iOS安全-切换后台背景模糊

导读
我们在双击切换到后台的时候，背景会有缩略图，而缩略图会暴漏用户的部分敏感数据，所以要求切换到后台后，缩略图需要做毛玻璃模糊处理。一些银行类的应用会要求这么做。比如招商银行。

切换到后台效果，可以看到时钟是没有模糊的。

实现思路也很简单在切换后台前，截取当前页面，然后做高斯模糊，然后加在window上。切到前台前，将这个加的页面移除出去。

关键代码：

//
//  PAFFBlurryView.m
//  Pods
//
//  Created by bolei on 16/10/9.
//
//

#import "BLBlurryView.h"
#import <Accelerate/Accelerate.h>

#define kBlurryTag 10009


@implementation BLBlurryView


- (instancetype)initWithFrame:(CGRect)frame blurryView:(UIView *)view {
    self = [super initWithFrame:frame];
    if (self) {
        UIImage *sourceImage = [self getCurrentImageWithView:view];
        if (sourceImage) {
            UIImage *image = [UIImage imageWithData:UIImageJPEGRepresentation(sourceImage, 1.0)];
            UIImage *sImage =  [self blurryImage:image withBlurLevel:0.1];
            UIImageView *bgView = [[UIImageView alloc] initWithFrame:frame];
            bgView.image = sImage;
            [self addSubview:bgView];
        }
        
    }
    return self;
}


+ (void)showBlurryViewInWindow {
    UIWindow *window = [UIApplication sharedApplication].delegate.window;
    BLBlurryView *view = [[BLBlurryView alloc] initWithFrame:window.frame blurryView:nil];
    view.tag = kBlurryTag;
    for (UIWindow *window in [[UIApplication sharedApplication] windows]) {
        if (window.windowLevel == UIWindowLevelNormal) {
            UIView *preView = [window viewWithTag:kBlurryTag];
            if (preView) {
                [preView removeFromSuperview];
            }
            [window addSubview:view];
        }
    }
}

+ (void)removeBlurryViewInWindow {
    for (UIWindow *window in [[UIApplication sharedApplication] windows]) {
        if (window.windowLevel == UIWindowLevelNormal) {
            UIView *view = [window viewWithTag:kBlurryTag];
            if (view) {
                [view removeFromSuperview];
            }
        }
    }
}


- (UIImage *)getCurrentImageWithView:(UIView *)view
{
    UIView *sourceView = view;
    if (sourceView == nil) {
        sourceView = [self getCurrentVisibleView];
    }
    
    if (sourceView == nil) {
        return nil;
    }
    

    UIGraphicsBeginImageContext(sourceView.bounds.size);
    [sourceView.layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}

- (UIImage *)blurryImage:(UIImage *)image withBlurLevel:(CGFloat)blur {
    if (blur < 0.f || blur > 1.f) {
        blur = 0.5f;
    }
    int boxSize = (int)(blur * 100);
    boxSize = boxSize - (boxSize % 2) + 1;
    
    CGImageRef img = image.CGImage;
    
    vImage_Buffer inBuffer, outBuffer;
    vImage_Error error;
    
    void *pixelBuffer;
    
    CGDataProviderRef inProvider = CGImageGetDataProvider(img);
    CFDataRef inBitmapData = CGDataProviderCopyData(inProvider);
    
    inBuffer.width = CGImageGetWidth(img);
    inBuffer.height = CGImageGetHeight(img);
    inBuffer.rowBytes = CGImageGetBytesPerRow(img);
    
    inBuffer.data = (void*)CFDataGetBytePtr(inBitmapData);
    
    pixelBuffer = malloc(CGImageGetBytesPerRow(img) *
                         CGImageGetHeight(img));
    
    if(pixelBuffer == NULL)
        NSLog(@"No pixelbuffer");
    
    outBuffer.data = pixelBuffer;
    outBuffer.width = CGImageGetWidth(img);
    outBuffer.height = CGImageGetHeight(img);
    outBuffer.rowBytes = CGImageGetBytesPerRow(img);
    
    error = vImageBoxConvolve_ARGB8888(&inBuffer,
                                       &outBuffer,
                                       NULL,
                                       0,
                                       0,
                                       boxSize,
                                       boxSize,
                                       NULL,
                                       kvImageEdgeExtend);
    
    
    if (error) {
        NSLog(@"error from convolution %ld", error);
    }
    
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef ctx = CGBitmapContextCreate(
                                             outBuffer.data,
                                             outBuffer.width,
                                             outBuffer.height,
                                             8,
                                             outBuffer.rowBytes,
                                             colorSpace,
                                             kCGImageAlphaNoneSkipLast);
    CGImageRef imageRef = CGBitmapContextCreateImage (ctx);
    UIImage *returnImage = [UIImage imageWithCGImage:imageRef];
    
    //clean up
    CGContextRelease(ctx);
    CGColorSpaceRelease(colorSpace);
    
    free(pixelBuffer);
    CFRelease(inBitmapData);
    
    CGColorSpaceRelease(colorSpace);
    CGImageRelease(imageRef);
    
    return returnImage;
}


- (UIView *)getCurrentVisibleView {
    UIWindow *window = [[UIApplication sharedApplication].delegate window];
    UIViewController *rootViewController = window.rootViewController;
    if ([rootViewController isKindOfClass:[UINavigationController class]]) {
        return ((UINavigationController *)rootViewController).visibleViewController.view;
    }
    return rootViewController.view;
}


/*
// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect {
    // Drawing code
}
*/

@end

逻辑并不复杂，主要麻烦的点在于什么时候调用，切到后台涉及到四个周期：

• applicationWillResignActive 失去焦点的时候会首先调用
• applicationDidEnterBackground 切换到后台后调用。在WillResignActive之后调用
• applicationWillEnterForeground 将要回到前台的时候调用
• applicationDidBecomeActive 已经到前台后调用

其中点击home，切换到后台，四个状态都会调用。但是双击home的话只会调用applicationWillResignActive和applicationDidBecomeActive

一般调用显示和消失需要成对调用。所以两个方案：

• 方案1：applicationWillResignActive调用消失，然后applicationWillEnterForeground调用显示
• 方案2：applicationDidEnterBackground调用消失，然后applicationDidBecomeActive调用显示

方案1的问题是：在调用applicationWillResignActive，需要截屏需要做模糊效果，如果这个调用时间过长，就不会显示模糊的效果，对于复杂的页面经常会出现这个问题，需要找更好的算法去解决，目前没找到好的方案。而且进入到APP会有比较久的时间的模糊效果。

方案2的问题是：在双击home的时候，这个模糊效果不会生效。相对来说更推荐方案二。