how to "prepare" a view in the background?

I have a two views, 1st is a simple view with some introduction about usage and by click of a button it opens the main view. The main view has many images and two customized tables with rows consists of text and image, thus the creation of the main view is quite slow. The profiler shows most of the time is consumed by imageIO (decode_mcu, png_read_filter_row, pmap_copy_page and decompress_onepass)

I want to optimize by creating the main view immediately after the 1st view is loaded, and when I click the button, it simply set that view to visible or bring that view to front.

I tried to alloc & init the main view in the first view's viewDidLoad

- (void)viewDidLoad
{
    [super viewDidLoad];
    rootVC = [[RootViewController alloc] initWithNibName:@"ViewController" bundle:nil];
    rootV开发者_Python百科C.delegate = self;


}

and do this in the button's action method

- (IBAction)buttonUp:(id)sender {
    [self.view addSubview: rootVC.view];
}

But this is not working, the loading still takes very long. How could I sacrifice some memory to make better user experience?

Thanks

Leo

You should:

1. Use Instruments to see where your code is spending the most time, and optimize there. It may not be where you think.

2. If you are using a UITableView, learn how to create UITableViewCells on demand (rather than preloading a large number) and recycle instances (rather than recreating them).

3. Only if Instruments points to this as the bottleneck: try loading/decompressing images in on a background queue, then updating the UIImageView (or whatever) on the main queue after loading.

In iOS 4.x or later, you can do this:

NSString *path = ...
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, NULL), ^{
    // This is happening in the background thread.
    UIImage *image = [UIImage imageWithContentsOfFile:path];
    dispatch_async(dispatch_get_main_queue(), ^{
        // This is happening on the main thread; all UI updates must happen here.
        imageView.image = image;
    });
});

P.S. By the way, UIViewController doesn't load the view on creation, but on demand. So based on your code above, the expression rootVC.view is probably triggering the view to load. If you want to force it to load earlier, just put rootVC.view somewhere else.

UIViewController tries to load its view lazily, i.e. the view isn't loaded until it's actually needed. If you really want to force your view controller to load its view, you just need to access its view property, e.g. rootVC.view.

However, you really shouldn't need to do that. Table views, for example, only create the cells that are visible on screen, so it's unlikely that the number of rows is significantly impacting the time to load the view. One caveat here is that there are some things that can really slow down a table view. For example, if you're using variable-height cells in a table with a lot of rows, that can really slow down table creation. Try not to do that.

@benzado gives some good pointers for improving performance. Trying to work against Cocoa-Touch is always a recipe for pain, so try instead to figure out why your view is taking so long to load. Where is your app spending all that time? What can you do to reduce that load?