5.1 运行循环和重绘原理
iOS 每次事件处理周期中只发送一次 drawRect: 消息。所以视图要重绘必须向其发送 setNeedDisplay 消息。
添加子视图，设置大小即可。类似一张大的画布，可以方便移动局部查看。
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CGRect screenRect = self.window.bounds;
CGRect bigRect = screenRect;
bigRect.size.width *= 2.0;
UIScrollView *scrollView = [[UIScrollView alloc] initWithFrame:screenRect];
[scrollView setPagingEnabled:YES];
[self.window addSubview:scrollView];
GWHypnosisView *hypnosisView = [[GWHypnosisView alloc] initWithFrame:screenRect];
[scrollView addSubview:hypnosisView];
screenRect.origin.x += screenRect.size.width;
GWHypnosisView *anotherView = [[GWHypnosisView alloc] initWithFrame:screenRect];
[scrollView addSubview:anotherView];
scrollView.contentSize = bigRect.size;
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