Windows in Action
|Column Tag:||C WORKSHOP
Windows in Action
By Robert B. Denny
Windows in Action
There is an initial hill that every Macintosh developer must climb before doing anything useful. Most applications require use of windows for presentation and control. Unfortunately for the developer, this fundamental requirement makes it necessary to understand a large part of the Macs internals. This information is presented piecemeal in Inside Macintosh (IM).
Last month, we covered the static aspects of windows, including data struc- tures and the functions of the Window Manager. Much of this information can be gleaned from IM relatively easily.
The dynamic aspects of windows, how- ever, are far more subtle. Most of what we learn about window dynamics comes from our experience implementing window-based applications. Take some time to observe how various Mac applications handle things like activating overlapped windows and dragging or re- sizing of windows. Some applications take an overkill approach, erasing the entire window and re-drawing everything. Others behave in a more refined fashion, updating only whats necessary.
The goal of this months column is to provide you with some insights into window dynamics and how to apply that knowledge to avoid over-updating and ghost images.
In particular, we will cover creating, updating, dragging, re-sizing, activating and deactivating of windows. The details of scroll-bars, textEdit and other window content items will be covered in next months column.
Before we go any further, lets quickly review the various regions involved in window construction and dynamics.
Fig. 1 shows the various component regions of a common edit window. The terminology used here and in Inside Macintosh is confusing. The go-away, drag and grow regions arent really regions in the QuickDraw sense. They are areas within the window that are handled specially by the Window Manager. For the purposes of compatibility with IM, however, we will use the same termin- ology.
The structure and content regions are true regions. The structure region encloses the entire window, frame and all. The frame consists of the entire title bar and the border (even the drop-shadow, if any). The content region is that area inside the frame. Note that the scroll bars and size box are located within the content region. These regions change only when the window is moved or re-sized.
The Update Region
There is another region that we will be dealing with extensively, the update region. Its not a structural part of the window as are the structure and content regions. Instead, it is a dynamic descrip- tion of the area(s) of the window which need updating at any given instant.
The update region, like the structure and content regions, is linked directly to the windowRecord. It is used by the Window Manager to control QuickDraw updates to the screen. Well look at this in more detail later.
When windows are shuffled front and back, or moved about the desktop, it can be confusing as to whom is responsible for re-drawing what parts of the window. What will the window manager do for you, and what must your application do itself? Here is a simple rule to remember:
The Window Manager handles the frame; your application must handle the contents.
The contents include any controls (e.g., scroll bars) and the grow icon in the grow region, as well as whatever you have placed elsewhere in the window area.
How a Window is Drawn
Lets look at the process of creating a new window, neglecting the programming needed and concentrating on what actually happens. Keep in mind the rule presented above regarding who handles what.
First, the Window Manager calls the definition procedure (DefProc) for the window type and asks it to draw the frame. In doing this, the DefProc manipulates the desktop visRgn and clipRgn to make sure that only the visible portion of the frame will be actually drawn. The DefProc may also draw a go-away box in the title bar area. If you are unclear about DefProcs, refer to last months C Workshop.
When the DefProc returns to the Window Manager (with the frame drawn), the Window Manager posts an update event for the application that requested the window to be drawn, then returns control to the application.
It is important to understand just how this update event gets generated. The Window Manager determines which areas of the windows content region need updating. This set of areas is accumu- lated into the windows update region, which is now no longer empty. When the Toolbox Event Manager sees a window with a non-empty update region, it queues an update event for that window.
Note that drawing and updating is supported for any window on the desktop. the window being drawn need not be frontmost nor the active window. It might not be visible at all.
Sometime later, when the application de-queues the update event just posted, it must draw the window contents. This could be tricky, since all or part of the window could be obscured, and if its being re-drawn, some of the image might not need updating.
The proper method of handling update events involves use of two very important Window Manager services, BeginUpdate() and EndUpdate() as follows:
* Handle update event
WindowPtr wp; /* Update this window */
... /* Draw everything */
BeginUpdate() calculates the intersec- tion of the windows visRgn and the update region. The result of this intersection is the area that is visible and needs updating. It then replaces the visRgn with this newly calculated region. Finally, it clears the update region so that the update event will not be repeated for the window. Then control is returned to the update event handler.
At this point your application may draw the entire window contents without fear. Actual drawing will be restricted to the (temporary) visRgn, which encloses only what may and should be drawn. Nothing else will be drawn.
After this has been done, call EndUpdate() to restore the original visRgn. This completes the actions needed to service an update event.
So the steps are, restrict actual drawing by hacking the visRgn, draw everything, then restore the real visRgn. This approach to window content maintenance is absolutely basic to successful Mac application development.
Dont be tempted to try analyzing the hacked visRgn calculated by BeginUpdate() and manually restrict your drawing to that region only. You may think youre saving time by not drawing invisible areas, but youre more likely to chew up at least that much time figuring out what you dont want to do!
Window contents must be updated whenever something happens to cause a portion of the windows image to become visible after being obscured. The Window Manager will automatically accumulate areas into a windows update region, and your application may manually add to the update region as well.
In either case, when the update event occurs, part or all of the window may need re-drawing. In order to do this properly, you must have a description of the windows contents stored somewhere. Think about the implications of this.
Lets say your application requests input of a set of 10 values that you use to draw a figure. You simply read the values and draw the figure as they are read in. Now lets say you drag the window partially off-screen, release it, then drag it back on screen. Youll get an update event for the area that was off-screen. How can you reproduce the figure? The values are gone.
You should have stored the values in an array so that they could be used by the update event handler to re-draw the figure as needed. And why draw any time except after an update event? This is a trivial example of an important rule in Mac programming:
Never store anything in the windows image. Always maintain the data needed to re-draw the window and draw only in response to an update event.
If you change something in the window, change the descriptive data then force an update event by calling InvalRect() or InvalRgn() to put the changed area into the update region. Let the update event handler do the actual drawing. This method of window maintenance is called deferred drawing because the changes are made to the descriptive data and the actual drawing is deferred to the time of the update event.
If you follow the above rule, it will usually simplify your programs logic, making it more reliable and less likely to leave fossils on the screen or blow holes in the window image.
Activation and Deactivation
The Macintosh User Interface Guideines specify that only one window on the screen may be active. What does active mean? Remember that any window on the screen can (and may) be updated at any time. But the user should be able to tell which window will react to keystrokes and/or mouse clicks. Which window is hooked up to the mouse and keyboard?
The active window is the one the user is working in at the moment. It is always the frontmost window (alerts and dialogs excepted) and has a distinctive highlighted appearance.
The Window Manager provides services to activate and deactivate windows. Also, the system delivers activation events to the the application so that it can make any changes to the window contents as a result of activation or deactivation. Remember, the Window Manager handles the frame and the application handles the contents.
Fig. 2 shows a document window in its deactivated state. The title bar (drag region) has no stripes and the go-away box is not present. These items are controlled by the Window Manager. The scroll bars are hidden and the grow icon is gone. These items are controlled by the applications deactivation event handler.
Fig. 3 shows an activated document window. The title bar has stripes showing and the go-away box is visible. The Window Manager handles these items. The scroll bars and the grow icon are visible. The applications activate event handler handles the latter items.
Activation and deactivation are signalled by the same activation event being queued for the window. The application must decode which action to take.
What your application should do in response to update and activation events can become confusing. Servicing update and activation events is a simple matter if you keep the following rule in mind:
Drawing should be done only for update events. Activation and deactivation should cause only changes in appearance of activation dependent items such as controls and menus.
If you follow this rule, it will simplify the logic of your program and prevent unnecessary drawing. Do actual drawing only in response to update events. Use activation events to change the appearance of items.
Putting it All Together
Lets take the concepts we have learned and apply them to some common situations. Fig. 4 shows the sequence of events when a window is activated and brought to the front. Look at the steps involved and note the roles of the update and activation events, and the formation of the update region.
There are some fine-points of handling the scroll bars and grow icon that we will discuss next month, when we cover controls and textEdit.
The next illustration shows what happens when a window which is partially off-screen is dragged back on to the screen. It was this case which turned the light on for me personally.
Interestingly enough, no update event is generated for a window which is dragged from one place to another on the screen. The window manager has a complete image of the window already on the screen, so it simply bit-blits (fast image copy) the image from one place to the other.
The surprise comes when you drag the window off-screen, release it, then drag it back on. Now part of the window must be re-constructed as if it was being drawn for the first time. An update event is posted and the application must re-draw the part that was off the screen.
The last movie, fig. 6, shows what happens when a window is re-sized. No activation events are posted for this action.
Note particularly the manual invalidation of the scroll bars by calling InvalRect() for each. This is needed because the window manager has no idea that the controls are going to be moved, invalidating the area covered by them.
Next month, well finish up the treatment of windows with a description of controls and textEdit. By treating these two subjects together, well learn a lot from their interrelatedness. Ill also include some C routines for handling update, activate, deactivate, drag and grow actions for windows.
In the near future, The C Workshop will contain a complete application written in C which supports multiple windows of two types, one of which supports fully functional scroll bars and textEdit. The other window type supports a crude blackboard which does not remember the blackboard contents. We hope youll find this useful as a basis for non-trivial applications.
For now, please be patient. I have not included much C in the last few columns, since the template application. I feel that there is so much to know before one can write even a trivial application that its best to hold off until the foundation is complete.