|Column Tag:||C Workshop
By Robert B. Denny
Layers and Interfaces
One of the most important things to come out of the computer world in the seventies is the concept of layered architecture. Layering provides us with a clean way to deal with the complexities of modern operating system and communications architectures. One of the most visible instances of layering is the ISO Open Systems Architecture which is widely used and quoted in standards for networks.
I consider layering to be a survival skill for software developers. Take time to consider the Macintosh from bottom to top. At the bottom, we have solid-state physics which attempts to explain the behavior of what might be particles or waves. At the top (users view) we have dragging, clicking, icons, windows, etc. Both the physics and the user concepts are abstract ideas. How many layers can you conceive of in a Macintosh?
Lets formalize this a bit. A layer is a structured set of related things that together provide a set of tools or services for implementing other things. An interface is a set of means for accessing the services provided by a layer. Finally a given layer might make use of the services provided by a lower layer in implementing its own services. It is this latter notion that makes layers have ranking.
How does all of this help us deal with complex systems? By restricting our thinking to the layer we are working on, we avoid cluttering our mind with unnecessary details. This kind of thinking comes naturally, but we can reinforce it by being aware of its importance.
Most of the developers I have talked with share the feeling that the Mac is a difficult system to learn. Inside Macintosh is a wealth of information, but it has little overall structure. There is a section called Inside Macintosh: A Road Map which does discuss the overall architecture, but this thread is not carried elsewhere, and one tends to dive in to a particular area of interest.
Here is my personal view of the Macintosh operating system as a set of layers. Below these layers are the low-level stuff I seldom deal with and above these layers are the programs I write. The boxes show the sections of the Mac system that I frequently encounter when developing user-oriented applications.
Another key concept needeed for Macintosh programming is that of data or table-driven software. This is another common technique that grew out of the seventies, when system complexity took a quantum jump.
A data-driven routine consists of a combination of code and data, split up so that variations are possible with no changes to the code. This technique is usually employed where there are a variety of related objects that need some sort of service or management that can be done with a common engine in code.
This is sort of a gray area; you might say that all routines are sensitive to data, or they wouldnt be useful. True, but a routine that stores no data of its own is special. It is usually called with a reference to a data structure which contains the state and attributes of the particular object that the routine is to work on. The data structure could even contain a reference to one or more action routines which are unique to the object in question. Examining the contents of such data structures can often provide great insight into the sublties of the routine or service.
Most of the managers in the Macintosh interface use this technique. The data structures are referred to as records from the Pascal terminology. In C, they can be implemented as structs.
The Window Manager
From the users viewpoint, a window is an object on the desktop that provides some sort of user interface. There is a whole set of guidelines describing how windows should look and behave. Window appearance and behavior is the essence of the Macintosh.
From the developers viewpoint, a window is a structured set of regions in a port under the control of the Window Manager. The Window Manager provides a layered interface to window services on the Macintosh, using QuickDraw to draw the pictures that create the abstract notion of windows on the screen.
Besides handling the windows themselves, the Window Manager handles shuffling, dragging and re-sizing of overlapping windows, generating events for owning applications to update window contents as required. This powerful and complex layer of the Mac architecture has a surprisingly simple interface.
The Window Manager uses a data structure, called a window record, to keep track of each window. It contains the currrent attributes and state of the corresponding window. The window records are kept on a linked list whose order corresponds to the planar ordering of the window images on the screen, as shown in figure 2.
If you havent looked over the section on the Window Manager in Inside Macintosh, now would be a good time to do so. In particular, study the section describing the window record and each of its fields. Well look at some of the more important fields here, but we cant cover them all. In C, the window record can be represented as follows:
C STRUCT FOR WINDOW RECORDS
#define boolean char
struct WindowRec *nextWindow;
Note that the first thing in the window record is a GrafPort structure. This is the data structure used by QuickDraw for manipulating ports in general. Of course, a window is a port, so it follows that the windows data would include (nested within) a GrafPort.
Before we dig any deeper into the window record, lets look at how it relates to other important data structures. Pointers and handles are represented alike in the interest of simplicity in Fig. 3.
PORTS, REGIONS AND WINDOWS
Looking at figure 3, observe the linkage of the region structs. The visRgn and clipRgn are hooked to the GrafPort, while the strucRgn, contRgn and updateRgn are hooked to the WindowRecord. The latter three regions are used only in the context of a window. If you are unclear about the role of regions, refer to Chris Derossis Pascal Procedures in MacTutor, Vol.1, No.3, February, 1985.
Note also that the ControlRecord for each control that is associated with the window is hooked to a linked list, and has a back-link pointing back to the owning windows WindowRecord. This implies that the Window Manager uses information about the controls, and that the Control Manager uses information about the owning window and its port. For example, the Window Manager function drawControls() traces down the linked list of ControlRecords, drawing each control in turn.
The various boolean fields in the WindowRecord indicate the current state of the windows visibility, highlighting (see below) and whether it has a go-away box. CAUTION: the Window Manager uses 255 as TRUE and 0 as FALSE. Typically C defines TRUE as 1. You should test these flags for not FALSE, rather than explicitly TRUE.
The three RgnHandles point to Rgn structs which describe important parts of the windows port. Well cover these later in the section on anatomy and next months column on window dynamics.
The windowDefProc field is a handle that points to a vectored set of routines that give the window its look. Associated with this is the dataHandle field, which points to private data used by the DefProc. DefProc is short for Definition Procedure, really a set of routines that perform the following:
Window Definition Procedure Tasks
Draw window frame
Return region where mouse was clicked
Calculate structure & content regions
Draw grow image of the window
Draw a size box in the content region
Note that the Window Manager has no idea what the window actually looks like. The appearance is completely governed by the DefProc, which is described by a handle in the WindowRecord. The six standard DefProcs handle windows of the types shown below in figure 5 with the symbolic value for the definition ID you pass to the Window Manager when creating the window.
Note that the documentProc flavor has the outlines for scroll bars. Scrollers are controls, not part of the basic windows definition. These are the standard window types. You could write a DefProc package for a custom window type. The hooks for this are described in Inside Macintosh in the Window Manager section.
The titleHandle field points to the windows title string and the titleWidth field contains the width of the title in pixels. ControlList is the listhead for the linked list of ControRecords for the controls associated with this window.
NextWindow is a pointer to the next window on the window list. As we stated earlier, the next window is the one behind this window. If this is the back-most window, NextWindow is NULL.
WindowPic is a handle to a QuickDraw picture for the window, or NULL if the application is responsible for updating the window. This implies that the window manager will automatically update a window if its contents are a QuickDraw Picture without generating update events.
Finally, there is a hack in the definitions used in Inside Macintosh. A WindowPtr is defined as a pointer to a grafPort, rather than a pointer to a WindowRecord. The name for the later is WindowPeek. It seems that the most frequent use of WindowRecord information involves the data contained in the embedded grafPort. Keep this quirk in mind.
Anatomy of a Document Window
The six standard window types are shown in figure 5. Of the six, the documentProc type is the most complex. In this section well look at the window as a set of regions. Viewing the window this way is essential when writing programs which manipulate them, particularly the routines that handle activation and updating of the window. A document window with scroll bars is shown in figure 6.
All windows have a structure region and a content region. The structure region covers the entire window, frame and all. The content region is the area inside the frame. These regions change only when the window is moved or re-sized.
The frame may include go-away and drag regions. The content region may include a grow region.These are not really regions in the formal sense, as defined by QuickDraw. The DefProc is responsible for handling these regions, not QuickDraw. The scroll bars are handled by the Control Manager, not the Window Manager.
The WindowRecord contains a handle to another region, the Update Region. This region describes the portion of the content region that needs updating, redrawing, after some change. Well deal with this in next months column on window dynamics.
One of the fine points not made clear in Inside Macintosh is the relationship between the grow image, the grow region and the scroll bars usually present in a document window. The grow image is the line-tracing of the window that appears when you click in the grow region.
The Control Manager can build scroll bars of any dimension. The convention is that scroll bars are 16 pixels wide. This matches the width of the scroller areas that appear in the grow image, and the width and height of the grow icon. These areas are fixed, set by the documentProc DefProc. Here is a fat-bits view of the lower right corner of the document window shown above (see fig. 7). Note that the scroll bar edges overlap the grow icon and the window frame by one pixel. The following fragment of C code computes the boundsRects for the scrollers, given the portRect for the window. Get the portRect from the grafPort that is embedded in the WindowRecord:
struct rect/* Define a rect */
struct grafPort *wp; /* Window Pointer */
struct rect *pr; /* -> portRect */
struct rect vs_rect; /* Vert. scroller */
struct rect hs_rect; /* Hor. scroller */
pr = &(wp->portRect);
* Compute bounds rect for vertical scroll
vs_rect.top = pr->top - 1;
vs_rect.left = pr->right - 15;
vs_rect.bottom = pr->bottom - 14;
vs_rect.right - pr->right + 1;
* Compute bounds rect for horiz. scroll
hs_rect.top = pr->bottom - 15;
hs_rect.left = pr->left - 1;
hs_rect.bottom = pr->bottom + 1;
hs_rect.right = pr->right - 14;
Next month well deal with window dynamics, dragging, growing, updating and activating. This is an area of Mac programming that seems complex on the surface. If you have tried your hand at window-oriented programs, you probably had many surprises and mysteries and you may have developed a few OPTs (old programmers tales). One of my most memorable surprises was discovering what happens if you omit the beginUpdate() and endUpdate() calls when handling an update event.
There is an underlying consistency to window dynamics. Its all there in Inside Macintosh, but in peices. Understanding window dynamics is essential if you are to get beyond trivial programs without unnecessary screen drawing or ugly hacks. See you then.