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copied!<p>I'm still trying to figure this out myself, so take this with some skepticism and forgive me if it contains errors.</p> <p><code>setNeedsLayout</code> is an easy one: it just sets a flag somewhere in the UIView that marks it as needing layout. That will force <code>layoutSubviews</code> to be called on the view before the next redraw happens. Note that in many cases you don't need to call this explicitly, because of the <code>autoresizesSubviews</code> property. If that's set (which it is by default) then any change to a view's frame will cause the view to lay out its subviews.</p> <p><code>layoutSubviews</code> is the method in which you do all the interesting stuff. It's the equivalent of <code>drawRect</code> for layout, if you will. A trivial example might be:</p> <pre><code>-(void)layoutSubviews { // Child's frame is always equal to our bounds inset by 8px self.subview1.frame = CGRectInset(self.bounds, 8.0, 8.0); // It seems likely that this is incorrect: // [self.subview1 layoutSubviews]; // ... and this is correct: [self.subview1 setNeedsLayout]; // but I don't claim to know definitively. } </code></pre> <p>AFAIK <code>layoutIfNeeded</code> isn't generally meant to be overridden in your subclass. It's a method that you're meant to call when you want a view to be laid out <em>right now</em>. Apple's implementation might look something like this:</p> <pre><code>-(void)layoutIfNeeded { if (self._needsLayout) { UIView *sv = self.superview; if (sv._needsLayout) { [sv layoutIfNeeded]; } else { [self layoutSubviews]; } } } </code></pre> <p>You would call <code>layoutIfNeeded</code> on a view to force it (and its superviews as necessary) to be laid out immediately.</p>

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