iOS中addSubview与removeFromSuperView耗时问题咨询

It’s totally understandable to be confused here—these methods look like they just update a parent pointer and modify an array, but under the hood, UIKit has to handle a ton of heavy lifting that adds up quickly, especially in cell lifecycle methods like cellWillAppear(_:) and prepareForReuse() that fire constantly during scrolling.

Here’s why these methods are eating up your time: #

• Layout cascade triggers: Adding or removing a view immediately triggers a layout update. If your cell uses Auto Layout, UIKit has to recalculate constraints, resolve layout priorities, and update the frame of every affected view (including nested subviews). This layout computation is far more expensive than a simple array modification.
• Layer tree and render loop work: Every UIView is backed by a CALayer. When you add/remove a view, UIKit has to update the layer hierarchy, which involves communicating with Core Animation. This can include invalidating the render loop, updating the visible layer tree, and even handling implicit animations (even if you think you’ve disabled them, some system-level animations might still run).
• Response chain and state cleanup: removeFromSuperview() doesn’t just detach the view—it also removes it from the UI responder chain, cleans up any associated gesture recognizers, cancels pending animations, and unregisters any layout observers. All these small operations add up when called hundreds of times during fast scrolling.
• High-frequency execution: prepareForReuse() and cellWillAppear(_:) are called repeatedly as cells scroll on/off screen. Doing add/remove operations here means you’re tearing down and rebuilding view hierarchies constantly, instead of reusing existing views.

Fixes to improve scrolling performance: #

• Use isHidden instead of add/remove: Initialize all your cell’s subviews once (in init(style:reuseIdentifier:) or awakeFromNib()), then toggle their visibility with isHidden when the cell’s content changes. This skips all the layout and layer overhead of adding/removing views.
• Precreate view pools if dynamic views are necessary: If you truly need dynamic subviews (e.g., variable numbers of elements), create a pool of reusable views in the cell. When the cell is reused, reassign existing views instead of adding new ones, and hide any unused ones.
• Simplify your Auto Layout: Complex constraint hierarchies (especially with nested stack views or conflicting priorities) make layout recalculations slower. Audit your constraints to remove unnecessary ones, use stack views efficiently, and avoid overconstraining views.
• Dig deeper with Instruments: Use the Time Profiler instrument to look at the full call stack under addSubview(_:)/removeFromSuperview()—you might find that a specific subview’s layout, gesture recognizer, or animation is the real bottleneck. The Core Animation instrument can also show you if layer updates are causing excessive render time.

内容的提问来源于stack exchange，提问作者Ren