DZNEmptyDataSet是iOS开发中常用的一个处理UITableView 或者 UICollectionView无数据视图的一个第三方框架,其使用方式可以参考iOS开发-使用DZNEmptyDataSet处理UITableView无数据情况

今天我们来主要来分析其实现思路

首先,设置 emptyDataSetSource 与 emptyDataSetDelegate

tableView.emptyDataSetSource = self;
tableView.emptyDataSetDelegate = self;

我们可以看一下这两个属性的setter实现

#pragma mark - Setters (Public)

- (void)setEmptyDataSetSource:(id<DZNEmptyDataSetSource>)datasource
{
    if (!datasource || ![self dzn_canDisplay]) {
        // 移除无数据视图
        [self dzn_invalidate];
    }
    // 此处为运行时的动态绑定,常见于为Categary添加属性的代码中
    objc_setAssociatedObject(self, kEmptyDataSetSource, [[DZNWeakObjectContainer alloc] initWithWeakObject:datasource], OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    
    // 我们需要注意的地方 
    // We add method sizzling for injecting -dzn_reloadData implementation to the native -reloadData implementation
    [self swizzleIfPossible:@selector(reloadData)];
    
    // Exclusively for UITableView, we also inject -dzn_reloadData to -endUpdates
    if ([self isKindOfClass:[UITableView class]]) {
        [self swizzleIfPossible:@selector(endUpdates)];
    }
}

- (void)setEmptyDataSetDelegate:(id<DZNEmptyDataSetDelegate>)delegate
{
    if (!delegate) {
        // 移除无数据视图
        [self dzn_invalidate];
    }
    // 此处为运行时的动态绑定,常见于为Categary添加属性的代码中
    objc_setAssociatedObject(self, kEmptyDataSetDelegate, [[DZNWeakObjectContainer alloc] initWithWeakObject:delegate], OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

我们需要注意的地方

// We add method sizzling for injecting -dzn_reloadData implementation to the native -reloadData implementation
    [self swizzleIfPossible:@selector(reloadData)];
    
    // Exclusively for UITableView, we also inject -dzn_reloadData to -endUpdates
    if ([self isKindOfClass:[UITableView class]]) {
        [self swizzleIfPossible:@selector(endUpdates)];
    }

先来看下这个方法的实现

- (void)swizzleIfPossible:(SEL)selector
{
    // Check if the target responds to selector
    if (![self respondsToSelector:selector]) {
        return;
    }
    
    // Create the lookup table
    if (!_impLookupTable) {
        _impLookupTable = [[NSMutableDictionary alloc] initWithCapacity:3]; // 3 represent the supported base classes
    }
    
    // We make sure that setImplementation is called once per class kind, UITableView or UICollectionView.
    for (NSDictionary *info in [_impLookupTable allValues]) {
        Class class = [info objectForKey:DZNSwizzleInfoOwnerKey];
        NSString *selectorName = [info objectForKey:DZNSwizzleInfoSelectorKey];
        
        if ([selectorName isEqualToString:NSStringFromSelector(selector)]) {
            if ([self isKindOfClass:class]) {
                return;
            }
        }
    }
    
    Class baseClass = dzn_baseClassToSwizzleForTarget(self);
    NSString *key = dzn_implementationKey(baseClass, selector);
    NSValue *impValue = [[_impLookupTable objectForKey:key] valueForKey:DZNSwizzleInfoPointerKey];
    
    // If the implementation for this class already exist, skip!!
    if (impValue || !key || !baseClass) {
        return;
    }
    
    // Swizzle by injecting additional implementation
    Method method = class_getInstanceMethod(baseClass, selector);
    IMP dzn_newImplementation = method_setImplementation(method, (IMP)dzn_original_implementation);
    
    // Store the new implementation in the lookup table
    NSDictionary *swizzledInfo = @{DZNSwizzleInfoOwnerKey: baseClass,
                                   DZNSwizzleInfoSelectorKey: NSStringFromSelector(selector),
                                   DZNSwizzleInfoPointerKey: [NSValue valueWithPointer:dzn_newImplementation]};
    
    [_impLookupTable setObject:swizzledInfo forKey:key];
}

核心部分

// Swizzle by injecting additional implementation
    Method method = class_getInstanceMethod(baseClass, selector);
    IMP dzn_newImplementation = method_setImplementation(method, (IMP)dzn_original_implementation);

此处使用运行时的方式交换方法的实现,当我们调用

[self swizzleIfPossible:@selector(reloadData)];

之后,实际上已经交换了UITableView的reloadData的实现,当tableview或者collectionview需要调用reloadData时,真正调用的便是我们交换后方法dzn_original_implementation

再进一步看下dzn_original_implementation的实现

void dzn_original_implementation(id self, SEL _cmd)
{
    // Fetch original implementation from lookup table
    Class baseClass = dzn_baseClassToSwizzleForTarget(self);
    NSString *key = dzn_implementationKey(baseClass, _cmd);
    
    NSDictionary *swizzleInfo = [_impLookupTable objectForKey:key];
    NSValue *impValue = [swizzleInfo valueForKey:DZNSwizzleInfoPointerKey];
    
    IMP impPointer = [impValue pointerValue];
    
    // We then inject the additional implementation for reloading the empty dataset
    // Doing it before calling the original implementation does update the 'isEmptyDataSetVisible' flag on time.
    [self dzn_reloadEmptyDataSet];
    
    // If found, call original implementation
    if (impPointer) {
        ((void(*)(id,SEL))impPointer)(self,_cmd);
    }
}

在里面主要执行两个内容,第一个就是加载无数据时视图

[self dzn_reloadEmptyDataSet];

第二个则是调用原本的方法reloadData 或者是 endUpdates

 // If found, call original implementation
    if (impPointer) {
        ((void(*)(id,SEL))impPointer)(self,_cmd);
    }

至此,DZNEmptyDataSet大致流程分析完毕,具体的部分大家可以下载源码去研读🐶