R – What’s the simplest way to override a Delphi property’s default value

default-valuedelphiproperties

I'm trying to write a TCustomDBGrid descendant that's designed to feel like a TListBox. One of the things I want to change is the Options property's defaults. TCustomDBGrid defines Options as:

property Options: TDBGridOptions read FOptions write SetOptions
  default [dgEditing, dgTitles, dgIndicator, dgColumnResize, dgColLines,
  dgRowLines, dgTabs, dgConfirmDelete, dgCancelOnExit];

Trying to override that in my class with

  property Options: TDBGridOptions default
     [dgTitles, dgTabs, dgRowSelect, dgAlwaysShowSelection, dgCancelOnExit];

doesn't work; the compiler expects read or write after the type, not default. Problem is, FOptions and SetOptions are both defined as private, not protected, in TCustomDBGrid.

Do I have to write my own get and set methods that invoke "inherited Options", or is there a simpler way to do this?

Best Answer

Don't specify the type. Then you can change the default. And remember that you also have to set the Options property to this default in the constructor. The code below does not actually set the default, it just lets it know to not stream the property value when the value is like that.

property Options default [dgTitles, dgTabs, dgRowSelect, dgAlwaysShowSelection, dgCancelOnExit];

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Delphi – What’s the simplest way to call Http GET url using Delphi

Calling a RESTful web service using Indy is pretty straight forward.

Add IdHTTP to your uses clause. Remember that IdHTTP needs the "HTTP://" prefix on your URLs.

function GetURLAsString(const aURL: string): string;
var
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begin
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Ios – What’s the difference between the atomic and nonatomic attributes

The last two are identical; "atomic" is the default behavior (~~note that it is not actually a keyword; it is specified only by the absence of nonatomic~~ -- atomic was added as a keyword in recent versions of llvm/clang).

Assuming that you are @synthesizing the method implementations, atomic vs. non-atomic changes the generated code. If you are writing your own setter/getters, atomic/nonatomic/retain/assign/copy are merely advisory. (Note: @synthesize is now the default behavior in recent versions of LLVM. There is also no need to declare instance variables; they will be synthesized automatically, too, and will have an _ prepended to their name to prevent accidental direct access).

With "atomic", the synthesized setter/getter will ensure that a whole value is always returned from the getter or set by the setter, regardless of setter activity on any other thread. That is, if thread A is in the middle of the getter while thread B calls the setter, an actual viable value -- an autoreleased object, most likely -- will be returned to the caller in A.

In nonatomic, no such guarantees are made. Thus, nonatomic is considerably faster than "atomic".

What "atomic" does not do is make any guarantees about thread safety. If thread A is calling the getter simultaneously with thread B and C calling the setter with different values, thread A may get any one of the three values returned -- the one prior to any setters being called or either of the values passed into the setters in B and C. Likewise, the object may end up with the value from B or C, no way to tell.

Ensuring data integrity -- one of the primary challenges of multi-threaded programming -- is achieved by other means.

Adding to this:

atomicity of a single property also cannot guarantee thread safety when multiple dependent properties are in play.

Consider:

 @property(atomic, copy) NSString *firstName;
 @property(atomic, copy) NSString *lastName;
 @property(readonly, atomic, copy) NSString *fullName;

In this case, thread A could be renaming the object by calling setFirstName: and then calling setLastName:. In the meantime, thread B may call fullName in between thread A's two calls and will receive the new first name coupled with the old last name.

To address this, you need a transactional model. I.e. some other kind of synchronization and/or exclusion that allows one to exclude access to fullName while the dependent properties are being updated.

Best Answer

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