Support for different types of indicators, including gif images

[jdmoreira]

This pull request is my proposed solution for issue #315.

I basically changed 'kf_indicator' from being a view to being of type 'Indicator', which is a protocol. Having a protocol allows us to add different kinds of Indicators.

I've already added a struct that conforms to 'Indicator' for Images (including GIF support) and another for NSProgressIndicator/UIActivivityIndicator.

Since the setter for 'kf_indicator' is private, the user chooses what indicator to use through the 'kf_indicatorType' enum. There are default values, plus values that receive parameters as associated values.
There's a .custom value that allows the user to pass any type that conforms to 'Indicator', allowing the user to have his own custom indicator view.

Also, since I needed to save any swift type that conforms to a protocol in the associated objected 'kf_indicator', I had to created a wrapped/lifted version of objc_set/objc_getAssociatedObject. That's my first commit, I added this to file WrappedAssociatedObjects.swift and replaced all objc_set/get_AssociatedObject calls with calls to the wrapped version.

All suggestions and changes are welcome.

[codecov-io]

Current coverage is 73.04% (diff: 71.08%)

Merging #425 into master will decrease coverage by 1.22%

@@             master       #425   diff @@
==========================================
  Files            14         16     +2   
  Lines          1691       1755    +64   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits           1256       1282    +26   
- Misses          435        473    +38   
  Partials          0          0          

Powered by Codecov. Last update b1fa34e...126f42a

[onevcat]

To follow Swift 2 API guideline, please use upper case for enum member here.

[onevcat]

Hi, @jdmoreira

Just had a brief look at it. Great implementation!

But I have some concern before merging it. You have removed kf_showIndicatorWhenLoading and IndicatorView typealias, which were marked as public before. I understand that by doing so could make code cleaner and expressive. However, as a framework, it is also important to keep back compatible as mush as possible. As semantic versioning requiring, we may need a major update for such API breaking. IMO, it is not a good idea to do so for this kind of change.

So before it could be merged, may I ask to add back the IndicatorView typealias as well as the kf_showIndicatorWhenLoading property, and make it behave as .activity is set when kf_indicatorType is set to .none. (and maybe mark this API as deprecated with warning).

I also commented in the code for a trivial style/convention. And it would be even greater if you can also make a p-r to swift3 branch (there you could do the breaking change by removing the kf_showIndicatorWhenLoading), since the main development of Kingfisher will continue there instead of Swift 2.x version.

Thanks again for it. It's impressive to me!

[jdmoreira]

Absolutely. I'll do it :)

[onevcat]

Fix and merged in #430

[hifall]

Looks like an awesome feature! I can't wait to add this in ASAP.

Will the document be updated soon?

[onevcat]

@hifall It's already in version 2.6.0. And I will continue work on it for Swift 3 later.