All UIWebViews share a single web thread.
When one of them is init-ed, removed from superview etc., they will attempt to lock the web thread from the main thread in a blocking fashion, thus temporarily freezing the run loop of the main thread.
If the web thread is busy, e.g. while doing a long synchronous XMLHttpRequest, this may block the main thread for a long time.
Is there a way to avoid this?
If I could modify UIWebView, I'd just make the lock attempt non-blocking, but obviously that's not the case, so I'm looking for other clever ideas.
The long and short of it is: avoid doing anything that blocks the web thread for a significant amount of time (window.alert, window.prompt, XMLHttpRequest.open('GET', url, false), possibly others)
Also, avoid calling methods that lock the web thread and then immediately doing something that takes a long time as the web thread is only unlocked once control is returned to the run loop. (Example: call -[UITextView setText:] then read a file synchronously on the main thread)
you cannot make synchronous xhr since this will lead to crossdomain policy errors.
来源：https://stackoverflow.com/questions/1431594/avoid-main-thread-freezes-when-uiwebview-tries-to-blockingly-lock-the-web-thread