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POCreating an autolayout-based metrics view
 

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    <p>I have a reusable view I will be using in <code>UITableViewCell's</code> and <code>UICollectionViewCell</code>'s, and need to get its dimensions for <code>tableView:heightForRowAtIndexPath:</code>. Some subviews have stuff going on inside <code>layoutSubviews</code> so I can't call <code>systemLayoutForContentSize:</code>, instead my plan is to:</p> <ol> <li>Instantiate the metrics view.</li> <li>Set the size to include the desired width.</li> <li>Populate it with data.</li> <li>Update constraints / Layout subviews.</li> <li>Grab the height of the view or an internal "sizing" view.</li> </ol> <p>The problem I'm running into is that I cannot force the view to layout without inserting it into the view and waiting for the runloop.</p> <p>I've distilled a rather boring example. Here's View.xib. The subview is misaligned to highlight that the view is never getting laid out even to the baseline position:</p> <p><img src="https://i.stack.imgur.com/mxBMA.png" alt="View.xib"></p> <p>On the main thread I call:</p> <pre><code>UIView *view = [[UINib nibWithNibName:@"View" bundle:nil] instantiateWithOwner:nil options:nil][0]; NSLog(@"Subviews: %@", view.subviews); view.frame = CGRectMake(0, 0, 200, 200); [view updateConstraints]; [view layoutSubviews]; NSLog(@"Subviews: %@", view.subviews); [self.view addSubview:view]; [view updateConstraints]; [view layoutSubviews]; NSLog(@"Subviews: %@", view.subviews); dispatch_async(dispatch_get_main_queue(), ^{ NSLog(@"Subviews: %@", view.subviews); }); </code></pre> <p>I get out the following view information:</p> <pre><code>1) "&lt;UIView: 0x8bad9e0; frame = (50 50; 220 468); autoresize = W+H; layer = &lt;CALayer: 0x8be0070&gt;&gt;" 2) "&lt;UIView: 0x8bad9e0; frame = (50 50; 220 468); autoresize = W+H; layer = &lt;CALayer: 0x8be0070&gt;&gt;" 3) "&lt;UIView: 0x8bad9e0; frame = (50 50; 220 468); autoresize = W+H; layer = &lt;CALayer: 0x8be0070&gt;&gt;" 4) "&lt;UIView: 0x8bad9e0; frame = (0 100; 100 100); autoresize = W+H; layer = &lt;CALayer: 0x8be0070&gt;&gt;" </code></pre> <p>1 indicates that the fresh-out-of-the-NIB view hasn't been laid out. 2 indicates that <code>updateConstraints/layoutSubviews</code> did nothing. 3 indicates that adding it to the view hierarchy did nothing. 4 finally indicates that adding to the view hierarchy and one pass through the main-loop laid out the view.</p> <p>I would like to get to the point where I can get the view's dimensions without having to let the application handle it or perform manual calculations (string height + constraint1 + constraint2) on my own.</p> <p><strong>Update</strong></p> <p>I've observed that if I place <code>view</code> inside a <code>UIWindow</code> I get a slight improvement:</p> <pre><code>UIView *view = [[UINib nibWithNibName:@"View" bundle:nil] instantiateWithOwner:nil options:nil][0]; UIWindow *window = [[UIWindow alloc] initWithFrame:CGRectMake(0, 0, 200, 200)]; view.frame = CGRectMake(0, 0, 200, 200); [window addSubview:view]; [view layoutSubviews]; </code></pre> <p>If <code>view.translatesAutoresizingMaskIntoConstraints == YES</code>, the view's immediate subviews will be laid out, but none of their children.</p>
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