OC：亮度调节

背景：有些场景，比如展示付款码的时候，需要将屏幕调到最亮以方便商家扫码，在支付完成退出当前页面或者App退到后台时，就需要将屏幕亮度恢复到原来的亮度。

MZBrightnessHelper.h

#import <Foundation/Foundation.h>

@import CoreGraphics;

@interface MZBrightnessHelper : NSObject

/** 逐步设置亮度 */
+ (void)mz_graduallySetBrightness:(CGFloat)value;

/** 逐步恢复亮度 */
+ (void)mz_graduallyResumeBrightness;

@end

MZBrightnessHelper.m

#import "MZBrightnessHelper.h"

#import <UIKit/UIKit.h>

static CGFloat _currentBrightness = 0;
static NSOperationQueue *_queue;

@implementation MZBrightnessHelper

+ (void)initialize {
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(saveDefaultBrightness) name:UIScreenBrightnessDidChangeNotification object:nil];
    [self saveDefaultBrightness];
}

+ (void)saveDefaultBrightness {
    _currentBrightness = [UIScreen mainScreen].brightness;
}

+ (void)mz_graduallySetBrightness:(CGFloat)value {
    if (!_queue) {
        _queue = [[NSOperationQueue alloc] init];
        _queue.maxConcurrentOperationCount = 1;
    }
    [_queue cancelAllOperations];
    
    CGFloat brightness = [UIScreen mainScreen].brightness;
    CGFloat step = 0.005 * ((value > brightness) ? 1 : -1);
    int times = fabs((value - brightness) / 0.005);
    for (CGFloat i = 1; i < times + 1; i++) {
        [_queue addOperationWithBlock:^{
            [NSThread sleepForTimeInterval:1 / 180.0];
            [UIScreen mainScreen].brightness = (i == times) ? ((value > brightness) ? value : _currentBrightness) : (brightness + i * step);
        }];
    }
}

+ (void)mz_graduallyResumeBrightness {
    [self mz_graduallySetBrightness:_currentBrightness];
}

@end

使用：注册应用要注册通知UIApplicationWillResignActiveNotification和UIApplicationDidBecomeActiveNotification。并在viewWillAppear:和viewWillDisappear:做相应设置即可。

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationWillResignActive) name:UIApplicationWillResignActiveNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(applicationDidBecomeActive) name:UIApplicationDidBecomeActiveNotification object:nil];

- (void)applicationWillResignActive {
    [MZBrightnessHelper mz_graduallyResumeBrightness];
}

- (void)applicationDidBecomeActive {
    [MZBrightnessHelper mz_graduallySetBrightness:kScreenBrightness];
}