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PromiEvent

To help web3 integrate into all kind of projects with different standards, we provide multiple ways to act on asynchronous functions.
Most web3.js objects allow a callback as the last parameter, as well as returning promises to chain functions.
Meter as a blockchain has different levels of finality and therefore needs to return multiple "stages" of an action.
To cope with this requirement, we return a "PromiEvent" for functions like web3.eth.sendTransaction or contract methods. These stages are encapsulated into a "PromiEvent", which combines a promise with an event emitter.
The event emitter fires an event for each of the finality stages.
An example of a function that benefits from a PromiEvent is the web3.eth.sendTransaction method:
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web3.eth.sendTransaction({from: '0x123...', data: '0x432...'})
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.once('transactionHash', function(hash){ ... })
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.once('receipt', function(receipt){ ... })
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.on('confirmation', function(confNumber, receipt){ ... })
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.on('error', function(error){ ... })
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.then(function(receipt){
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// will be fired once the receipt is mined
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});
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