Add more documentation for plugins

pkg/analyzer_plugin/doc/tutorial/introduction.md

+# Introduction
+
+The purpose of this page is to give you an overview of what an analyzer plugin
+is and what it can do.
+
+## What is a plugin?
+
+An analyzer plugin is a piece of code that communicates with the analysis server
+to provide additional analysis support. The additional support is often specific
+to a package or set of packages. For example, there is a plugin that provides
+analysis specific to the Angular framework. Plugins are not required to be
+specific to a package, but if the additional analysis is general enough, we
+would urge you to consider contributing it back to the Dart project so that
+everyone can more easily benefit from your work.
+
+Plugins are written in Dart. They are executed by the analysis server by running
+them in the same VM as the analysis server but each plugin is run in a separate

[devoncarew, 2017/08/01 22:19:16]

analysis server, but

[Brian Wilkerson, 2017/08/02 14:45:16]

Done

+isolate. The analysis server communicates with the plugins using a wire protocol
+that is specified in the [plugin API][pluginapi] document. This API is similar
+to the API used by the analysis server to communicate with clients.
+
+The API consists of three kinds of communication. When the analysis server needs
+information from the plugin, or needs to pass information to the plugin, it
+sends a *request*. The plugin is required to answer every request with a
+*response*. If the request was a request for information, then the response will
+contain the requested information. Otherwise, the response is merely an
+acknowledgement that the request was received. In addition, the plugin can send
+a *notification* to the server to provide information to the server.
+
+## What can plugins do?
+
+The scope of what a plugin can do is defined by the [plugin API][pluginapi], but
+it's useful to start with a high level overview.
+
+### Lifecycle management
+
+The API includes support for managing the lifecycle of a plugin. There is no
+guarantee about when plugins will be started or stopped relative to either the
+server or to each other.
+
+When a plugin is first started, the analysis server will send a
+`plugin.versionCheck` request to the plugin to verify that the plugin is using
+the same version of the API as the server and therefore can communicate with the
+server. This exchange also serves to communicate some other information between
+the two participants.
+
+When the server is asked to shut down, it will send a `plugin.shutdown` request
+to the plugin to shut it down. This gives the plugin an opportunity to release
+system resources or perform any other necessary actions. If a plugin encounters
+an error that causes it to need to shut down, it should send a `plugin.error`
+notification to the server to indicate that it is doing so.
+
+### Managing analysis
+
+The API includes support for managing which files are analyzed. There is no
+requirement for when a plugin should perform the analysis, but to optimize the
+user experience plugins should provide information to the server as quickly as
+possible.
+
+The analysis server sends an `analysis.setContextRoots` request to plugins to
+tell them which files to analyze. Each `ContextRoot` indicates the root
+directory containing the files to be analyzed (included) and any files and
+directories within the root that should *not* be analyzed (excluded). Plugins
+can read and use excluded files in the process of analyzing included files, but
+should not report results for excluded files.
+
+In order to improve the user experience, the analysis server will send an
+`analysis.setPriorityFiles` request to specify which files should take priority
+over other files. These are typically the files that are open and visible in the
+client.
+
+The analysis server will send an `analysis.handleWatchEvents` request to the
+plugin when one or more files (within a context root) have been modified. The
+plugin is expected to re-analyze those files in order to update the results for
+those files.
+
+The analysis server will send an `analysis.updateContent` request when the user
+has edited a file but the edited content has not yet been written to disk. This
+allows the plugin to provide analysis results as the user is typing.
+
+### Requesting Analysis Results
+
+In order to accommodate the workflow of clients, there are two ways for the
+server to request analysis results from a plugin.
+
+First, the server can send a request to request specific results for a specific
+file. This is typically used for client-side functionality that the user has to
+explicitly request and that clients will not retain long term. For example,
+there is a request to get code completion suggestions. These requests are
+discussed below.
+
+For functionality that is always available, or for results that can change
+without the client being aware that new data should be requested, there is a
+subscription model. The server will send an `analysis.setSubscriptions` request
+to the plugin. This request tells the plugin which results should be sent to the
+server when the results become available. The plugin does not send any results
+in the response to the request, but instead is expected to send a notification
+to the server when the results have been computed. The notifications that can be
+requested are also discussed below.
+
+If the server has explicitly requested results, either by a request or by a
+subscription, the plugin should provide those results even if the file is an
+excluded file. This exception to the general rule does *not* apply to the
+implicit subscription for diagnostics.
+
+Plugins should *not* send analysis results that duplicate the information
+computed by the analysis server itself. The expectation is for plugins to
+extend this information, not replicate it.
+
+### Diagnostics
+
+Plugins can generate diagnostics to make users aware of problems in the code
+that they have written. Diagnostics are typically displayed in the editor region
+and might also be displayed in a separate diagnostics view or as decorations on
+a directory structure view.
+
+Plugins are expected to send any diagnostics that they generate for any of the
+analyzed files (files that are included in a context root and not excluded).
+Essentially, there is an implicit subscription for errors for all (non-excluded)
+files. The plugin should send the errors in an `analysis.errors` notification.
+
+### Semantic highlighting
+
+Highlight information is used to color the content of the editor view.
+
+If the server has subscribed for highlighting information in some set of files,
+then the plugin should send the information in an `analysis.highlights`
+notification whenever the information needs to be updated.
+
+### Navigation
+
+Navigation information is used by clients to allow users to navigate to the
+location at which an identifier is defined.
+
+Navigation information can be requested both by an `analysis.getNavigation`
+request and by a subscription. If the server has subscribed for navigation
+information in some set of files, the the plugin should send the information in
+an `analysis.navigation` notification whenever the information needs to be
+updated.
+
+There is a tutorial explaining how to implement [navigation][navigation].
+
+### Mark occurrences
+
+Occurrences information is used by clients to highlight (or mark) all uses
+within a given file of a single identifier when the user selects one use of that
+identifier.
+
+If the server has subscribed for occurrences information in some set of files,
+then the plugin should send the information in an `analysis.occurrences`
+notification whenever the information needs to be updated.
+
+### Outline
+
+Outline information is typically used by clients to provide a tree indicating
+the nesting structure of declarations within the code.
+
+If the server has subscribed for outline information in some set of files, then
+the plugin should send the information in an `analysis.outline` notification
+whenever the information needs to be updated.
+
+### Folding
+
+Folding information is used to allow users to collapse regions of text.
+
+If the server has subscribed for folding information in some set of files, then
+the plugin should send the information in an `analysis.folding` notification
+whenever the information needs to be updated.
+
+### Code completion
+
+Code completion suggestions are used to provide possible completions at some
+point in the text.
+
+When the client request completion suggestions, the server will send a
+`completion.getSuggestions` request. The plugin should only send suggestions
+that would not also be returned by the server.
+
+There is a tutorial explaining how to implement [code completion][completion].
+
+### Fixes and assists
+
+Fixes and assists are a set of edits that users can choose to have applied to
+the code. They differ from a refactoring in that they cannot request additional
+information from the user. For example, a rename refactoring needs to know the
+new name, which requires prompting the user, and hence could not be implemented
+as either a fix or an assist.
+
+Fixes are associated with specific diagnostics, and hence should only be
+generated if the diagnostics with which they are associated have been generated.
+For example, if a diagnostic has been produced to indicate that a required
+semicolon is missing, a fix might be generated to insert a semicolon.
+
+The analysis server will request fixes by sending an `edit.getFixes` request.
+
+Plugins should provide fixes for as many of the diagnostics they generate as
+possible, but only when those fixes provide value to the user. (For example, a
+fix to insert a semicolon is arguably harder to use than simply typing the
+semicolon would be, and therefore is of questionable value.)
+
+There is a tutorial explaining how to implement [fixes][fixes].
+
+Assists are generally context-specific and hence should only be generated if the
+cursor is in the right context. For example, if there is an assist to convert an
+expression-style function body (one introduced by `=>`) into a block-style
+function body, it should only be generated if the cursor is within an
+expression-style function body.
+
+The analysis server will request assists by sending an `edit.getAssists`
+request.
+
+There is a tutorial explaining how to implement [assists][assists].
+
+[assists]: assists.md
+[completion]: completion.md
+[fixes]: fixes.md
+[navigation]: navigation.md
+[pluginapi]: https://htmlpreview.github.io/?https://github.com/dart-lang/sdk/blob/master/pkg/analyzer_plugin/doc/api.html