Lập trình Wrox Professional Xcode 3 cho Mac OS part 26 potx

You can revert your changes to any previous snapshot on a change - by - change basis, by fi le, or perform a wholesale restore of the entire project... Each snapshot has a name and an op

Snapshots

WHAT'S IN THIS CHAPTER?

Taking snapshots of your entire project Reviewing snapshots and comparing them with your working fi les Managing and restoring snapshots

If you ’ re like me, you hate to throw away code Most code, especially the stuff that works,

is a hard - won commodity that represents time, effort, expertise, and creativity The best way to improve your code, however, is to experiment with new approaches — which often means abandoning or discarding the code you ’ ve already written Making copies of your source fi les or saving snippets of old code as comments is both awkward and time consuming

Enter snapshots Snapshots let you have it both ways: you can keep your existing code while simultaneously replacing it with new, experimental code

In simple terms, a snapshot is an archived copy of the source fi les in your project It quickly

preserves their current state so you are free to make changes to your project — even radical ones — secure in the knowledge that you can always revert back to the saved version at any time You can make as many snapshots as you like You can easily see what changes you ’ ve made, as shown in Figure 11 - 1, by comparing your working project fi les to those in a snapshot You can revert your changes to any previous snapshot on a change - by - change basis,

by fi le, or perform a wholesale restore of the entire project

➤

➤

➤

11

Snapshots encourage experimentation by making it easy to save and contrast existing code with new

code There ’ s very little risk in rewriting a collection loop to improve performance when Xcode will

preserve the previous implementation Take a snapshot, rewrite the loop, and test its performance Is

it an improvement? Keep the new code Not an improvement? Restore the saved snapshot and move

on or try a different approach It ’ s that simple

Snapshots resemble a source control system, but shouldn ’ t be treated as one Each snapshot is

(conceptually) a complete and independent copy of your entire project Snapshots are not deltas,

so they won ’ t preserve a history or audit trail of your changes — although you can compare two

snapshots Snapshots are cached on your local fi lesystem; they cannot be transferred to another

user, nor is any snapshot information preserved in the project itself Snapshots are intended to be

a spontaneous method of marking your progress Snapshots can be used independently of, or in

addition to, the source control management features See the “ Source Control vs Snapshots ” section

of Chapter 21 about issues that can arise when mixing snapshots and source control operations

TAKING SNAPSHOTS

You have four ways of taking a snapshot:

Choose the File ➪ Make Snapshot (Control+Command+S) command Add a snapshot from the snapshot window

Take a snapshot via the refactoring tool Restore a snapshot

➤

➤

➤

➤

FIGURE 11 - 1

Snapshots occur immediately; there are no dialogs or other overt indications that a snapshot was taken Each snapshot has a name and an optional description; but when initially added all snapshots are given a generic name that describes how the snapshot was made (menu command, snapshot window, refactor transformation, or restore) and the date If you want to make a snapshot memorable, rename it or add a comment to the freshly created snapshot in the snapshot window (File ➪ Snapshots)

When Xcode takes a snapshot, it preserves the project source fi les contained

within your project folder If you have added source items to your project whose

fi les are located outside of your project folder, those fi les will not be included

in the snapshot If you want resources outside of your project folder (like other projects) to be included in your snapshots, consider relocating your project’s root folder (see Chapter 21) or use the snapshot feature of the organizer (see Chapter 22).

The minimal user interaction is intended to make snapshots a seamless part of your development workfl ow, not another interruption to it On modern computer systems, snapshots are fast and inexpensive Get into the habit of taking a snapshot before starting any atomic change to your project Taking snapshots should become second nature, just as many developers save their fi les at strategic times You will want to visit your snapshots occasionally to dispose of old and intermediate snapshots

MANAGING SNAPSHOTS

Snapshots are managed in the snapshot window (File ➪ Snapshots) The snapshot window is typically collapsed and shows only the summary of snapshots, as shown in Figure 11 - 2 Click the Show Files button in the toolbar to display change information that was shown in Figure 11 - 1

The snapshot window is the central interface for browsing, commenting, examining, comparing, and deleting your snapshots These actions are explained in the following sections

Commenting on Snapshots

Snapshots are initially named by the method they were

and when If you want to give a snapshot a more memorable

name, or make some notes about it, select the snapshot and

edit the information as shown in Figure 11 - 3

Remember that the snapshot represents the project as it

was before you make any additional changes Make your

comments from the perspective of how the project was (for

example, “ Performance baseline before any optimization ” ),

not how it will be (for example, “ Adding new slider

animation ” ) That way, the comments will make

more sense later

Examining Snapshots

You can examine the differences between a snapshot and the current state of your project by

selecting a snapshot and expanding the snapshot window, as was shown in Figure 11 - 1

The right side of the snapshots window will contain one or two panes:

A list of fi le changes

A differences browser When you select a snapshot, the fi les that have been changed, added, or removed to the project

since that snapshot was taken are listed on the right Selecting a fi le in the list presents a differences

viewer pane that shows the specifi c changes that were made Both are shown in Figure 11 - 4

➤

➤

FIGURE 11-4

FIGURE 11-3

When you select a single snapshot, the differences shown are all the changes that have occurred since that snapshot was taken

If you select exactly two snapshots, as shown in Figure 11 - 5, the fi le list shows the differences between those two snapshots

FIGURE 11-5

Restoring Changes

You can discard your changes and recover your original code from a snapshot in one of three ways

Restore Individual Changes

To recover an individual change, locate the change by selecting the snapshot and a fi le Locate the original text in the differences browser The original (snapshot) version will be on the left and your current version will be on the right, although you can change this in the Xcode preferences

Select and copy the original code to the clipboard, open the fi le in your project, and paste over the modifi ed code

The differences viewer can normally update the fi les directly, exchanging or combining changes directly For some reason, this feature is disabled in the snapshots window

Restore Individual Files

To restore all of the changes made to an individual fi le, and this includes adding and removing

a fi le, select the snapshot, select one or more fi les in the fi le list, and click the Restore button

in the toolbar

The snapshot fi les immediately replace those in the project If the change entailed removing a fi le,

that fi le is added back to the project If the change was adding a fi le, that fi le is deleted and removed

from the project

Restore an Entire Project

To restore an entire project, select the snapshot, but don ’ t select any fi les Clicking the Restore

button restores the entire project and all fi les to the state preserved in the snapshot

Undoing a Restore

If restoring your entire project from a snapshot — without any warning or confi rmation — sounds

hazardous, it ’ s not Every restore fi rst creates a pre - restore snapshot of your project, forming a

sort of redo stack If you performed any kind of restore by mistake, fi nd the pre - restore snapshot

and restore that Yes, this will create another pre - restore snapshot To discard your changes

permanently, restore from a snapshot and then delete the pre - restore snapshot created by the restore

DELETING SNAPSHOTS

Deleting a snapshot is the one truly destructive

snapshot action To delete one or more snapshots,

select them in the list and click the Delete button

in the toolbar Xcode confi rms that you want to delete

the selected snapshots, as shown in Figure 11 - 6

If you want to delete all snapshots for every project

at once, follow these steps:

Quit Xcode Locate and discard the fi le ~/Library/

Application Support/Developer/Shared/

SnapshotRepository.sparseimage

SUMMARY

Taking snapshots encourages experimentation, spontaneously illustrates your changes, and reduces

the chance of errors should you decide to revert to a previous incarnation of your code Using

➤

➤

FIGURE 11-6