Add observer

[AirChen]

Add Observer instance to receive all blocks.😁

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[ykjchen]

@AirChen your contribution is appreciated. Before we consider this further, could you create an issue and describe why you think this feature is necessary? That would help folks evaluate whether the benefit of this feature outweighs the long term maintenance cost, and help solidify any new API.

[AirChen]

@ykjchen Thanks for your advice, I will create an issue soon.