版本记录
| 版本号 | 时间 |
|---|---|
| V1.0 | 2018.02.24 |
前言
我们做APP，文字和图片是绝对不可缺少的元素，特别是图片一般存储在图床里面，一般公司可以委托第三方保存，NB的公司也可以自己存储图片，ios有很多图片加载的第三方框架，其中最优秀的莫过于SDWebImage，它几乎可以满足你所有的需求，用了好几年这个框架，今天想总结一下。感兴趣的可以看其他几篇。
1. SDWebImage探究（一）
2. SDWebImage探究（二）
3. SDWebImage探究（三）
4. SDWebImage探究（四）
5. SDWebImage探究（五）
6. SDWebImage探究（六） —— 图片类型判断深入研究
7. SDWebImage探究（七） —— 深入研究图片下载流程（一）之有关option的位移枚举的说明
8. SDWebImage探究（八） —— 深入研究图片下载流程（二）之开始下载并返回下载结果的总的方法
9. SDWebImage探究（九） —— 深入研究图片下载流程（三）之下载之前的缓存查询操作
10. SDWebImage探究（十） —— 深入研究图片下载流程（四）之查询缓存后的block回调处理
11. SDWebImage探究（十一） —— 深入研究图片下载流程（五）之SDWebImageDownloadToken和操作对象的生成和返回
12. SDWebImage探究（十二） —— 深入研究图片下载流程（六）之下载器到具体下载操作的代理分发实现
13. SDWebImage探究（十三） —— 深入研究图片下载流程（七）之NSURLSession中几个代理的基本用法和关系
SDWebImageDownloaderOperation中下载完成代理方法
下面我们就看一下SDWebImageDownloaderOperation中下载完成的代理方法的调用。
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error;
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
@synchronized(self) {
self.dataTask = nil;
__weak typeof(self) weakSelf = self;
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:weakSelf];
if (!error) {
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadFinishNotification object:weakSelf];
}
});
}
if (error) {
[self callCompletionBlocksWithError:error];
} else {
if ([self callbacksForKey:kCompletedCallbackKey].count > 0) {
/**
* If you specified to use `NSURLCache`, then the response you get here is what you need.
* if you specified to only use cached data via `SDWebImageDownloaderIgnoreCachedResponse`,
* the response data will be nil.
* So we don't need to check the cache option here, since the system will obey the cache option
*/
if (self.imageData) {
UIImage *image = [UIImage sd_imageWithData:self.imageData];
NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
image = [self scaledImageForKey:key image:image];
// Do not force decoding animated GIFs
if (!image.images) {
if (self.shouldDecompressImages) {
if (self.options & SDWebImageDownloaderScaleDownLargeImages) {
#if SD_UIKIT || SD_WATCH
image = [UIImage decodedAndScaledDownImageWithImage:image];
[self.imageData setData:UIImagePNGRepresentation(image)];
#endif
} else {
image = [UIImage decodedImageWithImage:image];
}
}
}
if (CGSizeEqualToSize(image.size, CGSizeZero)) {
[self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}]];
} else {
[self callCompletionBlocksWithImage:image imageData:self.imageData error:nil finished:YES];
}
} else {
[self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Image data is nil"}]];
}
}
}
[self done];
}
在这里代理方法中，主要做了如下工作：
- 在主线程发送下载停止
SDWebImageDownloadStopNotification和下载结束SDWebImageDownloadFinishNotification的通知。 - 进行错误error处理。
- 有错误，直接回调
completedBlock(image, imageData, error, finished)。 - 没错误，进行图像数据的相关解压缩处理。
- 有错误，直接回调
- 下载方法完成后的结束处理，调用
done自定义方法，初始化一些参数。
接下来我们就详细的看一下这个下载结束的处理过程。
发送通知
这里发送的通知就是在主线程发送下载结束的通知，并且在没有错误的情况下，再发一个下载完成的通知。
NSString *const SDWebImageDownloadStopNotification = @"SDWebImageDownloadStopNotification";
NSString *const SDWebImageDownloadFinishNotification = @"SDWebImageDownloadFinishNotification";
@synchronized(self) {
self.dataTask = nil;
__weak typeof(self) weakSelf = self;
dispatch_async(dispatch_get_main_queue(), ^{
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadStopNotification object:weakSelf];
if (!error) {
[[NSNotificationCenter defaultCenter] postNotificationName:SDWebImageDownloadFinishNotification object:weakSelf];
}
});
}
这里：
通知
SDWebImageDownloadStopNotification的作用是停止下载动画指示动画。通知
SDWebImageDownloadFinishNotification我进行了全局搜索，并没有找到add的那个方法，可能是留给用户的一个接口吧，可能是这样吧。
错误处理
1. 有错误
if (error) {
[self callCompletionBlocksWithError:error];
}
- (void)callCompletionBlocksWithError:(nullable NSError *)error {
[self callCompletionBlocksWithImage:nil imageData:nil error:error finished:YES];
}
- (void)callCompletionBlocksWithImage:(nullable UIImage *)image
imageData:(nullable NSData *)imageData
error:(nullable NSError *)error
finished:(BOOL)finished {
NSArray<id> *completionBlocks = [self callbacksForKey:kCompletedCallbackKey];
dispatch_main_async_safe(^{
for (SDWebImageDownloaderCompletedBlock completedBlock in completionBlocks) {
completedBlock(image, imageData, error, finished);
}
});
}
最后还是直接调用completedBlock(image, imageData, error, finished)进行了回调。
2. 无错误
没有错误这里处理的东西就比较多了，涉及到几种情况的几种判断，下面我们就一起看一下else里面的东西。
static NSString *const kCompletedCallbackKey = @"completed";
else {
if ([self callbacksForKey:kCompletedCallbackKey].count > 0) {
/**
* If you specified to use `NSURLCache`, then the response you get here is what you need.
* if you specified to only use cached data via `SDWebImageDownloaderIgnoreCachedResponse`,
* the response data will be nil.
* So we don't need to check the cache option here, since the system will obey the cache option
*/
if (self.imageData) {
UIImage *image = [UIImage sd_imageWithData:self.imageData];
NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
image = [self scaledImageForKey:key image:image];
// Do not force decoding animated GIFs
if (!image.images) {
if (self.shouldDecompressImages) {
if (self.options & SDWebImageDownloaderScaleDownLargeImages) {
#if SD_UIKIT || SD_WATCH
image = [UIImage decodedAndScaledDownImageWithImage:image];
[self.imageData setData:UIImagePNGRepresentation(image)];
#endif
} else {
image = [UIImage decodedImageWithImage:image];
}
}
}
if (CGSizeEqualToSize(image.size, CGSizeZero)) {
[self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}]];
} else {
[self callCompletionBlocksWithImage:image imageData:self.imageData error:nil finished:YES];
}
} else {
[self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Image data is nil"}]];
}
}
}
(a) 获取回调数组的大小
大家可以看到，else里面的处理都是在数组中元素个数大于0[self callbacksForKey:kCompletedCallbackKey].count > 0下进行的。
@property (strong, nonatomic, nonnull) NSMutableArray<SDCallbacksDictionary *> *callbackBlocks;
- (nullable NSArray<id> *)callbacksForKey:(NSString *)key {
__block NSMutableArray<id> *callbacks = nil;
dispatch_sync(self.barrierQueue, ^{
// We need to remove [NSNull null] because there might not always be a progress block for each callback
callbacks = [[self.callbackBlocks valueForKey:key] mutableCopy];
[callbacks removeObjectIdenticalTo:[NSNull null]];
});
return [callbacks copy]; // strip mutability here
}
这里移除了数组中的数组中的对象[NSNull null]。
(b) 存在图像数据imageData
如果你指定使用NSURLCache，那么你得到的响应就是你需要的。 如果您指定仅通过SDWebImageDownloaderIgnoreCachedResponse使用缓存数据，则响应数据将为nil。 所以我们不需要在这里检查缓存选项，因为系统将服从缓存选项cache option。
- 首先对图像进行转化和处理
UIImage *image = [UIImage sd_imageWithData:self.imageData];
NSString *key = [[SDWebImageManager sharedManager] cacheKeyForURL:self.request.URL];
image = [self scaledImageForKey:key image:image];
这里大家可以看到，首先做的就是将NSData数据类型转化为UIImage对象，使用的就是UIImage分类的一个方法sd_imageWithData:。
+ (nullable UIImage *)sd_imageWithData:(nullable NSData *)data {
if (!data) {
return nil;
}
UIImage *image;
SDImageFormat imageFormat = [NSData sd_imageFormatForImageData:data];
if (imageFormat == SDImageFormatGIF) {
image = [UIImage sd_animatedGIFWithData:data];
}
#ifdef SD_WEBP
else if (imageFormat == SDImageFormatWebP)
{
image = [UIImage sd_imageWithWebPData:data];
}
#endif
else {
image = [[UIImage alloc] initWithData:data];
#if SD_UIKIT || SD_WATCH
UIImageOrientation orientation = [self sd_imageOrientationFromImageData:data];
if (orientation != UIImageOrientationUp) {
image = [UIImage imageWithCGImage:image.CGImage
scale:image.scale
orientation:orientation];
}
#endif
}
return image;
}
+ (UIImage *)sd_animatedGIFWithData:(NSData *)data {
if (!data) {
return nil;
}
#if SD_MAC
return [[UIImage alloc] initWithData:data];
#endif
CGImageSourceRef source = CGImageSourceCreateWithData((__bridge CFDataRef)data, NULL);
size_t count = CGImageSourceGetCount(source);
UIImage *staticImage;
if (count <= 1) {
staticImage = [[UIImage alloc] initWithData:data];
} else {
// we will only retrieve the 1st frame. the full GIF support is available via the FLAnimatedImageView category.
// this here is only code to allow drawing animated images as static ones
#if SD_WATCH
CGFloat scale = 1;
scale = [WKInterfaceDevice currentDevice].screenScale;
#elif SD_UIKIT
CGFloat scale = 1;
scale = [UIScreen mainScreen].scale;
#endif
CGImageRef CGImage = CGImageSourceCreateImageAtIndex(source, 0, NULL);
#if SD_UIKIT || SD_WATCH
UIImage *frameImage = [UIImage imageWithCGImage:CGImage scale:scale orientation:UIImageOrientationUp];
staticImage = [UIImage animatedImageWithImages:@[frameImage] duration:0.0f];
#endif
CGImageRelease(CGImage);
}
CFRelease(source);
return staticImage;
}
这里需要注意的是，首先判断了数据格式，对于GIF格式的做了特殊的处理，根据sd_animatedGIFWithData:方法返回对应的image。
接下来就是对图像进行缩放，利用方法scaledImageForKey: image:。
- (nullable UIImage *)scaledImageForKey:(nullable NSString *)key image:(nullable UIImage *)image {
return SDScaledImageForKey(key, image);
}
inline UIImage *SDScaledImageForKey(NSString * _Nullable key, UIImage * _Nullable image) {
if (!image) {
return nil;
}
#if SD_MAC
return image;
#elif SD_UIKIT || SD_WATCH
if ((image.images).count > 0) {
NSMutableArray<UIImage *> *scaledImages = [NSMutableArray array];
for (UIImage *tempImage in image.images) {
[scaledImages addObject:SDScaledImageForKey(key, tempImage)];
}
UIImage *animatedImage = [UIImage animatedImageWithImages:scaledImages duration:image.duration];
#ifdef SD_WEBP
if (animatedImage) {
SEL sd_webpLoopCount = NSSelectorFromString(@"sd_webpLoopCount");
NSNumber *value = objc_getAssociatedObject(image, sd_webpLoopCount);
NSInteger loopCount = value.integerValue;
if (loopCount) {
objc_setAssociatedObject(animatedImage, sd_webpLoopCount, @(loopCount), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
}
#endif
return animatedImage;
} else {
#if SD_WATCH
if ([[WKInterfaceDevice currentDevice] respondsToSelector:@selector(screenScale)]) {
#elif SD_UIKIT
if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)]) {
#endif
CGFloat scale = 1;
if (key.length >= 8) {
NSRange range = [key rangeOfString:@"@2x."];
if (range.location != NSNotFound) {
scale = 2.0;
}
range = [key rangeOfString:@"@3x."];
if (range.location != NSNotFound) {
scale = 3.0;
}
}
UIImage *scaledImage = [[UIImage alloc] initWithCGImage:image.CGImage scale:scale orientation:image.imageOrientation];
image = scaledImage;
}
return image;
}
#endif
}
- 对于非GIF图像的特殊处理
// Do not force decoding animated GIFs
if (!image.images) {
if (self.shouldDecompressImages) {
if (self.options & SDWebImageDownloaderScaleDownLargeImages) {
#if SD_UIKIT || SD_WATCH
image = [UIImage decodedAndScaledDownImageWithImage:image];
[self.imageData setData:UIImagePNGRepresentation(image)];
#endif
} else {
image = [UIImage decodedImageWithImage:image];
}
}
}
这里是对非GIF图片的特殊处理，首先判断如果需要进行图像解压缩self.shouldDecompressImages，然后判断options是否是SDWebImageDownloaderScaleDownLargeImages。如果是SDWebImageDownloaderScaleDownLargeImages，那么就对图像进行解压缩以及缩小等处理，使用方法image = [UIImage decodedAndScaledDownImageWithImage:image。
+ (nullable UIImage *)decodedAndScaledDownImageWithImage:(nullable UIImage *)image {
if (![UIImage shouldDecodeImage:image]) {
return image;
}
if (![UIImage shouldScaleDownImage:image]) {
return [UIImage decodedImageWithImage:image];
}
CGContextRef destContext;
// autorelease the bitmap context and all vars to help system to free memory when there are memory warning.
// on iOS7, do not forget to call [[SDImageCache sharedImageCache] clearMemory];
@autoreleasepool {
CGImageRef sourceImageRef = image.CGImage;
CGSize sourceResolution = CGSizeZero;
sourceResolution.width = CGImageGetWidth(sourceImageRef);
sourceResolution.height = CGImageGetHeight(sourceImageRef);
float sourceTotalPixels = sourceResolution.width * sourceResolution.height;
// Determine the scale ratio to apply to the input image
// that results in an output image of the defined size.
// see kDestImageSizeMB, and how it relates to destTotalPixels.
float imageScale = kDestTotalPixels / sourceTotalPixels;
CGSize destResolution = CGSizeZero;
destResolution.width = (int)(sourceResolution.width*imageScale);
destResolution.height = (int)(sourceResolution.height*imageScale);
// current color space
CGColorSpaceRef colorspaceRef = [UIImage colorSpaceForImageRef:sourceImageRef];
size_t bytesPerRow = kBytesPerPixel * destResolution.width;
// Allocate enough pixel data to hold the output image.
void* destBitmapData = malloc( bytesPerRow * destResolution.height );
if (destBitmapData == NULL) {
return image;
}
// kCGImageAlphaNone is not supported in CGBitmapContextCreate.
// Since the original image here has no alpha info, use kCGImageAlphaNoneSkipLast
// to create bitmap graphics contexts without alpha info.
destContext = CGBitmapContextCreate(destBitmapData,
destResolution.width,
destResolution.height,
kBitsPerComponent,
bytesPerRow,
colorspaceRef,
kCGBitmapByteOrderDefault|kCGImageAlphaNoneSkipLast);
if (destContext == NULL) {
free(destBitmapData);
return image;
}
CGContextSetInterpolationQuality(destContext, kCGInterpolationHigh);
// Now define the size of the rectangle to be used for the
// incremental blits from the input image to the output image.
// we use a source tile width equal to the width of the source
// image due to the way that iOS retrieves image data from disk.
// iOS must decode an image from disk in full width 'bands', even
// if current graphics context is clipped to a subrect within that
// band. Therefore we fully utilize all of the pixel data that results
// from a decoding opertion by achnoring our tile size to the full
// width of the input image.
CGRect sourceTile = CGRectZero;
sourceTile.size.width = sourceResolution.width;
// The source tile height is dynamic. Since we specified the size
// of the source tile in MB, see how many rows of pixels high it
// can be given the input image width.
sourceTile.size.height = (int)(kTileTotalPixels / sourceTile.size.width );
sourceTile.origin.x = 0.0f;
// The output tile is the same proportions as the input tile, but
// scaled to image scale.
CGRect destTile;
destTile.size.width = destResolution.width;
destTile.size.height = sourceTile.size.height * imageScale;
destTile.origin.x = 0.0f;
// The source seem overlap is proportionate to the destination seem overlap.
// this is the amount of pixels to overlap each tile as we assemble the ouput image.
float sourceSeemOverlap = (int)((kDestSeemOverlap/destResolution.height)*sourceResolution.height);
CGImageRef sourceTileImageRef;
// calculate the number of read/write operations required to assemble the
// output image.
int iterations = (int)( sourceResolution.height / sourceTile.size.height );
// If tile height doesn't divide the image height evenly, add another iteration
// to account for the remaining pixels.
int remainder = (int)sourceResolution.height % (int)sourceTile.size.height;
if(remainder) {
iterations++;
}
// Add seem overlaps to the tiles, but save the original tile height for y coordinate calculations.
float sourceTileHeightMinusOverlap = sourceTile.size.height;
sourceTile.size.height += sourceSeemOverlap;
destTile.size.height += kDestSeemOverlap;
for( int y = 0; y < iterations; ++y ) {
@autoreleasepool {
sourceTile.origin.y = y * sourceTileHeightMinusOverlap + sourceSeemOverlap;
destTile.origin.y = destResolution.height - (( y + 1 ) * sourceTileHeightMinusOverlap * imageScale + kDestSeemOverlap);
sourceTileImageRef = CGImageCreateWithImageInRect( sourceImageRef, sourceTile );
if( y == iterations - 1 && remainder ) {
float dify = destTile.size.height;
destTile.size.height = CGImageGetHeight( sourceTileImageRef ) * imageScale;
dify -= destTile.size.height;
destTile.origin.y += dify;
}
CGContextDrawImage( destContext, destTile, sourceTileImageRef );
CGImageRelease( sourceTileImageRef );
}
}
CGImageRef destImageRef = CGBitmapContextCreateImage(destContext);
CGContextRelease(destContext);
if (destImageRef == NULL) {
return image;
}
UIImage *destImage = [UIImage imageWithCGImage:destImageRef scale:image.scale orientation:image.imageOrientation];
CGImageRelease(destImageRef);
if (destImage == nil) {
return image;
}
return destImage;
}
}
+ (BOOL)shouldDecodeImage:(nullable UIImage *)image {
// Prevent "CGBitmapContextCreateImage: invalid context 0x0" error
if (image == nil) {
return NO;
}
// do not decode animated images
if (image.images != nil) {
return NO;
}
CGImageRef imageRef = image.CGImage;
CGImageAlphaInfo alpha = CGImageGetAlphaInfo(imageRef);
BOOL anyAlpha = (alpha == kCGImageAlphaFirst ||
alpha == kCGImageAlphaLast ||
alpha == kCGImageAlphaPremultipliedFirst ||
alpha == kCGImageAlphaPremultipliedLast);
// do not decode images with alpha
if (anyAlpha) {
return NO;
}
return YES;
}
+ (BOOL)shouldScaleDownImage:(nonnull UIImage *)image {
BOOL shouldScaleDown = YES;
CGImageRef sourceImageRef = image.CGImage;
CGSize sourceResolution = CGSizeZero;
sourceResolution.width = CGImageGetWidth(sourceImageRef);
sourceResolution.height = CGImageGetHeight(sourceImageRef);
float sourceTotalPixels = sourceResolution.width * sourceResolution.height;
float imageScale = kDestTotalPixels / sourceTotalPixels;
if (imageScale < 1) {
shouldScaleDown = YES;
} else {
shouldScaleDown = NO;
}
return shouldScaleDown;
}
并将处理后的图像数据赋值给self.imageData。
如果不是SDWebImageDownloaderScaleDownLargeImages，那么就只进行解压缩，不进行大小的处理。
+ (nullable UIImage *)decodedImageWithImage:(nullable UIImage *)image {
if (![UIImage shouldDecodeImage:image]) {
return image;
}
// autorelease the bitmap context and all vars to help system to free memory when there are memory warning.
// on iOS7, do not forget to call [[SDImageCache sharedImageCache] clearMemory];
@autoreleasepool{
CGImageRef imageRef = image.CGImage;
CGColorSpaceRef colorspaceRef = [UIImage colorSpaceForImageRef:imageRef];
size_t width = CGImageGetWidth(imageRef);
size_t height = CGImageGetHeight(imageRef);
size_t bytesPerRow = kBytesPerPixel * width;
// kCGImageAlphaNone is not supported in CGBitmapContextCreate.
// Since the original image here has no alpha info, use kCGImageAlphaNoneSkipLast
// to create bitmap graphics contexts without alpha info.
CGContextRef context = CGBitmapContextCreate(NULL,
width,
height,
kBitsPerComponent,
bytesPerRow,
colorspaceRef,
kCGBitmapByteOrderDefault|kCGImageAlphaNoneSkipLast);
if (context == NULL) {
return image;
}
// Draw the image into the context and retrieve the new bitmap image without alpha
CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
CGImageRef imageRefWithoutAlpha = CGBitmapContextCreateImage(context);
UIImage *imageWithoutAlpha = [UIImage imageWithCGImage:imageRefWithoutAlpha
scale:image.scale
orientation:image.imageOrientation];
CGContextRelease(context);
CGImageRelease(imageRefWithoutAlpha);
return imageWithoutAlpha;
}
}
- 对图像尺寸进行判断并回调
if (CGSizeEqualToSize(image.size, CGSizeZero)) {
[self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}]];
}
else {
[self callCompletionBlocksWithImage:image imageData:self.imageData error:nil finished:YES];
}
这里进行了判断，如果图像尺寸为0，那么就会调用callCompletionBlocksWithError:方法进行回调并给出错误信息。如果图像的尺寸不为0，那么就会调用[self callCompletionBlocksWithImage:image imageData:self.imageData error:nil finished:YES];方法，同样进行了回调。
(c) 不存在图像数据imageData
如果不存在数据，那么就调用完成包含错误域和错误信息的方法。这个掉用的方法和error有错误的时候调用的方法是一样的，调用方式如下：
[self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Image data is nil"}]];
下载完成的处理
下面要做的就是下载完成代理方法调用最后要做的处理[self done];，调用自定义的方法done。
- (void)done {
self.finished = YES;
self.executing = NO;
[self reset];
}
- (void)reset {
dispatch_barrier_async(self.barrierQueue, ^{
[self.callbackBlocks removeAllObjects];
});
self.dataTask = nil;
self.imageData = nil;
if (self.ownedSession) {
[self.ownedSession invalidateAndCancel];
self.ownedSession = nil;
}
}
上面就是最后做的处理，将相关的状态重置为初始状态，看一眼代码就很清晰了，不多说了。
后记
这一篇主要是对
NSURLSessionTaskDelegate代理方法中的方法- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error的作用进行详细的解析。希望大家能喜欢，后面会对另外的几个实现的代理方法进行说明和解析。
