Context, ...) error {
  ctx.Event("rewards", {"...": "..."}, ...)
  
  // ...
}

func msgHandler(ctx sdk.Context, msg sdk.Msg, ...) error {
  ctx.Event("transfer", {"...": "..."}, ...)

  // ...
}

func (ba *BaseApp) runTx() {
    return abci.Response{
        Events: ctx.CollectEvents()
    }
}

So three new methods on the context:

type Context struct {
    // ...
    events map[string][]cmn.KVPair
}

func (ctx Context) Event(eType string, attrs ...cmn.KVPair) {}
func (ctx Context) CollectEvents() []abci.Event {}

// not necessary, but may be helpful
func (ctx Context) FlushEvents() {}

replaces: #3122
replaces: #3956

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