设置视图的alpha值，而子视图不透明

最近想整一下导航栏渐变，然后发现UIColor的一个方法: colorWithAlphaComponent: 通过此方法给视图一个alpha值，改变该视图的透明度，但其上的自视图控件透明度不受影响。

// Returns a color in the same color space as the receiver with the specified alpha component.
- (UIColor *)colorWithAlphaComponent:(CGFloat)alpha;

创建两个视图UIView:

#pragma mark -懒加载
- (UIView *)fView
{
    if (!_fView) {
        _fView = [[UIView alloc] initWithFrame:CGRectMake(10, 100, SCREEN_Width - 20, 200)];
        _fView.backgroundColor = [UIColor blueColor];
    }
    return _fView;
}

- (UIView *)subView
{
    if (!_subView) {
        _subView = [[UIView alloc] initWithFrame:CGRectMake(20, 20, SCREEN_Width - 60, 160)];
        _subView.backgroundColor = [UIColor redColor];
    }
    return _subView;
}

添加到控制器视图上，_subView添加到_fView上:

[self.view addSubview:self.fView];
[self.fView addSubview:self.subView];

默认效果:

default.png

1. 直接设置视图的alpha值，其上的子控件透明度也会改变:

self.fView.alpha = 0.3;

1.png

2. 通过UIColor的类方法+ (UIColor *)colorWithWhite:(CGFloat)white alpha:(CGFloat)alpha;

self.fView.backgroundColor = [UIColor colorWithWhite:1 alpha:0.3];

2.png

3. 通过UIColor的实例方法- (UIColor *)colorWithAlphaComponent:(CGFloat)alpha;

self.fView.backgroundColor = [[UIColor blueColor] colorWithAlphaComponent:0.1];

3.png

4.通过UIColor方法:+ (UIColor *)colorWithRed:(CGFloat)red green:(CGFloat)green blue:(CGFloat)blue alpha:(CGFloat)alpha;

self.fView.backgroundColor = [UIColor colorWithRed:0 green:0 blue:1 alpha:0.3];

4.png

5. 16进制方式设置颜色时 设置alpha,实质还是通过RGBA设置颜色:

self.fView.backgroundColor = [Tools colorWithHex:0x0000FF alpha:0.3];

5.png

这样看，除了直接设置视图的alpha会影响到子视图的透明度，其他方式均不影响到其子视图的透明度。

#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>

@interface Tools : NSObject

+ (UIColor *)colorWithHex:(unsigned int)hex;

+ (UIColor *)colorWithHex:(unsigned int)hex alpha:(CGFloat)alpha;

@end

#import "Tools.h"

@implementation Tools

+ (UIColor *)colorWithHex:(unsigned int)hex alpha:(CGFloat)alpha{
    
    return [UIColor colorWithRed:((float)((hex & 0xFF0000) >> 16)) / 255.0
                           green:((float)((hex & 0xFF00) >> 8)) / 255.0
                            blue:((float)(hex & 0xFF)) / 255.0
                           alpha:alpha];
    
}

+ (UIColor *)colorWithHex:(unsigned int)hex
{
    return [self colorWithHex:hex alpha:1];
}

@end