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Metacello User Guide

In this guide we'll take a walk through a couple of common development scenarios and highlight some of the features of the Metacello Scripting API.

For installation and more detailed documentation on the Metacello Scripting API, see the Metacello Scripting API Documentation.

Introduction

The number one job of the Metacello Scripting API is to simplify the job of loading projects into your image. As you are probably all too aware, today it's a two step process where you first load the configuration into your image using Gofer and then load your project using Metacello:

Gofer new
  package: 'ConfigurationOfSeaside3';
  smalltalkhubUser: 'Seaside' project: 'Seaside31';
  load.
((Smalltalk at: #ConfigurationOfSeaside3) version: #stable) load.

In the early days of Metacello (and Gofer) this was a great improvement over the alternatives, but today, 3 years after the introduction of Metacello, there should be a better way...and there is. Using the Metacello Scripting API the above expression reduces to the following:

Metacello new
  configuration: 'Seaside3';
  load.

Loading

In this example of the load command we are leveraging a couple of default values, namely the version of the project and the repository where the ConfigurationOfSeaside package can be found:

Metacello new
  configuration: 'Seaside3';
  load.

Here is a variant of the same expression with the default version explicitly specified (the default repository description varies depending upon the platform that you are running on):

Metacello new
  configuration: 'Seaside3';
  version: #stable;
  smalltalkhubUser: 'Seaside' project: 'Seaside31';
  load.

The version attribute can be any legal version number. smalltalkhubUser:project: is a repository shortcut. You can also specify the full repository description as an URL:

Metacello new
  configuration: 'Seaside3';
  version: #stable;
  repository: 'http://smalltalkhub.com/mc/Seaside/Seaside31/main';
  load.

Load Conflicts

Load conflicts occur if any one of the load attributes is changed when doing an indirect load of a project that has already been loaded into your image.

A direct load is a project load occurs when the project is named in the Metacello load expression. In the following expression the version 3.1.1 of Seaside is directly loaded:

Metacello new
  configuration: 'Seaside3';
  version: '3.1.1';
  repository: 'http://smalltalkhub.com/mc/Seaside/Seaside31/main';
  load.

An indirect load is a project load that occurs as a result of a project dependency. In the following expression a version (3.1.3 as of this writing) of Seaside is indirectly loaded:

Metacello new
  configuration: 'Magritte3';
  version: '3.1.4';
  repository: 'http://smalltalkhub.com/mc/Magritte/Magritte3/main';
  load: 'Seaside'.

Load attributes that count towards conflicts are:

  • configuration versus baseline
  • version
  • repository

Version conflicts are divided into two categories: upgrade and downgrade.

Upgrade

When a later version of an already loaded project is indirecty loaded, a MetacelloAllowProjectUpgrade exception is signaled. By default, upgrades are allowed (useIncoming).

If you want to override the default behavior you can use one the following messages:

  1. Disallow the upgrade of the Seaside3 project with the #disallowUpgrades: message:

    Metacello new
      configuration: 'Magritte3';
      version: '3.1.4';
      repository: 'http://smalltalkhub.com/mc/Magritte/Magritte3/main';
      onUpgradeUseLoaded: #('Seaside3');
      load: 'Seaside'.
  2. Dsallow upgrades for all projects with the #disallowUpgrades message:

    Metacello new
      configuration: 'Magritte3';
      version: '3.1.4';
      repository: 'http://smalltalkhub.com/mc/Magritte/Magritte3/main';
      onUpgradeUseLoaded;
      load: 'Seaside'.
  3. Access the underlying exception and the loaded and incoming Metacello regitrations with the #onUpgrade: message:

    Metacello new
      configuration: 'Magritte3';
      version: '3.1.4';
      repository: 'http://smalltalkhub.com/mc/Magritte/Magritte3/main';
      onUpgrade: [:ex :loaded :incoming |
        incoming baseName = 'Seaside3'
          ifTrue: [ ex useLoaded ]
          ifFalse: [ 
            "default"
            ex useIncoming ];
      load: 'Seaside'.

Downgrade

When an earlier version of an already loaded project is indirectly loaded, a MetacelloAllowProjectDowngrade exception is signaled. By default, downgrades are disallowed (useLoaded).

If you want to override the default behavior you can use one of the following expressions:

  1. Allow a downgrade of Seaside3 project with the allowDowngrades: message:

    Metacello new
      configuration: 'Magritte3';
      version: '3.1.4';
      repository: 'http://smalltalkhub.com/mc/Magritte/Magritte3/main';
      onDowngradeUseIncoming: #('Seaside3');
      load: 'Seaside'.
  2. Allow downgrades for all projects with the allowDowngrades message:

    Metacello new
      configuration: 'Magritte3';
      version: '3.1.4';
      repository: 'http://smalltalkhub.com/mc/Magritte/Magritte3/main';
      onDowngradeUseIncoming;
      load: 'Seaside'.
  3. Access the underlying exception and the loaded and incoming Metacello regitrations with the #onDowngrade: message:

    Metacello new
      configuration: 'Magritte3';
      version: '3.1.4';
      repository: 'http://smalltalkhub.com/mc/Magritte/Magritte3/main';
      onDowngrade: [:ex :loaded :incoming |
        incoming baseName = 'Seaside3'
          ifTrue: [ ex useIncoming ]
          ifFalse: [ 
            "default"
            ex useLoaded ];
      load: 'Seaside'.

Conflicts

When the type of project (configuration or baseline) or repository for an already loaded project is changed while doing an indirect load, a MetacelloAllowConflictingProjectUpgrade exception is signaled. By default, a MetacelloConflictingProjectError is signalled if the MetacelloAllowConflictingProjectUpgrade exception is unhandled. As a consequence, you must decide how you want to have the conflict resolved:

  • use the incoming project specification
  • use the loaded project specification

  1. The onConflictUseIncoming:useLoaded: message can be used to specify the names of the projects for which you want to use the incoming or loaded project specifications:

    Metacello new
      configuration: 'Magritte3';
      version: '3.1.4';
      repository: 'http://smalltalkhub.com/mc/Magritte/Magritte3/main';
      onConflictUseIncoming: #('Seaside3') useLoaded: #('Grease');
      load: 'Seaside'.
  2. Use the incoming project specifications for all projects with the onConflictsUseIncoming message:

    Metacello new
      configuration: 'Magritte3';
      version: '3.1.4';
      repository: 'http://smalltalkhub.com/mc/Magritte/Magritte3/main';
      onConflictUseIncoming;
      load: 'Seaside'.
  3. Use the loaded project specifications for all projects with the onConflictsUseLoaded message:

    Metacello new
      configuration: 'Magritte3';
      version: '3.1.4';
      repository: 'http://smalltalkhub.com/mc/Magritte/Magritte3/main';
      onConflictUseLoaded;
      load: 'Seaside'.
  4. Access the underlying exception and the loaded and incoming Metacello regitrations with the #onConflict: message:

    Metacello new
      configuration: 'Magritte3';
      version: '3.1.4';
      repository: 'http://smalltalkhub.com/mc/Magritte/Magritte3/main';
      onConflict: [:ex :loaded :incoming |
        incoming baseName = 'Seaside3'
          ifTrue: [ ex useIncoming ]
          ifFalse: [ 
            "default - throw error"
            ex pass ];
      load: 'Seaside'.

Listing

Once you've loaded one or more projects into your image, you may want to list them. The following is an example of the list command:

Metacello image
  configuration: [:spec | true ];
  list.

The image message tells Metacello that you'd like to look at only loaded configurations.

The block argument to the configuration: message is used to select against the list of loaded MetacelloProjectSpec instances in the registry.

The list command itself returns a list of MetacelloProjectSpec instances that can be printed, inspected or otherwise manipulated.

In addition to a select block, you can specify a select collection specifying the names of the projects you'd like to select:

Metacello registry
  configuration: #('Seaside3' 'MetacelloPreview');
  list.

The registry message tells Metacello that you'd like to look at all projects in the registry whether or not they are loaded.

The collection argument to the configuration: message is used to select against the list of project names in the registry.

The list command can also be used to look at configurations in Monticello repositories. For example:

Metacello new
  configuration: [:spec | spec name beginsWith: 'Seaside'];
  smalltalkhubUser: 'Seaside' project: 'Seaside31';
  list.

lists the configurations whose names (sans the ConfigurationOf) begin with Seaside in the smalltalk hub repostory.

Getting

Once you've loaded a project into your image the next logical step is upgrading your project to a new version.

Let's say that a new #stable version of Seaside3 has been released and that you want to upgrade. This is a two step process:

Get a new version of the configuration

The following expression gets the latest version of the configuration:

Metacello image
  configuration: 'Seaside3';
  get.

By using the image message, you can leverage the fact that the registry remembers from which repository you loaded the original version of the configuration.

The get command simply downloads the latest version of the configuration package from that repository.

You may download the configuration from a different repository:

Metacello image
  configuration: 'Seaside3';
  smalltalkhubUser: 'Seaside' project: 'MetacelloConfigurations';
  get.

The get command will update the registry with the new repository location information.

You may also use the get command to load a configuration for a project into your image without actually loading the project itself:

Metacello image
  configuration: 'Magritte3';
  repository: 'http://smalltalkhub.com/mc/Magritte/Magritte3/main';
  get.

The 'Magritte3' project information will be registered in the registry and marked as unloaded.

Load the new version

Once you've got a new copy of the Seaside30 configuration loaded into your image, you may upgrade your image with the following expression:

Metacello image
  configuration: 'Seaside3';
  version: #stable;
  load.

By using the image message, you are asking Metacello to look the project up in the registry before performing the operation, so it isn't necessary to supply all of the project details for every command operation.

Of course, the load command updates the registry.

If you want to load a project for which you've already done a get (like the Magritte project earlier), you can do the following:

Metacello registry
  configuration: 'Magritte';
  version: #stable;
  load.

In this case you use the registry message to indicate that you are interested in both loaded and unloaded projects.

Locking

Let's say that you are using an older version of Seaside3 (say 3.1.1) instead of the #stable version (3.1.3) and that your application doesn't work with newer versions of Seaside3 (you've tried and it's more work to get you application to work with the newer version of Seaside3 than it's worth).

Let's also say that you want to try out something in the Magritte project, but when you try loading Magritte, you end up having Seaside 3.1.3 loaded as well.

This is an unfortunate side effect of Metacello trying to do the right thing, only in your case it is the wrong thing.

Fortunately, the lock command can give you control. First you need to lock the Seaside3 project:

Metacello image
  configuration: 'Seaside3';
  lock.

The image message tells Metacello to do a lookup in the list of loaded projects and then to put a lock on the loaded version of the project.

If you want you can specify which version of the project you want locked:

Metacello image
  configuration: 'Seaside3';
  version: '3.1.2';
  lock.

After a project is locked Metacello will always use the locked project specification (version and repository) for resolving packages and dependent projects.

If a different version or repository is specified during a load, a Warning is raised notifying you that the locked project specification will be used.

If you don't want an interactive Warning to be raised during your load, you can use onWarningLog to log and resume the Warning:

Metacello registry
  configuration: 'Magritte3';
  onWarningLog;
  load.

If you want to track the use of locks explicitly you can use onLock: which is only triggered when a locked project is involved:

Metacello registry
  configuration: 'Magritte3';
  onLock: [:ex :existing :new | 
    Transcript cr; show: 'Locked project: ', existing projectName printString.
    ex honor];
  load.

Breaking locks

Let's say that when you load the Magritte3 project you have decided that in this particular case you would like to bypass the lock and let the version of Seaside specified by the Magritte3 project to be loaded.

We'll use onLock: to break the lock and allow the new version of the Seaside project to be loaded:

Metacello registry
  configuration: 'Magritte3';
  onLock: [:ex :existing :new | 
    existing baseName = 'Seaside3'
      ifTrue: [ ex break ].
    ex pass ];
  load.

use the message honor if you want to honor the lock and not load the new version.

break is a synonym for allow and honor is a synonym for disallow.

With onLockBreak: you only need to supply the name of the projects for which you want the locks broken:

Metacello registry
  configuration: 'Magritte3';
  onLockBreak: #( 'Seaside3' );
  load.

If you want to break all locks uncoditionally then use onLockBreak:

Metacello registry
  configuration: 'Magritte3';
  onLockBreak;
  load.

When a lock is broken, the project is not unlocked. The lock remains and is applied to the to the freshly loaded version and repository.

Upgrading a locked project

If you want to explicitly upgrade a locked project, you can use the load command. The following command will upgrade Seaside3 to version 3.1.2 even if it is locked:

Metacello image
  configuration: 'Seaside3';
  version: '3.1.2';
  lock.

The newly loaded version of the project will continue to be locked.

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