I'm a newer to Swift. UITableViewDataSource and UITableViewDelegate have so many functions. If I have a UIViewController to adopt them, why don't I have to implement all the functions? In other languages such as Java, the class has to implement all the functions declared in the interface.

CodePudding user response：

That's because UITableViewDataSource and UITableViewDelegate are Objective-C protocols, not Swift protocols.

The distinction is important, because Objective-C protocols allow you to mark some methods as optional, meaning they do not have to be implemented by whoever conforms to them.

There is an old but still useful Apple doc on the topic here, but essentially:

@protocol XYZPieChartViewDataSource
- (NSUInteger)numberOfSegments;
- (CGFloat)sizeOfSegmentAtIndex:(NSUInteger)segmentIndex;
@optional
- (NSString *)titleForSegmentAtIndex:(NSUInteger)segmentIndex;
- (BOOL)shouldExplodeSegmentAtIndex:(NSUInteger)segmentIndex;
@required
- (UIColor *)colorForSegmentAtIndex:(NSUInteger)segmentIndex;
@end

(Taken from the link above)

The XYZPieChartViewDataSource protocol has three required methods. If they are not marked as either @optional or @required, they are required by default. Note that the attribute is applied in cascade until finding another attribute, so both - (NSString *)titleForSegmentAtIndex:(NSUInteger)segmentIndex; and - (BOOL)shouldExplodeSegmentAtIndex:(NSUInteger)segmentIndex; are optional.

The methods of UITableViewDataSource and UITableViewDelegate that you are not required to implement have the @optional attribute.

In Swift, it is possible to have protocols with optional implementations as well, but you need to expose your protocol to Objective-C with the @objc attribute, and the methods that are marked as optional must be exposed to Objective-C as well. This is what XYZPieChartViewDataSource would look like in Swift:

@objc protocol XYZPieChartViewDataSource {
    @objc func numberOfSegments() -> Int
    @objc func sizeOfSegmentAtIndex(_ segmentIndex: Int) -> CGFloat
    
    @objc optional func titleForSegmentAtIndex(_ segmentIndex: Int) -> String
    @objc optional func shouldExplodeSegmentAtIndex(_ segmentIndex: Int) -> Bool
    
    @objc func colorForSegmentAtIndex(_ segmentIndex: Int) -> UIColor
}

CodePudding user response：

In the documentation you'll notice the functions are often marked as optional. For example; tableView(_:didSelectRowAt:) has "optional" before the method declaration meaning you do not have to implement it.