iOS开发 容易犯错的知识点和不错的细小知识点(持续更新)

CGImage和CGImageRef这两个应当是用来重绘图形的类，它们在应用时是按照图像的像素矩阵来绘制图片的，它们可以用来处理bitmap。

CGImageRef与UIImage的互转

 

CGImageRef转换成UIImage CGImageRef

iOffscreen = CGBitmapContextCreateImage(context);

UIImage* image = [UIImage imageWithCGImage: iOffscreen];

 

UIImage转换成CGImageRef

UIImage *loadImage=[UIImage imageNamed:@"comicsplash.png"];

CGImageRef cgimage=loadImage.CGImage;

NSData *data;

        if (UIImagePNGRepresentation(image) == nil) {

            data = UIImageJPEGRepresentation(image, 1);

        } else {

            data = UIImagePNGRepresentation(image);

        }

UIImagePNGRepresentation转换PNG格式的图片为二进制，如果图片的格式为JPEG则返回nil；
 [fileManager createFileAtPath:[filePath stringByAppendingString:@"/image.png"] contents:data attributes:nil];    将图片保存为PNG格式

 [fileManager createFileAtPath:[filePath stringByAppendingString:@"/image.jpg"] contents:data attributes:nil];   将图片保存为JPEG格式



我们也可以写成下面的格式存储图片

NSString *pngImage = [filePath stringByAppendingPathComponent:@"Documents/image.png"];

NSString *jpgImage = [filePath stringByAppendingPathComponent:@"Documents/image.jpg"];



[data writeToFile:pngImage atomically:YES];

[data writeToFile:jpgImage atomically:YES];

 button.titleLabel.font = [UIFont systemFontOfSize: 28.0];

 CLLocation *orig=[[CLLocation alloc] initWithLatitude:[mainDelegate.latitude_self doubleValue]  longitude:[mainDelegate.longitude_self doubleValue]];  
 CLLocation* dist=[[CLLocation alloc] initWithLatitude:[tmpNewsModel.latitude doubleValue] longitude:[tmpNewsModel.longitude doubleValue]];  
      
 CLLocationDistance kilometers=[orig distanceFromLocation:dist]/1000;  
 NSLog(@"距离:",kilometers);  

最近开发iPhone程序，获得开发签名后在真机器上部署测试，在点击Build And Run后，提示The executable was signed with invalid entitlements.（The entitlements specified in your application’s Code Signing Entitlements file do not match those specified in your provisioning profile.）错误->(0xE8008016)，没有发布成功。在网上查询资料后，按照其中一个方法成功解决了此问题，具体如下：
 
第一步：在工程中添加文件new file，选择Code Signing 中的Entitlements，添加后名称为Entitlements.plist。
 
第二步：点击Entitlements.plist进行编辑，删除所有Root下的Key，然后添加一个Boolean类型，名称为get-task-allow的Key，状态为CHECKED
 
第三步：在Targets中的Info中的Build选项卡中的Code Signing Entitlements的值设为Entitlements.plist，重新Build即可解决

+ (UIImage *)getFirstFrameOfVideoWithVideoURL:(NSURL *)videoURL
{
    AVURLAsset *asset = [[AVURLAsset alloc] initWithURL:videoURL options:nil];
    NSParameterAssert(asset);
    AVAssetImageGenerator *assetIG = [[AVAssetImageGenerator alloc] initWithAsset:asset];
    
    assetIG.appliesPreferredTrackTransform = YES;
    assetIG.apertureMode = AVAssetImageGeneratorApertureModeEncodedPixels;
    
    CGImageRef thumbnailImageRef = NULL;
    CFTimeInterval thumbnailImageTime = 60;
    NSError *igError = nil;
    thumbnailImageRef = [assetIG copyCGImageAtTime:CMTimeMake(thumbnailImageTime, 60) actualTime:NULL error:&igError];
    
    if (!thumbnailImageRef)
        NSLog(@"thumbnailImageGenerationError %@", igError );
    
    UIImage *thumbnailImage = thumbnailImageRef ? [[UIImage alloc] initWithCGImage:thumbnailImageRef] : nil;
    
    return thumbnailImage;
}

很多人想到的可能是取消AutoLayout,这样确实能滑动,但不提倡这样做,应该:
- (void)viewDidLayoutSubviews
{    
    [super viewDidLayoutSubviews];
    self.mScrollView.frame = CGRectMake(0, 0, kScreenWidth, kScreenHeight);
    self.mScrollView.contentSize = CGSizeMake(kScreenWidth, 568);   
}

- (void) viewDidAppear:(BOOL)animated
{
       self.scrollView.frame = CGRectMake(0, 0, 320, 480);
       [self.scrollView setContentSize:CGSizeMake(320, 1000)];
}

我觉得,目测是storyboard中默认使用的AutoLayout会使得scrollview重置为CGRectZero

+ (UIColor *)colorFromHexString:(NSString *)hexString
{
    
    if ([[hexString substringToIndex:1] isEqualToString:@"#"]) {
        hexString = [hexString substringFromIndex:1];
    }
    
    unsigned rgbValue = 0;
    NSScanner *scanner = [NSScanner scannerWithString:hexString];
    [scanner scanHexInt:&rgbValue];
    
    return [UIColor colorWithRed:((rgbValue & 0xFF0000) >> 16)/255.0 green:((rgbValue & 0xFF00) >> 8)/255.0 blue:(rgbValue & 0xFF)/255.0 alpha:1.0];
}

+ (UIImage *)compressImage:(UIImage *)imgSrc withSize:(CGSize)size
{
    UIGraphicsBeginImageContext(size);
    CGRect rect = {{0,0}, size};
    [imgSrc drawInRect:rect];
    UIImage *compressedImg = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return compressedImg;
}

frame（帧）：一个gif可以简单认为是多张image组成的动画，一帧就是其中一张图片image.

frameCount（帧数）： 就是一个gif有多少帧

loopCount（播放次数）：有些gif播放到一定次数就停止了，如果为0就代表gif一直循环播放。

delayTime（延迟时间）：每一帧播放的时间，也就是说这帧显示到delayTime就转到下一帧。

 
所以gif播放主要就是把每一帧image解析出来，然后每一帧显示它对应的delaytime，然后再显示下一张。如此循环下去。

 
下面是纯粹实现由系统提供的解码：

-(void)decodeWithFilePath:(NSString *)filePath

{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^() {

        NSData *data = [NSData dataWithContentsOfFile:self.path];

        [self decodeWithData:data];

    });

}

-(void)decodeWithData:(NSData *)data
{
    CGImageSourceRef src = CGImageSourceCreateWithData((CFDataRef) data, NULL);
    if (src)
    {
        //获取gif的帧数
        NSUInteger frameCount = CGImageSourceGetCount(src);
        //获取GfiImage的基本数据
        NSDictionary *gifProperties = (NSDictionary *) CGImageSourceCopyProperties(src, NULL);
        if(gifProperties)
        {
            //由GfiImage的基本数据获取gif数据
            NSDictionary *gifDictionary =[gifProperties objectForKey:(NSString*)kCGImagePropertyGIFDictionary];
            //获取gif的播放次数
            NSUInteger loopCount = [[gifDictionary objectForKey:(NSString*)kCGImagePropertyGIFLoopCount] integerValue];
            for (NSUInteger i = 0; i < frameCount; i++)
            {
                 //得到每一帧的CGImage
                CGImageRef img = CGImageSourceCreateImageAtIndex(src, (size_t) i, NULL);
                if (img)
                {
                    //把CGImage转化为UIImage
                    UIImage *frameImage = [UIImage imageWithCGImage:img];
                    //获取每一帧的图片信息
                    NSDictionary *frameProperties = (NSDictionary *) CGImageSourceCopyPropertiesAtIndex(src, (size_t) i, NULL);
                    if (frameProperties)
                    {
                        //由每一帧的图片信息获取gif信息
                        NSDictionary *frameDictionary = [frameProperties objectForKey:(NSString*)kCGImagePropertyGIFDictionary];
                        //取出每一帧的delaytime
                        CGFloat delayTime = [[frameDictionary objectForKey:(NSString*)kCGImagePropertyGIFDelayTime] floatValue];

                       //TODO 这里可以实现边解码边回调播放或者把每一帧image和delayTime存储起来
                        CFRelease(frameProperties);
                    }
                    CGImageRelease(img);
                }
            }
            CFRelease(gifProperties);
        }
        CFRelease(src);
    }    
}

可能你会发现这个类的父类是NSMutableSet。纳尼？不是说NSMutableSet是不可以储存重复对象的吗。其实NSCountedSet也是不能储存重复的对象的，查看Apple文档中对这个类的描述有这么一句：

Each distinct object inserted into an NSCountedSet object has a counter associated with it.
插入NSCountedSet对象的每个不同的对象都有一个与之相关的计数器

也就是说如果遇到重复对象的加入，这个对象的计数器就会+1。所以可以到这个类有个名叫
- (NSUInteger)countForObject:(id)object    的方法来统计重复对象的个数。

NSArray *array = @[@1, @2, @2, @1];
NSCountedSet *set = [[NSCountedSet alloc]initWithArray:array];
    
[set enumerateObjectsUsingBlock:^(id obj, BOOL *stop) {
	NSLog(@"%@ => %d", obj, [set countForObject:obj]);
}];

[[UIBarButtonItem appearance] setBackButtonTitlePositionAdjustment:UIOffsetMake(0, -60)
                                                     forBarMetrics:UIBarMetricsDefault];

UIGraphicsBeginImageContext(self.view.bounds.size);

 [self.view.layer renderInContext:UIGraphicsGetCurrentContext()];

 UIImage *image= UIGraphicsGetImageFromCurrentImageContext();

 UIGraphicsEndImageContext();

 UIImageWriteToSavedPhotosAlbum(image, self, nil, nil);

- (UIImage *)creatImageWithView:(UIView *)view rect:(CGRect)rect
{
    CGSize size = view.bounds.size;
    UIGraphicsBeginImageContextWithOptions(size, NO, 0);
    [view.layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *viewImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    CGImageRef viewImageRef = viewImage.CGImage;
    CGImageRef imageRef = CGImageCreateWithImageInRect(viewImageRef, rect);
    
    UIImage *image = [[UIImage alloc] initWithCGImage:imageRef];
    CGImageRelease(imageRef);
    return image;
}

- (double)radians:(double)degress
{
    return degress * M_PI/180;
}