[TOC]
本文为笔者在阅读 SDWebImage 时的笔记,帮助自己加深印象,让学习一步一个脚印,也希望能帮到其他人。
学习 SDWebImage 就不得不看官方提供的流程图和 UML 图:
阅读源码之前,首先通过对源文件进行分组来查看源码的文件结构,分组方式1(SDWebImage 源码解析):
分组方式2:
或参考以下 Workflow(搬好小板凳看SDWebImage源码解析(一)):
再来看一个框架的调用流程图(SDWebImage源码解析):
UIImageView +WebCache
暴露了很多调用灵活的接口,其最终都会调用到 UIView+WebCache
分类的如下方法中:
- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
operationKey:(nullable NSString *)operationKey
setImageBlock:(nullable SDSetImageBlock)setImageBlock
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDExternalCompletionBlock)completedBlock;
因为 SDWebImage 框架也支持 UIButton 的下载图片等方法,所以需要在它们的父类 UIView
里面统一一个下载方法(SDWebImage源码解析)。
该方法里执行了如下操作:
- 先取消正在进行的operation
- options != SDWebImageDelayPlaceholder 就先展示占位图
- url存在:
- 创建加载指示器菊花
- 获取manager
- 调用manager的loadImage:::方法,并返回operation,loadImage:::结束的回调里执行如下步骤:
- 停掉加载指示器(菊花)
- 根据需要标记为progress的完成状态
- 不需要设置图片的情况:
- 如果需要调用completedBlock回调,就在主线程中执行completedBlock回调
- 需要设置图片的情况:
- 图片存在,且没有设置SDWebImageAvoidAutoSetImage:targetImage = image, targetData = data;
- 图片不存在,且设置了延迟展示占位图:targetImage = placeholder, targetData = nil;
- 加载结束 且 (options为图片强制转场 或 cacheType为None),就设置transition = sself.sd_imageTransition;
- 回到主线程中设置图片:即调用sd_setImage:imageData:basedOnClassOrViaCustomSetImageBlock:transition:cacheType:imageURL:方法,如果group存在,则设置图片和完成回调的的操作需要加入group,不存在group则直接调用设置图片的方法,最后主线程调用回调block
- load操作结束后,为UIView绑定新的operation,因为之前把UIView上的操作取消了
- url不存在:
- 停掉加载指示器(菊花)
- 调用completedBlock回调,在其中返回错误信息
/*
先取消正在进行的operation
options != SDWebImageDelayPlaceholder 就先展示占位图
url存在:
创建加载指示器菊花
获取manager
调用manager的loadImage:::方法,并返回operation,loadImage:::结束的回调里执行如下步骤:
停掉加载指示器(菊花)
根据需要标记为progress的完成状态
不需要设置图片的情况:
如果需要调用completedBlock回调,就在主线程中执行completedBlock回调
需要设置图片的情况:
图片存在,且没有设置SDWebImageAvoidAutoSetImage:targetImage = image, targetData = data;
图片不存在,且设置了延迟展示占位图:targetImage = placeholder, targetData = nil;
加载结束 且 (options为图片强制转场 或 cacheType为None),就设置transition = sself.sd_imageTransition;
回到主线程中设置图片:即调用sd_setImage:imageData:basedOnClassOrViaCustomSetImageBlock:transition:cacheType:imageURL:方法,如果group存在,则设置图片和完成回调的的操作需要加入group,不存在group则直接调用设置图片的方法,最后主线程调用回调block
load操作结束后,为UIView绑定新的operation,因为之前把UIView上的操作取消了
url不存在:
停掉加载指示器(菊花)
调用completedBlock回调,在其中返回错误信息
*/
- (void)sd_internalSetImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
operationKey:(nullable NSString *)operationKey
setImageBlock:(nullable SDSetImageBlock)setImageBlock
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDExternalCompletionBlock)completedBlock
context:(nullable NSDictionary<NSString *, id> *)context {
//valid key:UIImageView || UIButton || NSButton || UIView
NSString *validOperationKey = operationKey ?: NSStringFromClass([self class]);
//下面这行代码是保证没有当前正在进行的异步下载操作, 使它不会与即将进行的操作发生冲突(取消UIImageView(UIView)上当前正在进行的异步下载,确保每个 UIImageView 对象中永远只存在一个 operation,当前只允许一个图片网络请求,该 operation 负责从缓存中获取 image 或者是重新下载 image)
[self sd_cancelImageLoadOperationWithKey:validOperationKey];
//用imageURLKey做key, 将url作为属性绑定到UIView上(类似于NSDictionary,其中 static char imageURLKey;)
objc_setAssociatedObject(self, &imageURLKey, url, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
//“&”在这里是位运算符,if括号里面的运算表示:options的值不是“延迟占位图展示”的话就成立
if (!(options & SDWebImageDelayPlaceholder)) {
if ([context valueForKey:SDWebImageInternalSetImageGroupKey]) {
dispatch_group_t group = [context valueForKey:SDWebImageInternalSetImageGroupKey];
dispatch_group_enter(group);
}
//dispatch_main_async_safe是一个宏,保证在主线程安全执行
dispatch_main_async_safe(^{
//先展示占位图
[self sd_setImage:placeholder imageData:nil basedOnClassOrViaCustomSetImageBlock:setImageBlock];
});
}
if (url) {
#if SD_UIKIT
// check if activityView is enabled or not
//设置了activityView(正在加载指示器)为显示的话,那么就调用显示activityView的方法...
if ([self sd_showActivityIndicatorView]) {
//去创建菊花(activityView)然后转起来...
[self sd_addActivityIndicator];
}
#endif
// reset the progress
self.sd_imageProgress.totalUnitCount = 0;
self.sd_imageProgress.completedUnitCount = 0;
//获得manager
SDWebImageManager *manager;
if ([context valueForKey:SDWebImageExternalCustomManagerKey]) {
manager = (SDWebImageManager *)[context valueForKey:SDWebImageExternalCustomManagerKey];
} else {
manager = [SDWebImageManager sharedManager];
}
__weak __typeof(self)wself = self;
//创建了一个block,会作为参数传入manager的loadImageWithURL:::方法
SDWebImageDownloaderProgressBlock combinedProgressBlock = ^(NSInteger receivedSize, NSInteger expectedSize, NSURL * _Nullable targetURL) {
wself.sd_imageProgress.totalUnitCount = expectedSize;
wself.sd_imageProgress.completedUnitCount = receivedSize;
if (progressBlock) {
progressBlock(receivedSize, expectedSize, targetURL);
}
};
//load操作开始,加载图片(从缓存中取或下载),并返回operation对象
id <SDWebImageOperation> operation = [manager loadImageWithURL:url options:options progress:combinedProgressBlock completed:^(UIImage *image, NSData *data, NSError *error, SDImageCacheType cacheType, BOOL finished, NSURL *imageURL) {
__strong __typeof (wself) sself = wself;
//如果 self被提前释放了,直接返回
if (!sself) { return; }
#if SD_UIKIT
//先把菊花停掉...
[sself sd_removeActivityIndicator];
#endif
// if the progress not been updated, mark it to complete state
//加载结束 且 没有错误 且 progress的值没发生变化,说明没有更新progress,就把progress标记为完成状态
if (finished && !error && sself.sd_imageProgress.totalUnitCount == 0 && sself.sd_imageProgress.completedUnitCount == 0) {
sself.sd_imageProgress.totalUnitCount = SDWebImageProgressUnitCountUnknown;
sself.sd_imageProgress.completedUnitCount = SDWebImageProgressUnitCountUnknown;
}
//加载结束了 或者 options为不自动设置图片,标记应该调用完成回调
BOOL shouldCallCompletedBlock = finished || (options & SDWebImageAvoidAutoSetImage);
//(图片存在 但 options为不自动设置图片) 或者 (图片不存在 且 options为延迟设置占位图),标记不设置图片
BOOL shouldNotSetImage = ((image && (options & SDWebImageAvoidAutoSetImage)) ||
(!image && !(options & SDWebImageDelayPlaceholder)));
SDWebImageNoParamsBlock callCompletedBlockClojure = ^{
//如果 self被提前释放了,直接返回
if (!sself) { return; }
//需要直接设置图片的话,直接调用setNeedsLayout
if (!shouldNotSetImage) {
[sself sd_setNeedsLayout];
}
//执行completedBlock回调,并将图片用block带回去
if (completedBlock && shouldCallCompletedBlock) {
completedBlock(image, error, cacheType, url);
}
};
// case 1a: we got an image, but the SDWebImageAvoidAutoSetImage flag is set
// OR
// case 1b: we got no image and the SDWebImageDelayPlaceholder is not set
if (shouldNotSetImage) {
//如果不需要设置图片,则在主线程中执行上面的 callCompletedBlockClojure block
dispatch_main_async_safe(callCompletedBlockClojure);
return;
}
//以下均为需要设置图片的情况
UIImage *targetImage = nil;
NSData *targetData = nil;
if (image) { //图片存在,且没有设置SDWebImageAvoidAutoSetImage(因为图片存在 且 设置了SDWebImageAvoidAutoSetImage的情况已经在上面return掉了)
// case 2a: we got an image and the SDWebImageAvoidAutoSetImage is not set
targetImage = image;
targetData = data;
} else if (options & SDWebImageDelayPlaceholder) { //图片不存在,且设置了延迟展示占位图
// case 2b: we got no image and the SDWebImageDelayPlaceholder flag is set
targetImage = placeholder;
targetData = nil;
}
#if SD_UIKIT || SD_MAC
// check whether we should use the image transition
SDWebImageTransition *transition = nil;
//加载结束 且 (options为图片强制转场 或 cacheType为None)
if (finished && (options & SDWebImageForceTransition || cacheType == SDImageCacheTypeNone)) {
transition = sself.sd_imageTransition;
}
#endif
if ([context valueForKey:SDWebImageInternalSetImageGroupKey]) {
//如果group存在,则使用group
dispatch_group_t group = [context valueForKey:SDWebImageInternalSetImageGroupKey];
dispatch_group_enter(group);
dispatch_main_async_safe(^{
#if SD_UIKIT || SD_MAC
[sself sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock transition:transition cacheType:cacheType imageURL:imageURL];
#else
[sself sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock];
#endif
});
// ensure completion block is called after custom setImage process finish
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
callCompletedBlockClojure();
});
} else {
//group不存在,直接回到主线程中设置图片
dispatch_main_async_safe(^{
#if SD_UIKIT || SD_MAC
[sself sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock transition:transition cacheType:cacheType imageURL:imageURL];
#else
[sself sd_setImage:targetImage imageData:targetData basedOnClassOrViaCustomSetImageBlock:setImageBlock];
#endif
callCompletedBlockClojure();
});
}
}];
//load操作结束后,为UIView绑定新的操作,因为之前把UIView上的操作取消了。
[self sd_setImageLoadOperation:operation forKey:validOperationKey];
} else {
//url不存在的情况
dispatch_main_async_safe(^{
#if SD_UIKIT
//把菊花停掉...
[self sd_removeActivityIndicator];
#endif
//执行completedBlock回调,并带入url为空的错误提示
if (completedBlock) {
NSError *error = [NSError errorWithDomain:SDWebImageErrorDomain code:-1 userInfo:@{NSLocalizedDescriptionKey : @"Trying to load a nil url"}];
completedBlock(nil, error, SDImageCacheTypeNone, url);
}
});
}
}
在 sd_internalSetImageWithURL
的一开始执行了一个取消操作 [self sd_cancelImageLoadOperationWithKey:validOperationKey];
,取消 UIImageView(UIView) 上当前正在进行的异步下载,确保每个 UIImageView 对象中永远只存在一个 operation,当前只允许一个图片网络请求,该 operation 负责从缓存中获取 image 或者是重新下载 image。sd_cancelImageLoadOperationWithKey
的方法实现位于 UIView+WebCacheOperation
, UIView+WebCacheOperation.m
中的内容如下:
#import "UIView+WebCacheOperation.h"
#import "objc/runtime.h"
static char loadOperationKey;
// key is copy, value is weak because operation instance is retained by SDWebImageManager's runningOperations property
// we should use lock to keep thread-safe because these method may not be acessed from main queue
typedef NSMapTable<NSString *, id<SDWebImageOperation>> SDOperationsDictionary;
//这些方法用来取消 UIView 的图像加载,它们是内部使用的,而不是公开的。所有这些存储型operations是weak的,所以,图像加载完毕后他们就会销毁,如果你需要存储这些operations,请使用你自己的类强引用他们
//可以这么理解,UIView有个NSMapTable类型的属性operations,它通过键值的方式来保存opration
@implementation UIView (WebCacheOperation)
//相当于通过Associate为UIView添加了一个NSMapTable类型的属性
- (SDOperationsDictionary *)sd_operationDictionary {
@synchronized(self) {
//operations的真实类型是NSMapTable
SDOperationsDictionary *operations = objc_getAssociatedObject(self, &loadOperationKey);
if (operations) {
return operations;
}
//operations的key是强引用,value是弱引用
operations = [[NSMapTable alloc] initWithKeyOptions:NSPointerFunctionsStrongMemory valueOptions:NSPointerFunctionsWeakMemory capacity:0];
//通过loadOperationKey为UIView绑定一个operations(类似于NSDictionary)
objc_setAssociatedObject(self, &loadOperationKey, operations, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
return operations;
}
}
//存储图像加载的operation(该operation通过一个weak的 map table 存储在UIView中)
- (void)sd_setImageLoadOperation:(nullable id<SDWebImageOperation>)operation forKey:(nullable NSString *)key {
if (key) {
//先取消
[self sd_cancelImageLoadOperationWithKey:key];
if (operation) {
//获取UIView通过Associate添加的NSMapTable类型的绑定对象operationDictionary
SDOperationsDictionary *operationDictionary = [self sd_operationDictionary];
@synchronized (self) {
//将operation通过key添加到UIView的绑定对象operationDictionary里
[operationDictionary setObject:operation forKey:key];
}
}
}
}
//取消UIView对应key的所有operations
- (void)sd_cancelImageLoadOperationWithKey:(nullable NSString *)key {
// Cancel in progress downloader from queue
//相当于通过Associate获得UIView的一个叫sd_operationDictionary的属性,其真实类型是NSMapTable,key是强引用,value是弱引用
SDOperationsDictionary *operationDictionary = [self sd_operationDictionary];
id<SDWebImageOperation> operation; //operation是一个遵守了SDWebImageOperation协议的对象
@synchronized (self) {
//operation是通过key存储在UIView的一个NSMapTable里
operation = [operationDictionary objectForKey:key];
}
if (operation) {
if ([operation conformsToProtocol:@protocol(SDWebImageOperation)]){
//先取消
[operation cancel];
}
@synchronized (self) {
//再移除
[operationDictionary removeObjectForKey:key];
}
}
}
//根据当前的UIVIew和key移除operations,而不取消
- (void)sd_removeImageLoadOperationWithKey:(nullable NSString *)key {
if (key) {
SDOperationsDictionary *operationDictionary = [self sd_operationDictionary];
@synchronized (self) {
[operationDictionary removeObjectForKey:key];
}
}
}
@end
/*
url的类型检查
创建一个SDWebImageCombinedOperation类型的operation
如果url不合法,或失败过且失败了也不重试,则执行一个url不合法的Completion回调
(接下来为url合法的情况)
将operation加入self.runningOperations
去缓存中查找图片,且返回一个NSOperation对象,保存在SDWebImageCombinedOperation的实例operation的cacheOperation属性里
(接下来为查找结果的回调)
需要从网络下载图片:
执行回调callCompletionBlockForOperation
从网络下载图片,返回一个SDWebImageDownloadToken类型的对象,保存在SDWebImageCombinedOperation的实例operation的downloadToken属性里
(下载结束的回调)
operation被提前释放了,或者operation的任务被取消了,什么都不做
(否则,下载出错了)
执行回调callCompletionBlockForOperation,传入错误信息
判断是否要把失败的url存入self.failedURLs
(其他情况)
如果options是失败后重试,则把url从self.failedURLs移除掉
... ...
不需要从网络下载图片,且在缓存中找到了图片,执行回调callCompletionBlockForOperation
不需要从网络下载图片,且没有在缓存中找到图片,执行回调callCompletionBlockForOperation
返回一个保存了查询队列NSOperation的SDWebImageCombinedOperation实例operation
*/