Mobile UI understanding and generation require both fine-grained component perception and reliable structured synthesis. Inspired by the unified direction introduced by UI-UG, we present UI-UG 2, a compact 4B-parameter multimodal large language model (MLLM) that advances mobile UI Understanding and Generation with broader task coverage, a front-end-aligned taxonomy, and online render-aware reinforcement learning. UI-UG 2 constructs a multi-source understanding corpus from RICO, VINS, Android in the Wild, and Android Control, annotated with an 80-category two-level UI taxonomy aligned with front-end component semantics, and pairs it with over 5,000 requirement-screenshot-JSON tuples for progressive structured UI generation through a domain-specific language (DSL). For understanding, UI-UG 2 introduces decomposed rewards for category prediction, OCR fidelity, color recognition, dense grounding, and GUI grounding. Unlike the offline Direct Preference Optimization (DPO)-style generation optimization in UI-UG, UI-UG 2 optimizes generation through end-to-end online Group Relative Policy Optimization (GRPO): sampled DSL outputs are rendered with a Playwright-based server-side pipeline and evaluated using LLM-as-a-Judge and contrastive preference ranking. Experiments show that UI-UG 2 achieves the strongest referring and dense-grounding results among the evaluated models on our 80-category benchmark, and achieves the best composite generation score among compared models at a compact scale (4B). More importantly, we observe a consistent understanding-to-generation transfer: adding fine-grained UI perception supervision improves generation quality under a fixed training budget, suggesting that component-level semantics provide a shared representational foundation for both tasks.

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