Online education research experienced a surge of empirical work on student participation in open networked environments triggered by the popularity of MOOCs and developments in collecting and processing online traces of student activity. Thus far, studies of MOOCs have been exploratory in nature, failing to embrace existing educational theories in a way that addresses the specificities of learning in non-formal digital educational environments. As such, the causal links between the learning-related factors and desired learning outcomes remain unexplored, lacking solid frameworks for explanatory analyses of learning. To better understand the current state and approaches to model learning, we conducted a systematic literature review of MOOC research. The first part of this paper analyzes a wide-range of learning related constructs used to predict and measure student engagement. More importantly, building on the analyses of 38 studies included in the review, we put forward a theoretical framework for studying learning and interpreting observed learning indicators in non-formal digital environments. The structure of the framework stems from a well-established model of student engagement. To make the framework suitable to the open online context, we have redefined and re- operationalized its elements according to the findings of the systematic review. More specifically, we propose a model for studying the association between contextual factors (i.e., demographic, classroom, and individual needs), student engagement (i.e., academic, behavioral, cognitive, and affective engagement metrics) and learning outcome (i.e., academic, social, and affective). This study explains hypothesized association between the three constructs of context, engagement and outcomes, as well as presents a list of metrics used to operationalize elements of the proposed model. Further research aligned by the suggested framework would allow for further comparisons between the studies, as well as with different (more traditional) forms of education.