ios - UIView 的 opaque、alpha 和 backgroundColor 之间有什么关系？

Question

例如:

1. 我将 UIView 的 alpha 属性设置为 alpha = 0，这是否意味着它的 opaque 属性被视为 opaque=YES？

2. opaque，alpha 和 backgroundColor 如何影响性能？

3. 还有什么吗？...

类似问题

• UIView performance: opaque, backgroundColor, clearsContextBeforeDrawing?
• Is UIView's opaque property with a value of YES in conflict with its backgroundColor property with a value of [UIColor clearColor]?
• Cocoa/iPhone: BackgroundColor and Opaque Properties

Answer 1

backgroundColor 是出现在 UIView 背面的颜色。默认情况下，它是 nil，从而为 UIView 生成清晰的背景色。要为您的 UIView 分配背景，请像这样分配一个 UIColor:

myView.backgroundColor = [UIColor greenColor];

您还可以将 backgroundColor 设置为图像:

myView.backgroundColor = [UIColor colorWithPatternImage:[UIImage imageNamed@"myImage"]];

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alpha 是 View 的不透明度。您可以使用此属性淡化 UIView 并使其部分透明。该值的范围从 0(完全不可见)到 1(完全不透明)。

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opaque 是一个 bool 值，指示 UIView 是否不透明。如果您将 alpha 设置为任何小于 1 的值，那么您应该将 opaque 设置为 NO:

myView.opaque = NO;

Apple 的 documentation解释 opaque 属性工作的原因和方式:

This property provides a hint to the drawing system as to how it should treat the view. If set to YES, the drawing system treats the view as fully opaque, which allows the drawing system to optimize some drawing operations and improve performance. If set to NO, the drawing system composites the view normally with other content. The default value of this property is YES.

An opaque view is expected to fill its bounds with entirely opaque content—that is, the content should have an alpha value of 1.0. If the view is opaque and either does not fill its bounds or contains wholly or partially transparent content, the results are unpredictable. You should always set the value of this property to NO if the view is fully or partially transparent.