Lập trình Wrox Professional Xcode 3 cho Mac OS part 18 potx

134 ❘ CHAPTER 7 SYNTAX-AWARE EDITING CODE FOLDING Code folding highlights blocks of structured text and allows them to be collapsed — or “ folded ” — so that you can concentrate on the higher - level structure of your code. Code folding is supported in all C - like languages (C + + , Objective - C, Java) and XML, as shown in Figure 7 - 5. To see and use code folding, you must have the Show Code Folding Ribbon option turned on in the Text Editing pane of the Xcode Preferences. FIGURE 7-5 The code folding ribbon appears between the gutter and the text. The depth of the blocks in your text is graphed using gradations of grey; the darker the ribbon, the deeper the text in that line is nested. Hovering your cursor over the ribbon presents collapse controls for the block of text that begins on that line. Code Focus Code focus highlights a block of text in the editor, making it easier to see what the block encompasses, as shown in the bottom window in Figure 7 - 5. Code focus appears when you hover your cursor over the code folding ribbon. Uncheck the Code Focus option in the preferences to disable this behavior. If you fi nd code focus really useful, even temporarily, and are tired of moving your cursor over the code folding ribbon, you can have code focus follow your current text selection by choosing View ➪ Code Folding ➪ Focus Follows Selection (Control + Option + Command + F). Now, the code focus animation will continuously highlight the block containing the current selection or insertion point. Invoke the command a second time to toggle the feature off again. c07.indd 134c07.indd 134 1/22/10 12:24:23 PM1/22/10 12:24:23 PM Download at getcoolebook.com Code Folding ❘ 135 Folding Code You have four ways to fold a block of code, listed in the following table. ACTION OR COMMAND EFFECT Collapse button in ribbon Collapses highlighted block View ➪ Code Folding ➪ Fold Collapses block containing the current selection View ➪ Code Folding ➪ Fold Methods/Functions Collapses the outmost block of every method or function View ➪ Code Folding ➪ Fold Comment Blocks Collapses all multi - line comments Folded code is collapsed and replaced with an ellipsis placeholder, as shown in Figure 7 - 6. FIGURE 7-6 The Fold command folds the block containing the current text selection or insertion point, and is the only fold action that uses the current selection. The Collapse button folds the highlighted block in the ribbon, and the other two commands apply globally to the entire fi le. Folding comment blocks only applies to multi - line comments (like / * … * / ). The command does not recognize, or fold, successive lines of single - line comments ( // ). Have no fear; the text in the folded region is still there. Including a folded block placeholder in a text selection includes all of the text in that block, just as if it weren ’ t folded. Unfolding Code You have six ways to unfold code, listed in the following table. c07.indd 135c07.indd 135 1/22/10 12:24:24 PM1/22/10 12:24:24 PM Download at getcoolebook.com 136 ❘ CHAPTER 7 SYNTAX-AWARE EDITING ACTION OR COMMAND EFFECT Expand button in ribbon Expands collapsed block View ➪ Code Folding ➪ Unfold Expands blocks contained in the selected text or in the selected block View ➪ Code Folding ➪ Unfold Methods/Functions Expands the outmost block of every method or function View ➪ Code Folding ➪ Unfold Comment Blocks Expands all multi - line comments View ➪ Code Folding ➪ Unfold All Expands all collapsed blocks Display any text in a collapsed block Expands the block containing the text The fi rst four actions are symmetric complements to the code folding actions in the previous section. In addition, the Unfold All command unfolds all collapsed blocks of code. Finally, any action that you take that causes some text to be selected or revealed — searching for text or jumping to a symbol defi nition — automatically expands the block so that the text is visible. INDENTING TEXT Over the years, programmers have developed coding styles that visually refl ect the structured nature of the languages in which they are programming. One of these conventions is indenting. These conventions have become so common and consistent, that many can be automated. Indenting is accomplished by starting a line with tab or space characters. Using spaces to indent lines is the most predictable method, but creates unnecessarily large fi les. Tab characters are more effi cient, but are problematic because there is no standard that dictates how many column positions a tab character represents. If you decide that tab stops should be every four columns and you open a text fi le from a programmer who thought tabs should be every eight columns, the text won ’ t line up correctly. You ’ ll achieve the greatest compatibility by adopting four - column tab stops, which seems to be the most common setting these days. You can also alter the tab width for individual fi les. The default tab settings for fi les are confi gured in the Indentation tab of the Xcode Preferences. Xcode ’ s tab width, indenting, and auto - formatting behaviors are all set globally in the preference ’ s Indenting pane, shown in Figure 7 - 7. The tab widths are default for fi les, but individual fi les can override these global settings. The remaining options in this preferences pane are global and apply to all source fi les. c07.indd 136c07.indd 136 1/22/10 12:24:25 PM1/22/10 12:24:25 PM Download at getcoolebook.com Indenting Text ❘ 137 Setting Tab Width and Indent Width Xcode makes a distinction between the width of the tab stops and the amount by which each line of code is indented. The two numbers do not have to be the same. The Tab Width setting, shown in Figure 7 - 7, determines the width of the tab stops in characters. The Indent Width setting is the number of columns ’ worth of white space added to the beginning of a line to indent it one level. Xcode uses the most effi cient combination of spaces and tabs that it can. If the indent width is an integer multiple of the tab width, only tab characters are used to indent lines. If not, a combination of tabs and spaces is used to achieve the desired indentation. Say you have a tab width of 8 and an indent width of 4. To indent a line one level requires four space characters. Indenting two levels would cause these spaces to be replaced by a single tab. The global tab and indent width settings can be overridden for an individual fi le either in the Info window for the fi le, or by choosing the View ➪ Text ➪ Tab Settings command, both of which are shown in Figure 7 - 8. FIGURE 7-7 FIGURE 7-8 c07.indd 137c07.indd 137 1/22/10 12:24:25 PM1/22/10 12:24:25 PM Download at getcoolebook.com 138 ❘ CHAPTER 7 SYNTAX-AWARE EDITING The Editor Uses Tabs option permits the use of tab characters to indent lines. Turning it off forces Xcode to use spaces for all indenting. This is the safest option when you ’ re sharing code with other programmers who might not be using the same tab width, but it also makes your fi les substantially larger. I also think it makes deleting the white space in lines awkward, and also makes it easier to inadvertently align text at odd indentations. The Reset to Text Editing Defaults button returns the fi le to using the global settings in the Indentation preferences panel. File - specifi c editor settings are maintained for the fi le in its project document. The settings will not be remembered for fi les that are not assets of a project, and the settings are independent of other projects. Automatic Indentation Automatic indenting occurs whenever you start a new line by pressing the Return key. The editor inserts the same amount of indentation on the new line as was present on the line above it. This feature is always active. To circumvent it for a single line, type Option ϩ Return at the end of the line. Option + Return starts a new line with no indentation. There is no simple option to turn off auto - indentation globally, but it can be defeated by changing the key binding for the Return key, as explained in Chapter 23. Syntax - Aware Indentation Enabling Syntax - Aware Indenting in the Indentation pane of the Xcode Preferences turns on a number of additional features that will aid you in indenting source code and forming code blocks. You can have Xcode do any of the following: Automatically indent lines when you press the Tab key Automatically indent code blocks Automatically create a balanced code block when you type a { character Automatically re - indent a line when you type one of several punctuation characters Automatically indent C - style comments Each feature can be independently confi gured or disabled, as desired. Syntax - Aware Tab Indenting The Tab Indents option determines what action occurs when you press the Tab key. It has three settings, as described in the following table. ➤ ➤ ➤ ➤ ➤ c07.indd 138c07.indd 138 1/22/10 12:24:28 PM1/22/10 12:24:28 PM Download at getcoolebook.com Indenting Text ❘ 139 TAB INDENTS SETTING TAB KEY ACTION In Leading White Space When the text cursor is at the beginning, in, or immediately to the right of any white space at the beginning of the line, pressing the Tab key causes the entire line to be indented one level. When not in this leading non – white space portion of the line, the Tab key inserts a single tab character. Always Regardless of where the cursor position is within the line, pressing the Tab key causes the line to be re - indented. Never Tab key inserts a tab character. The In Leading White Space setting is the most useful when you ’ re programming in block - structured languages. At the beginning of the line, pressing the Tab key indents the line. Within the line, typically at the end of a statement where you want to begin a comment, the Tab key simply inserts a tab character. “ White space ” refers to the non - graphic, printable characters in the Unicode character set. Practically, that means the tab, space, and end - of - line characters, but, technically, it also includes characters like the non - breaking space (Unicode 0x00A0 ). White space characters get their name from the fact that they cause subsequent characters in the fi le to be shifted to new lines or column positions, but are not themselves visible. In other words, they just leave a white space on the page. Note that the fi rst two settings are not variations on one another. Xcode has two indentation functions: indent and re - indent. Indent increases the indentation level of a line by inserting more white space. Re - indention looks at the indention of the line with respect to the indentation of the line that precedes it, and uses syntax - aware rules to determine what its indentation level should be. Repeatedly indenting a line continues to shift the line further and further to the right. Repeatedly re - indenting a line does nothing. Until you change the contents of the line, or the preceding line, Xcode repeatedly reformats the text to the same indentation level. You can circumvent the settings for the Tab key at any time by using Option + Tab or Control + I. Option + Tab always inserts a tab character, regardless of the syntax - aware tab settings. Control + I always performs a re - indent of the currently selected line, or lines, and is the same as the Edit ➪ Format ➪ Re - indent command. Indenting Solitary “ { ” Returning to the Indentations tab of the Xcode Preferences (see Figure 7 - 7), the Indent Solo “ { ” option determines what happens when an opening brace is the fi rst non – white space character of a line. The option determines how much the line will be indented relative to the previous line. This is independent of the Indent Width setting, but if non - zero it ’ s typically the same value — the Horstmann style illustrates an exception. The following table illustrates some popular coding styles that can benefi t from this option. The table assumes an indent width of 4. c07.indd 139c07.indd 139 1/22/10 12:24:28 PM1/22/10 12:24:28 PM Download at getcoolebook.com 140 ❘ CHAPTER 7 SYNTAX-AWARE EDITING STYLE INDENT SOLO “ { ” EXAMPLE Allman 0 if (a==b) { foo(); } Whitesmiths 4 if (a==b) { foo(); } GNU 2 if (a==b) { foo(); } Horstmann 0 if (a==b) { foo(); } The indentation amount is always relative to the indentation of the previous line, not the indentation level of the line containing the leading { character. In other words, if the Indent Solo “ { ” setting is 4 characters and the line was already indented four characters more than the previous line, a leading { changes nothing. In fact, the indentation level could decrease if the Indent Solo “ { ” setting is lower than the nominal indentation amount for the line. Automatically Close { } Blocks The Automatically Insert Closing “ } ” option does two things when you type a { character: it inserts a line with a matching } ahead of the cursor, and then it advances to the next line as if the Return key had been pressed. The effect is simple: typing a { creates a balanced block with the cursor positioned at the fi rst line of the block. Figure 7 - 9 shows an if statement before and after a single { character was typed. Controlling When Automatic Indentation Occurs The Automatically Indented Characters setting selects the characters that trigger an automatic re - indentation of the current line. When a character in the set is checked, typing one of FIGURE 7-9 c07.indd 140c07.indd 140 1/22/10 12:24:35 PM1/22/10 12:24:35 PM Download at getcoolebook.com those characters is the same as invoking the Edit ➪ Format ➪ Re - indent command after typing the character. Note that this is a re - indent , not an indent; Xcode reinterprets the line using its syntax - aware rules and determines the appropriate indentation level for the line based on its new content. This may, or may not, change the indentation of the line. This re - indentation occurs after the character has been inserted. Thus, the Return key calculates the indentation of the new line it just created, not the line the Return key was pressed on. Indenting Comments The two // Comments options apply to C/C + + - style comments when a line is re - indented. They are not applied when comments are being typed. The Indent // Comments option treats // comments like any other program statement and indents them to the same level as the surrounding code. If off, // comments are left at whatever indentation level the programmer typed them. The Align Consecutive // Comments option indents a // comment that appears alone on a line to the same indentation level as the // comment in the previous line. This is most signifi cant when // comments are started to the right of a program statement and continued in another // comment on the next line. The following three listings illustrate the effects of the two comment formatting options: No // Comment Formatting int main (int argc, char * const argv[]) { // Check the environment char * server_addr = getenv(“SERVER”); if (server_addr==NULL) exit(3); // terminate immediately // returning a status of 3 Indent // Comments Only int main (int argc, char * const argv[]) { // Check the environment char * server_addr = getenv(“SERVER”); if (server_addr==NULL) exit(3); // terminate immediately // returning a status of 3 Indent // Comments and Align Consecutive // Comments int main (int argc, char * const argv[]) { // Check the environment char * server_addr = getenv(“SERVER”); if (server_addr==NULL) Indenting Text ❘ 141 c07.indd 141c07.indd 141 1/22/10 12:24:36 PM1/22/10 12:24:36 PM Download at getcoolebook.com 142 ❘ CHAPTER 7 SYNTAX-AWARE EDITING exit(3); // terminate immediately // returning a status of 3 When you use both comment formatting options, you should get in the habit of inserting a blank line before starting a new C/C + + - style comment on its own line. Otherwise, it may get indented to the level of the comment on the previous line. Re - indenting Text As mentioned earlier, re - indenting text uses the language - specifi c rules to determine the appropriate indentation level for each line. This can happen automatically as you type, depending on which syntax - aware indentation options you have enabled, or manually when you use Control + I or the Edit ➪ Format ➪ Re - indent command. It also happens whenever text is pasted from the clipboard. You can always manually alter the indentation level of a line, or lines, using the Edit ➪ Format ➪ Shift Right (Command + ]) and Edit ➪ Format ➪ Shift Left (Command + [) commands. These either increase or decrease the indentation of every line in the selection by indentation width. Lines that are already at the left margin cannot be decreased any further. This is just a manual adjustment of the amount of white space at the beginning of the line. A subsequent re - indent recalculates the appropriate indentation level based on the language rules, undoing any effect of Shift Right or Shift Left. CODE COMPLETION The most sophisticated feature of syntax - aware editing is Code Completion, Xcode ’ s auto - completion technology. Code completion analyzes the context of the code you are typing and interactively offers suggestions for completing the statement. Code completion supports the C, C + + , Objective - C, Objective - C + + , Java, and AppleScript languages. It works by assembling a list of symbol names from the Code Sense index that would be appropriate at that point in your code. If you are starting a new statement, the appropriate symbols will include any global variable, function, class name, or language keyword, but if you typed a class or structure name, the appropriate symbols would be only the member functions or instance variables in that class or structure. Figure 7 - 10 shows the available completions for the CGSize structure nested inside a CGRect structure. FIGURE 710 c07.indd 142c07.indd 142 1/22/10 12:24:37 PM1/22/10 12:24:37 PM Download at getcoolebook.com . time to toggle the feature off again. c07.indd 134 c07.indd 134 1/22/10 12:24: 23 PM1/22/10 12:24: 23 PM Download at getcoolebook.com Code Folding ❘ 135 Folding Code You have four ways to fold a. source fi les. c07.indd 136 c07.indd 136 1/22/10 12:24:25 PM1/22/10 12:24:25 PM Download at getcoolebook.com Indenting Text ❘ 137 Setting Tab Width and Indent Width Xcode makes a distinction. Info window for the fi le, or by choosing the View ➪ Text ➪ Tab Settings command, both of which are shown in Figure 7 - 8. FIGURE 7-7 FIGURE 7-8 c07.indd 137 c07.indd 137 1/22/10 12:24:25 PM1/22/10