Why is 'didset' called on a property when I set the property of that property?

Question

In this code when the text changes, titleEditingChanged is called (as expected). But when it executes the line

investment?.title = sender.text!         


        

Answer 1

Property observers of value types (e.g. structures) are called also when an underlying property of the type instance is set; simply since the value of the instance itself is updated. The same does not hold for reference types; as long as the reference itself is not mutated, the property observer will not be called (i.e., the reference itself can be considered the value of reference types).

From Language Guide - Properties - Property Observers we read:

Property observers observe and respond to changes in a property’s value. Property observers are called every time a property’s value is set, even if the new value is the same as the property’s current value

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To verify the above, consider the following example:

/* reference type */
class InvestmentC {
    var title: String = "Foo"
}

/* value type */
struct InvestmentS {
    var title: String = "bar"
}

class InvestmentContainer {
    var investmentC : InvestmentC {
        didSet {
            print("did set a property of 'InvestmentC' instance (ref. type)")
        }
    }

    var investmentS : InvestmentS {
        didSet {
            print("did set a property of 'InvestmentS' instance (val. type)")
        }
    }

    init() {
        investmentC = InvestmentC()
        investmentS = InvestmentS()
    }
}

/* Example: property observer called only when setting a property
            of the value type instance 'investmentC'              */
let foo = InvestmentContainer()
foo.investmentC.title = "foobar" // prints: nothing
foo.investmentS.title = "foobar" // prints: "did set a property of 'InvestmentS' instance (val. type)"

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Hence, we can infer that your custom type Investment is a value type (a structure), and the didSet property observer of the instance investment of this type (in your UITableViewCell subclass) will be called even if you only set/update underlying properties of investment. If you'd like to avoid this, change Investment into a reference type (class), in which case the didSet property observer will only be changed if the investment instance itself is set/updated.