After I upgraded my android studio from 0.86 and 1.2. I tried to build my project, I got a lot of errors like appcompat-v7:21.0.0': No resource found that matches the given name: attr: android :xxxx"
Actually, that is because appcompat-v7:21.0.0' required API 21. So, I need to install API 21 Platform SDK, Extra> Google Support Repository and Google Support Library.
The most important part, the part I missed, in the build.gradle set complie sdk to 21

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I am not an expert in Xamarin. I only have spent 6 months on this programming platform. Thus, please feel free to response this post.
My frist preference is to Shared Projects. That is easier to manage. I can share xmal files across each project. That is simple, I think that is good to use in a small and one-off project.
However, if you have more focus on reuse, I think you choose PCL. That is more like a standard control library. You can use the controls in the library easily! Also, you are building a larger scale project, I think that is safer to use PCL, because PCL is more self-contained.
I think using PCL or Shared Projects is depending your project scale and goals.

Recently, I built an iOS app and I have packed it as ipa. Then I used the Application Loader to upload it to itunecconnect. Then I got an error you are not authorised to use this service. This is a very silly, that is because I have reset my password recently, I need to do click the area I clicked in the picture. And then I will have a dialog box popup then I entered the new login details and then it works!

In Xamarin Form, you have to know how to use CustomRender. You need to use it to preform some platform specific functions. However, we need to access some properties from CustomRenderer to Xamarin Form Control, such as its height and width, That is very easy to do, you just need to use Element property.
e.g.

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After the WWDC last week, watchOS has announced, there is a watchKit for developers. We can build apps on running appleWatch. Moreover, there is AndroidWear in the market for some time, On 2007, Apple released iPhone. It opens a market for independent developers to build apps running on phones. That was a game changer! The most of people have access to software applications in anywhere. Now, that is the second level, this is a wearable smart device. The users can have easier access to software applications. That will be another big changes