Windows are not cheap things

Even though Windows The operating system is window centric , But the window itself is not cheap . what's more ,1985 In, there will be strict memory restrictions , Forcing system designers to make different design decisions .

Let's take the list box control as an example .
In modern design , You can design the list box control as a series of sub window lists , Each sub window represents an entry in the list . that , have 20000 The list box of items will have 20000 Sub window . However , This design concept , stay 1985 Years are completely ridiculous .

Think about it , At that time Windows Around 16 Built by bit processor . Window handle is 16 A value , The interior is just close 64K Pointer to heap memory . A window object is 88 Bytes （ I really counted ）, This means that you can create at most about before running out of memory 700 About windows .

what's more , Menus are created in the same 64K In the memory heap , So the actual limit is much lower . Even if the window manager uses more than 64K A pile of （Windows 95 That's what it does ）,20000 Windows are also more than 1.5MB Of memory . because 8086 The maximum address space of is 1MB, Even if you use every byte of memory for window objects , You still don't have enough memory .

Besides , Making each list box item a window means that each list box will be a variable height list box , This brings the complexity of managing containers with variable height items .

This violates the API Two general principles of design ：（1） Simple things should be simple ,（2）“ Paid games ”, If you do simple things , You should not pay the cost of complex things .

Filling the list box with the actual window will also make “ Virtual list box ” The design of becomes more difficult . Use existing design , You can say “ There are onemillion projects ”, Without actually creating them . （ This is why the window space is divided into “ Customer ” and “ Non customers ” Areas rather than non customer areas are made up of small windows .）

In order to keep up with 16 position Windows Program compatibility （ Thanks a lot WOW layer , These programs can still be used in Windows XP function ）, Window handles in the system cannot exceed 65536 individual , Because exceeding this number will affect 16 Normal operation of bit program .（ Once created 65537 A window , There will be two with the same 16 Bit handle value window , This is due to the pigeon nest principle .）（ Yes , Even today ,16/32 Bit interoperability is still important .）

If the upper limit of windows that the system can create is 65536 A window handle , It contains 100000 The list of files will encounter serious problems . As new functions are added to the window manager , The size of the window object increases over time . Today it is even better than in the past 88 Bytes are even larger . It's best not to create unnecessary windows . If your application design requires you to create thousands of windows , Then you should consider migrating to the windowless model (windowless model), for example Internet Explorer、Word、 List box 、 Tree view 、 List view , Even our scrollbar sample program .

Through windowless model , You get rid of the overhead of a complete window handle , And all the baggage that comes with it . Because the window handle is visible to all processes , Therefore, centralized management of window lists will incur a lot of overhead . If your application has no windows , Then the only program that can access your content is your own program . You don't have to worry about marshalling 、 Cross process synchronization 、Unicode/ANSI transformation 、 External subclassing 、 hook …… If you want , You can also use 1GB Memory to track your windowless data , Because windowless controls do not affect any other processes .

The fact that other processes can access window handles , There is a practical limit on how many window handles can be created without affecting the entire system .

I Believe WinFX Used “ Everything on the screen is an element ” Model of . As far as I know , They built a windowless framework , So you don't have to do it yourself . （ however , I'm not sure about that , I am not WinFX people .）

summary

We need to be alert to the sharp increase in the number of windows , On the one hand, it will increase the difficulty of management , On the other hand , Process GDI There are too many objects , It will also significantly reduce runtime performance .
Interaction design , Make it simple , It's simpler , Flowery is not what we pursue , Good procedure , It should be used up and go .

Last

Raymond Chen Of 《The Old New Thing》 It's one of my favorite blogs , There's a lot about Windows Little knowledge , For the vast Windows For platform developers , It's really helpful .
This article is from ：《Windows are not cheap objects》