iOS-Capture Screenshot in Background

iOS系统，App运行在前台时，截屏的方式有很多，但是当被切到后台，或者锁屏时，截屏就会出现问题。

解决方案如下：

后台运行

iOS系统后台运行的方式参考：iOS-Background Execution。

截屏方式

一般情况下，使用以下方式是最好的:

- (UIImage *)snapshot{
    CGSize videoSize = CGSizeMake(150, 100);
    UIImage *captured = nil;
    UIGraphicsBeginImageContextWithOptions(videoSize, NO, 1);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetInterpolationQuality(context, kCGInterpolationLow);
    UIWindow *window = [[UIApplication sharedApplication] delegate].window;
    [window drawViewHierarchyInRect:CGRectMake(0, 0, videoSize.width, videoSize.height) afterScreenUpdates:NO];
    captured = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return captured;
}

但是，这种方式在切到后台，或者锁屏时，得到的图片的内容是空白，所以，这种情况下，我们直接利用CLLayer来渲染，生成图片。但是，利用CLLayer渲染生成的图片是跟手机屏幕大小一致的，为了得到我们想要的大小，需要进行拉伸，调用CGContextScaleCTM来完成：

CGContextScaleCTM(UIGraphicsGetCurrentContext(), videoSize.width / SCREEN_WIDTH, videoSize.height / SCREEN_HEIGHT);
[window.layer renderInContext:context];

OpenGL

用上述的方式截屏时，由于OpenGL的特殊性，无法截取到其渲染内容，所以，我们需要进一步进行修改：

创建一个CAEAGLLayer的子类：

CaptureCAEAGLLayer.h文件：

#import <QuartzCore/QuartzCore.h>

@protocol CaptureDelegate <NSObject>
- (void) renderInContext: (CGContextRef) context;
@end

@interface CaptureCAEAGLLayer : CAEAGLLayer
@property (nonatomic, assign) id<CaptureDelegate> captureDelegate;
@end

CaptureCAEAGLLayer.m文件：

#import "CaptureCAEAGLLayer.h"

@implementation CaptureCAEAGLLayer
- (void)renderInContext:(CGContextRef)ctx
{
    [super renderInContext:ctx];
    [self.captureDelegate renderInContext:ctx];
}
@end

对使用CAEAGLLayer的View进行修改：

CaptureCAEAGLLayer *eaglLayer = (CaptureCAEAGLLayer *)self.layer;
eaglLayer.captureDelegate = self;

+ (Class)layerClass
{
    return [CaptureCAEAGLLayer class];
}

- (void) renderInContext: (CGContextRef) context
{
    GLint backingWidth, backingHeight;
    
    // Bind the color renderbuffer used to render the OpenGL ES view
    // If your application only creates a single color renderbuffer which is already bound at this point,
    // this call is redundant, but it is needed if you're dealing with multiple renderbuffers.
    // Note, replace "_colorRenderbuffer" with the actual name of the renderbuffer object defined in your class.
    glBindRenderbufferOES(GL_RENDERBUFFER_OES, _colorRenderbuffer);
    
    // Get the size of the backing CAEAGLLayer
    glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_WIDTH_OES, &backingWidth);
    glGetRenderbufferParameterivOES(GL_RENDERBUFFER_OES, GL_RENDERBUFFER_HEIGHT_OES, &backingHeight);
    
    NSInteger x = 0, y = 0, width = backingWidth, height = backingHeight;
    NSInteger dataLength = width * height * 4;
    GLubyte *data = (GLubyte*)malloc(dataLength * sizeof(GLubyte));
    
    // Read pixel data from the framebuffer
    glPixelStorei(GL_PACK_ALIGNMENT, 4);
    glReadPixels(x, y, width, height, GL_RGBA, GL_UNSIGNED_BYTE, data);
    
    // Create a CGImage with the pixel data
    // If your OpenGL ES content is opaque, use kCGImageAlphaNoneSkipLast to ignore the alpha channel
    // otherwise, use kCGImageAlphaPremultipliedLast
    CGDataProviderRef ref = CGDataProviderCreateWithData(NULL, data, dataLength, NULL);
    CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
    CGImageRef iref = CGImageCreate(width, height, 8, 32, width * 4, colorspace, kCGBitmapByteOrder32Big | kCGImageAlphaPremultipliedLast,
                                    ref, NULL, true, kCGRenderingIntentDefault);
    
    // OpenGL ES measures data in PIXELS
    // Create a graphics context with the target size measured in POINTS
    NSInteger widthInPoints, heightInPoints;
    if (NULL != UIGraphicsBeginImageContextWithOptions) {
        // On iOS 4 and later, use UIGraphicsBeginImageContextWithOptions to take the scale into consideration
        // Set the scale parameter to your OpenGL ES view's contentScaleFactor
        // so that you get a high-resolution snapshot when its value is greater than 1.0
        CGFloat scale = self.contentScaleFactor;
        widthInPoints = width / scale;
        heightInPoints = height / scale;
    }
    else {
        // On iOS prior to 4, fall back to use UIGraphicsBeginImageContext
        widthInPoints = width;
        heightInPoints = height;
    }
    
    // UIKit coordinate system is upside down to GL/Quartz coordinate system
    // Flip the CGImage by rendering it to the flipped bitmap context
    // The size of the destination area is measured in POINTS
    CGContextSetBlendMode(context, kCGBlendModeCopy);
    CGContextDrawImage(context, CGRectMake(0.0, 0.0, widthInPoints, heightInPoints), iref);
    
    
    // Clean up
    free(data);
    CFRelease(ref);
    CFRelease(colorspace);
    CGImageRelease(iref);
}