Fix passing onLoad overwrites, and call onLoad inside onImageLoad

[johnsonsu]

Issue: Currently, there is no way to retrieve the onLoad event object with this library and passing an onLoad prop to LazyLoadImage will overwrite the handler on line 47 of LazyLoadImage.jsx causing afterLoad to never get called.

Description
This PR fixes passing onLoad to LazyLoadImage causing afterLoad() to never fire:

• Fix onLoad getting overwritten by props which causes onImageLoad() to never get called.
• preserve the functionality of onLoad by adding a callback.

[Aljullu]

Thanks for your contribution, @johnsonsu! I agree that it would be a good idea to pass the event object. However, it's not clear to me whether we need to introduce a new prop for this. Could we simply use afterLoad instead (maybe renaming it to onLoad)? Would that work for your use-case?

[johnsonsu]

Hi @Aljullu, hope you are doing well! Yes, that will work for my use case. I can update my code to include the event in the afterLoad callback like:

return e => {
    this.props.afterLoad(e);
    ...

However, I think it's still a good idea to preserve the onLoad for consistency because your library carries over all other props of img and having onLoad as an exception is confusing. In fact, I created this PR because I was expecting onLoad to work just like a regular img but it didn't. What do you think?

[Aljullu]

Good points, @johnsonsu. I'm thinking we should maybe simply rename the afterLoad to onLoad for the reasons you mention. And perhaps keep afterLoad as an alias of onLoad to keep backwards compatibility. Does it make sense to you? Are you up to giving it a spin? Ideally, we would update the docs as well.

[johnsonsu]

Good points, @johnsonsu. I'm thinking we should maybe simply rename the afterLoad to onLoad for the reasons you mention. And perhaps keep afterLoad as an alias of onLoad to keep backwards compatibility. Does it make sense to you? Are you up to giving it a spin? Ideally, we would update the docs as well.

I think that make sense. I will update the code and docs and push it to this PR in a bit.

[johnsonsu]

@Aljullu This is ready for your review.

[Aljullu]

Overall looks good, thanks again for working on this. I left one comment inline with a suggestion. Also, you will need to add onLoad to the propTypes and defaultProps of that file.

Besides that, it might be a good idea to duplicate this test with the onLoad prop.

[johnsonsu]

I think that's a good suggestion.

I've updated the code and labelled afterLoad as deprecated in the document and with comments.
I also updated the test to include both onLoad and afterLoad in the same test. This way the test can verify that passing onLoad no longer overwrites.

[Aljullu]

Awesome, this is looking good, @johnsonsu! Thanks a lot for your contribution! 🙌