为什么简单的模态视图控制器在显示和关闭时会滞后？

Question

The main view of my app is a UIImagePickerController camera view. When the app becomes active (in didBecomeActive), I present a modal view controller that shows some settings generated from a network request. (Note that for debugging purposes, I took the network request out and am currently just showing a dummy view)

The modal view animates in smoothly, but after loading it freezes for 3 seconds then responds normally. After dismissing the view (also animates smoothly), my image picker controller pauses for 2 seconds then resumes normally.

I have removed all functionality from the modal view controller to make sure there was no operations clogging the main thread. I am presenting the most basic of controllers, and still get the choppy ui. I would suspect that this is from my presenting view controller calling viewDidLoad/Unload or something similar, but my search did not give me any information on what delegate methods are called in the presenting view controller when a modal view is shown.

My problem can be solved by answering:

• What delegate methods are called in the presenter when a modal view is shown?
• (If any ^) How can I not call those methods, or make them run smoother?
• What common pitfalls are associated with modal view controllers?

Answer 1

There are multiple methods invoked. ViewDidLoad ViewWillAppear ViewDidAppear ViewWillDisappear ViewDidDisappear. Check all of those methods. Also, check any subviews you have created and see if they are doing any thing on their thread involving image loading in the methods i stated. Also does this occur in the simulator as well as a test device?

ModalViewControllers do not have too many pitfalls but understanding how many views are allocated on things like navigation stacks and how many views you have on top of each other. When you get rid of the modal viewcontroller do you call dismissviewcontroller?

Answer 2

This is probably because you are making a lot of processing in the main thread (usually when UI stops, it's because main thread processing). Try to provide us some code, specifically the one you think is the most heavy processing code! Sorry about my poor english :P! Try dispatching most heavy code to another thread with

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    //your heavy code here =)
});

Regards, Lucas

Answer 3

One thing that might be a contributor to some slight lag is reloading the same viewController from scratch each time.

BProfileTableViewController * _profileViewController = [[UIStoryboard storyboardWithName:@"Profile" bundle:[NSBundle chatUIBundle]] instantiateInitialViewController];
UINavigationController * profileNavigationController = [[UINavigationController alloc] initWithRootViewController:profileViewController];
[self.navigationController presentViewController:profileNavigationController animated:YES completion:nil];

You can see here that if this is on a tableView click then each time the app needs to create the viewController again. If instead we just reuse the view then it gets rid of some of that lag.

Add this in the header file

BProfileTableViewController * _profileView;

Then the modal view load code changes to:

// Open the users profile
if (!_profileView) {
    _profileView = [[UIStoryboard storyboardWithName:@"Profile" bundle:[NSBundle chatUIBundle]] instantiateInitialViewController];
}

UINavigationController * profileNavigationController = [[UINavigationController alloc] initWithRootViewController:_profileView];
[self.navigationController presentViewController:profileNavigationController animated:YES completion:nil];

This means we are just reloading the view instead of recreating the view