Create a Setter Only in Swift

Use didSet instead:

var masterFrame:CGRect {
    didSet {
        _imageView.frame = masterFrame
        _scrollView.frame = masterFrame
    }
}

From the docs:

The getter is used to read the value, and the setter is used to write the value. The setter clause is optional, and when only a getter is needed, you can omit both clauses and simply return the requested value directly, as described in Read-Only Computed Properties. But if you provide a setter clause, you must also provide a getter clause.

Well if I really have to, I would use this.

Swift compiler supports some attributes on getters, so you can use @available(*, unavailable):

public subscript(index: Int) -> T {
    @available(*, unavailable)
    get {
        fatalError("You cannot read from this object.")
    }
    set(v) {
    }
}

This will clearly deliver your intention to the code users.