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The core XBlock library is built around the assumption that Runtimes will be implemented for each application. Because of that design intention, there are methods on Runtimes such as handler_url and local_resource_url which really only make sense to be implemented by the hosting application. Unfortunately, in the existing edx-platform implementation, some of the XBlock runtimes are actually supplied by the Modulestore (upon construction of XBlocks and XModuleDescriptors), rather than being supplied by the runtime. This leads to a number of workarounds for those application specific methods, as well as an amount of trickery and juggling so that XBlocks, XModules, and XModuleDescriptors are provided with both the appropriate Runtime and appropriate FieldData for the Application context that they are being used in.
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