DZNEmptyDataSet的理解笔记

对第三方框架DZNEmptyDataSet中的UIScrollView+EmptyDataSet的解读
一, UIScrollView+EmptyDataSet.h文件提供了DZNEmptyDataSetSource和DZNEmptyDataSetDelegate协议中大量的方法，还提供了三个属性和一个公有的方法：
emptyDataSetSource；emptyDataSetDelegate ；emptyDataSetVisible；-（void）reloadEmptyDataSet（只重新加载empty的数据）
二，UIScrollView+EmptyDataSet.m文件中提供了三个类的实现

1, UIView+DZNConstraintBasedLayoutExtensions
只提供了以下一个方法; 用来实现约束

- (NSLayoutConstraint *)equallyRelatedConstraintWithView:(UIView *)view attribute:(NSLayoutAttribute)attribute
{
    return [NSLayoutConstraint constraintWithItem:view
                                        attribute:attribute
                                        relatedBy:NSLayoutRelationEqual
                                           toItem:self
                                        attribute:attribute
                                       multiplier:1.0
                                         constant:0.0];
}

2,DZNWeakObjectContainer : NSObject只提供了一个方法和属性；如下

@interface DZNWeakObjectContainer : NSObject

@property (nonatomic, readonly, weak) id weakObject;

- (instancetype)initWithWeakObject:(id)object;

@end

@implementation DZNWeakObjectContainer

- (instancetype)initWithWeakObject:(id)object
{
    self = [super init];
    if (self) {
        _weakObject = object;
    }
    return self;
}

@end

3,DZNEmptyDataSetView : UIView

@interface DZNEmptyDataSetView : UIView

@property (nonatomic, readonly) UIView *contentView;
@property (nonatomic, readonly) UILabel *titleLabel;
@property (nonatomic, readonly) UILabel *detailLabel;
@property (nonatomic, readonly) UIImageView *imageView;
@property (nonatomic, readonly) UIButton *button;
@property (nonatomic, strong) UIView *customView;
@property (nonatomic, strong) UITapGestureRecognizer *tapGesture;

@property (nonatomic, assign) CGFloat verticalOffset;
@property (nonatomic, assign) CGFloat verticalSpace;

@property (nonatomic, assign) BOOL fadeInOnDisplay;

- (void)setupConstraints;
- (void)prepareForReuse;

@end

//这里的didMoveToSuperView用到的渐变方法我感觉很棒
- (void)didMoveToSuperview
{
    self.frame = self.superview.bounds;
    
    void(^fadeInBlock)(void) = ^{_contentView.alpha = 1.0;};
    
    if (self.fadeInOnDisplay) {
        [UIView animateWithDuration:0.25
                         animations:fadeInBlock
                         completion:NULL];
    }
    else {
        fadeInBlock();
    }
}

/***
一些视图的懒加载方法，以及相关属性的配置
***/

//子视图的按钮点击后如何响应父视图的方法；就像如下操作
- (void)didTapButton:(id)sender
{
    SEL selector = NSSelectorFromString(@"dzn_didTapDataButton:");
    
    if ([self.superview respondsToSelector:selector]) {
        [self.superview performSelector:selector withObject:sender afterDelay:0.0f];
    }
}

//这里给各个视图约束条件，采用的是HVL语法，但真正被调用的时候却是在父视图中被调用此方法
- (void)setupConstraints
{
    // First, configure the content view constaints
    // The content view must alway be centered to its superview
    NSLayoutConstraint *centerXConstraint = [self equallyRelatedConstraintWithView:self.contentView attribute:NSLayoutAttributeCenterX];
    NSLayoutConstraint *centerYConstraint = [self equallyRelatedConstraintWithView:self.contentView attribute:NSLayoutAttributeCenterY];
    
    [self addConstraint:centerXConstraint];
    [self addConstraint:centerYConstraint];
    [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[contentView]|" options:0 metrics:nil views:@{@"contentView": self.contentView}]];
    
    // When a custom offset is available, we adjust the vertical constraints' constants
    if (self.verticalOffset != 0 && self.constraints.count > 0) {
        centerYConstraint.constant = self.verticalOffset;
    }
    
    // If applicable, set the custom view's constraints
    if (_customView) {
        [self.contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[customView]|" options:0 metrics:nil views:@{@"customView":_customView}]];
        [self.contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[customView]|" options:0 metrics:nil views:@{@"customView":_customView}]];
    }
    else {
        CGFloat width = CGRectGetWidth(self.frame) ? : CGRectGetWidth([UIScreen mainScreen].bounds);
        CGFloat padding = roundf(width/16.0);
        CGFloat verticalSpace = self.verticalSpace ? : 11.0; // Default is 11 pts
        
        NSMutableArray *subviewStrings = [NSMutableArray array];
        NSMutableDictionary *views = [NSMutableDictionary dictionary];
        NSDictionary *metrics = @{@"padding": @(padding)};
        
        // Assign the image view's horizontal constraints
        if (_imageView.superview) {
            
            [subviewStrings addObject:@"imageView"];
            views[[subviewStrings lastObject]] = _imageView;
            
            [self.contentView addConstraint:[self.contentView equallyRelatedConstraintWithView:_imageView attribute:NSLayoutAttributeCenterX]];
        }
        
        // Assign the title label's horizontal constraints
        if ([self canShowTitle]) {
            
            [subviewStrings addObject:@"titleLabel"];
            views[[subviewStrings lastObject]] = _titleLabel;
            
            [self.contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-(padding@750)-[titleLabel(>=0)]-(padding@750)-|"
                                                                                     options:0 metrics:metrics views:views]];
        }
        // or removes from its superview
        else {
            [_titleLabel removeFromSuperview];
            _titleLabel = nil;
        }
        
        // Assign the detail label's horizontal constraints
        if ([self canShowDetail]) {
            
            [subviewStrings addObject:@"detailLabel"];
            views[[subviewStrings lastObject]] = _detailLabel;
            
            [self.contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-(padding@750)-[detailLabel(>=0)]-(padding@750)-|"
                                                                                     options:0 metrics:metrics views:views]];
        }
        // or removes from its superview
        else {
            [_detailLabel removeFromSuperview];
            _detailLabel = nil;
        }
        
        // Assign the button's horizontal constraints
        if ([self canShowButton]) {
            
            [subviewStrings addObject:@"button"];
            views[[subviewStrings lastObject]] = _button;
            
            [self.contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-(padding@750)-[button(>=0)]-(padding@750)-|"
                                                                                     options:0 metrics:metrics views:views]];
        }
        // or removes from its superview
        else {
            [_button removeFromSuperview];
            _button = nil;
        }
        
        
        NSMutableString *verticalFormat = [NSMutableString new];
        
        // Build a dynamic string format for the vertical constraints, adding a margin between each element. Default is 11 pts.
        for (int i = 0; i < subviewStrings.count; i++) {
            
            NSString *string = subviewStrings[i];
            [verticalFormat appendFormat:@"[%@]", string];
            
            if (i < subviewStrings.count-1) {
                [verticalFormat appendFormat:@"-(%.f@750)-", verticalSpace];
            }
        }
        
        // Assign the vertical constraints to the content view
        if (verticalFormat.length > 0) {
            [self.contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:[NSString stringWithFormat:@"V:|%@|", verticalFormat]
                                                                                     options:0 metrics:metrics views:views]];
        }
    }
}

4,UIScrollView+DZNEmptyDataSet所有的关键就在下面这段代码里，swizzle的使用，让-dzn_reloadData inject reloadData 和 endUpdates中就可以了

- (void)setEmptyDataSetSource:(id<DZNEmptyDataSetSource>)datasource
{
    if (!datasource || ![self dzn_canDisplay]) {
        [self dzn_invalidate];
    }
    
    objc_setAssociatedObject(self, kEmptyDataSetSource, [[DZNWeakObjectContainer alloc] initWithWeakObject:datasource], OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    
    // 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)];
    }
}
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