Any allocations (e.g. events being registered) should be done in onAppear or onMount, and they should be "undone" in onUnmount or onDisappear.
The constructor of an item should not perform any allocations. (The counterpart to the constructor is garbage collection, which cannot perform any custom deallocations.)
It is allowed to call render multiple times during its parent's render (e.g. to figure out the optimal widths of columns in a grid).
It is not allowed to render the same item into multiple parents simultaneously.
It is allowed to "move" an item from one parent to another. In that case, onUnmount is called before onMount.
Methods starting with on, should, or do (and also getContextModifications, getPrepareContextModifications, getMaxRenderCacheSize) are "virtual" methods meant for overriding. Other methods (such as render and forceRender) should not be overridden in subclasses of Component (they are only there to be called). You don't have to override any methods, though: A component which does not define any methods is okay (although it won't actually do much).
Components should not manually install event listeners on child components. That happens automatically (with guaranteed cleanup), so that any child events are re-triggered on the parent. Any custom event handling beyond that can be done by overriding onChildEvent. Event handlers on the component itself should be defined via the onChange property.