DZNEmptyDataSet 框架阅读

DZNEmptyDataSet

1.代码使用简单：主要逻辑在UIScrollView+EmptyDataSet分类中完成。使用时只需要设置控制器为其数据源和代理，并实现相应的代理方法即可。
2.对runtime合理使用：利用runtime的关联功能实现分类中属性的getter、setter；利用runtime的method 的 IMP指针交互功能 进行reloadData等方法的扩展， 注入了额外的逻辑代码。
3.修改对空白列表占位视图的响应链传递路径。
4.采用NSLayoutConstraint+VFL（Visual Format Language）“可视化格式语言”进行设置约束

// Based on Bryce Buchanan's swizzling technique http://blog.newrelic.com/2014/04/16/right-way-to-swizzle/
// And Juzzin's ideas https://github.com/juzzin/JUSEmptyViewController

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);
    }
}

NSString *dzn_implementationKey(Class class, SEL selector)
{
    if (!class || !selector) {
        return nil;
    }
    
    NSString *className = NSStringFromClass([class class]);
    
    NSString *selectorName = NSStringFromSelector(selector);
    return [NSString stringWithFormat:@"%@_%@",className,selectorName];
}

Class dzn_baseClassToSwizzleForTarget(id target)
{
    if ([target isKindOfClass:[UITableView class]]) {
        return [UITableView class];
    }
    else if ([target isKindOfClass:[UICollectionView class]]) {
        return [UICollectionView class];
    }
    else if ([target isKindOfClass:[UIScrollView class]]) {
        return [UIScrollView class];
    }
    
    return nil;
}

- (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];
}