初始化：

UIImageView *_imageView = [[UIImageView alloc] init];
//其他初始化
/*
initWithFrame:<#(CGRect)#>
initWithCoder:<#(nonnull NSCoder *)#>
initWithImage:<#(nullable UIImage *)#>
initWithImage:<#(nullable UIImage *)#> highlightedImage:<#(nullable UIImage *)#>
*/

显示方式：居中、左、右，填充、拉伸、适应，类似设置屏保

_imageView.contentMode = UIViewContentModeScaleToFill;
//其他方式
/*
typedef NS_ENUM(NSInteger, UIViewContentMode) {
    UIViewContentModeScaleToFill,
    UIViewContentModeScaleAspectFit,      // contents scaled to fit with fixed aspect. remainder is transparent
    UIViewContentModeScaleAspectFill,     // contents scaled to fill with fixed aspect. some portion of content may be clipped.
    UIViewContentModeRedraw,              // redraw on bounds change (calls -setNeedsDisplay)
    UIViewContentModeCenter,              // contents remain same size. positioned adjusted.
    UIViewContentModeTop,
    UIViewContentModeBottom,
    UIViewContentModeLeft,
    UIViewContentModeRight,
    UIViewContentModeTopLeft,
    UIViewContentModeTopRight,
    UIViewContentModeBottomLeft,
    UIViewContentModeBottomRight,
};
*/

自动布局：

        _imageView.autoresizingMask =
        UIViewAutoresizingFlexibleLeftMargin | UIViewAutoresizingFlexibleTopMargin | UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;

/*
typedef NS_OPTIONS(NSUInteger, UIViewAutoresizing) {
    UIViewAutoresizingNone                 = 0,
    UIViewAutoresizingFlexibleLeftMargin   = 1 << 0,
    UIViewAutoresizingFlexibleWidth        = 1 << 1,
    UIViewAutoresizingFlexibleRightMargin  = 1 << 2,
    UIViewAutoresizingFlexibleTopMargin    = 1 << 3,
    UIViewAutoresizingFlexibleHeight       = 1 << 4,
    UIViewAutoresizingFlexibleBottomMargin = 1 << 5
};
*/

循环播放图片：

    UIImage *image1 = [UIImage imageNamed:@"1"];
    UIImage *image2 = [UIImage imageNamed:@"2"];
    UIImage *image3 = [UIImage imageNamed:@"3"];
    NSArray *imagesArray = @[image1,image2,image3];
    
    _imageView.animationImages = imagesArray;
    _imageView.animationDuration = [imagesArray count];
    //循环次数
    _imageView.animationRepeatCount = 0;
    [_imageView startAnimating];

加载图片：

    UIImage *image1 = [UIImage imageNamed:@"1"];
    UIImage *image2 = [UIImage imageWithContentsOfFile:@"filePath"];
    UIImage *image3 = [UIImage imageWithData:<#(nonnull NSData *)#>];
//image1、image2 会在内存中做缓存，图片多了消耗内存，但同一张图片只缓存一次，可重复利用。
//image3 以数据方式加载，展示网络图片，或者存储数据库展示图片，

实例：选择图片

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    
    _imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 200, 200)];
    [self.view addSubview:_imageView];
    _imageView.backgroundColor = [UIColor redColor];
    //
    
    _imageView.userInteractionEnabled = YES;
    
    UITapGestureRecognizer *tap = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(choosePictrue)];
    //tap.numberOfTapsRequired  = 2;
    [_imageView addGestureRecognizer:tap];
    [tap release];
    [_imageView release];
        
}

- (void)choosePictrue{
//调用相机拍照片或者直接获取相册中的照片
    UIImagePickerController *picc = [[UIImagePickerController alloc] init];
    picc.delegate = self;
    picc.sourceType = UIImagePickerControllerSourceTypePhotoLibrary;
    [self presentViewController:picc animated:YES completion:nil];
    [picc release];
}

- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info{
    _imageView.image = [info objectForKey:UIImagePickerControllerOriginalImage];
    [picker dismissViewControllerAnimated:YES completion:nil];
    
}

- (void)imagePickerControllerDidCancel:(UIImagePickerController *)picker{

    [picker dismissViewControllerAnimated:YES completion:nil];
}
@end

如何理解图片下载？

一般使用懒加载和图片缓存的模式，懒加载就是需要的时候才去临时加载。节省内存空间和用户流量。缓存是把url地址做成md5存在沙盒中以便下次直接使用。

SDWebImage的原理？

一个基于NSOperation/NSInvokeOperation的一个多线程图片缓存框架。每次通过[uiimageview setimageWithUrl:]就启动一个异步线程，负责下载网络图片。一旦下载完成就把网址使用md5做一个校验，然后存在沙盒管理目录中，以便下次直接从沙盒中读取。而不需从网络上下载，节省了流量，加快图片访问速度。

如果界⾯比较卡怎么办

1. 图⽚要⽤用异步下载,并且要缓存起来
2. 对于图⽚的加载到界⾯面上 要复⽤ 为什么要用异步? 异步的主要是为了界⾯面流畅
   http://blog.csdn.net/wave_1102/article/details/49913825