read and write Indicates that the property should be considered read / write. This attribute is the default value. The block @implementationrequires both the getter method and setter. If you use the directive @synthesizein the implementation block, the getter and setter methods are synthesized.
read-only Indicates that the property is read-only. If you specify read-only, @implementationonly the getter method is required in the block . If you use the directive @synthesizein a block @implementation, only the getter method is synthesized. Moreover, if you try to assign a value using the point syntax, you will get a compiler error.
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