onLayerAppears

onLayerAppears is an event-driven concept used in data visualization and geographic information system libraries that support layered rendering. It refers to a callback or listener that fires when a specific layer becomes visible or active within the rendering context. The exact moment the event triggers can vary by implementation: it may occur when a layer is added to a view, when a layer becomes visible due to panning or zooming, or when the layer finishes its initial rendering.

In practice, an onLayerAppears handler is used to run code that should execute only after the layer is shown. Common use cases include initializing resources tied to the layer, loading or refreshing data, starting animations, or updating UI elements related to that layer. The event payload typically includes identifiers and metadata for the layer, such as a layerId, a name or type, and sometimes the current viewport or visibility state.

Usage patterns differ by library. Some environments attach a callback directly to the layer configuration as onLayerAppears: (layer) => { ... }. Others expose a generic event system where developers register listeners like map.on('layerAppears', handler). Because implementations are not standardized, developers should consult the specific library’s documentation to understand the exact event name, payload, and lifecycle.

Common considerations include handling multiple firings if a layer toggles visibility, and avoiding redundant work by guarding against repeated execution or by debouncing. Related events include onLayerAdded, onLayerLoaded, and onLayerVisible.