# PhoneGap Documentation

# Developing a Plugin on iOS

A plugin is an Objective-C class that extends the `CDVPlugin` class.

Each plugin class must be registered using the config.xml file, as a <plugin> tag under the <plugins> key.

## Plugin Class Mapping

The JavaScript portion of a plugin always uses the `cordova.exec` method as follows:

    exec(<successFunction>, <failFunction>, <service>, <action>, [<args>]);

This marshals a request from the `UIWebView` to the iOS native side, more or less boiling down to calling the `action` method on the `service` class, with the arguments passed in the `args` array.

The plugin must be added under the `<plugins>` tag of your Cordova-iOS application's project's `config.xml` file.

    <feature name="LocalStorage">
        <param name="ios-package" value="CDVLocalStorage" />
    </feature>

The feature name='name' should match what you use in the JavaScript `exec` call, and the value matches the name of the plugin's Objective-C class. param name should always be "ios-package". Otherwise the plugin may compile but would not be reachable by Cordova.

## Plugin Initialization and Lifetime

There is one instance of a plugin object that is created per-UIWebView, and the lifetime of the instance is tied to the UIWebView. Plugins are not instantiated until they are first referenced by a call from JS, unless the `onload` attribute set within config.xml. E.g.:

    <plugin name="Echo" value="Echo" onload="true" />

There is no designated initializer for plugins. Instead, plugins should use the `pluginInitialize` method for their start-up logic.

Plugins with long-running requests, background activity (e.g. playing media), listeners or internal state should implement the `onReset` method and stop or clean up those activities. This method is run when the `UIWebView` navigates to a new page or refreshes, which reloads the JavaScript.

## Writing an iOS Cordova Plugin

We have JavaScript fire off a plugin request to the native side. We have the iOS Objective-C plugin mapped properly via the `config.xml` file. So what does the final iOS Objective-C Plugin class look like?

What gets dispatched to the plugin via JavaScript's `exec` function gets passed into the corresponding Plugin class's `action` method. A plugin method has this signature:

    - (void)myMethod:(CDVInvokedUrlCommand*)command
    {
        CDVPluginResult* pluginResult = nil;
        NSString* myarg = [command.arguments objectAtIndex:0];

        if (myarg != nil) {
            pluginResult = [CDVPluginResult resultWithStatus:CDVCommandStatus_OK];
        } else {
            pluginResult = [CDVPluginResult resultWithStatus:CDVCommandStatus_ERROR messageAsString:@"Arg was null"];
        }
        [self.commandDelegate sendPluginResult:pluginResult callbackId:command.callbackId];
    }

1. CDVInvokedUrlCommand.h
2. CDVPluginResult.h
3. CDVCommandDelegate.h

## iOS CDVPluginResult message types

Using CDVPluginResult you can return a variety of result types back to your JavaScript callbacks, using class methods that look like:

    + (CDVPluginResult*)resultWithStatus:(CDVCommandStatus)statusOrdinal messageAs...

You can create `String`, `Int`, `Double`, `Bool`, `Array`, `Dictionary`, `ArrayBuffer`, and `Multipart` types. Or, don't attach any arguments (just send a status). Or, return an Error. You can even choose to not send any plugin result at all, in which case the callback does not fire.

### Notes

- `messageAsArrayBuffer` expects `NSData*` and converts to an `ArrayBuffer` for your JavaScript callback (and `ArrayBuffers` sent to a plugin from JavaScript are converted to `NSData*`).
- `messageAsMultipart` expects an `NSArray*` containing any of the other supported types, and sends the whole array as the `arguments` to your JavaScript callback.
  - Quirk: this is not just syntactic sugar (though it is sweet). This way, all of the arguments are serialized/deserialized as necessary. e.g. it is safe to return `NSData*` as multipart, but not as `Array`/`Dictionary`.

## Plugin Signatures

The new signature supported beginning in Cordova 2.1.0 is:

        - (void)myMethod:(CDVInvokedUrlCommand*)command;

The old (deprecated) signature is:

        - (void)myMethod:(NSMutableArray*)arguments withDict:(NSMutableDictionary*)options;

Basically, the options dictionary has been removed for the new signature, and the callbackId is not the 0th index item for the arguments array, but it is now in a separate property.

## Echo Plugin iOS Plugin

We would add the following to the `<plugins>` tag of the project's `config.xml` file:

    <plugin name="Echo" value="Echo" />

Then we would add the following files (`Echo.h` and `Echo.m`) to the Plugins folder inside our Cordova-iOS application folder:

    /********* Echo.h Cordova Plugin Header *******/

    #import <Cordova/CDV.h>

    @interface Echo : CDVPlugin

    - (void)echo:(CDVInvokedUrlCommand*)command;

    @end

    /********* Echo.m Cordova Plugin Implementation *******/

    #import "Echo.h"
    #import <Cordova/CDV.h>

    @implementation Echo

    - (void)echo:(CDVInvokedUrlCommand*)command
    {
        CDVPluginResult* pluginResult = nil;
        NSString* echo = [command.arguments objectAtIndex:0];

        if (echo != nil && [echo length] > 0) {
            pluginResult = [CDVPluginResult resultWithStatus:CDVCommandStatus_OK messageAsString:echo];
        } else {
            pluginResult = [CDVPluginResult resultWithStatus:CDVCommandStatus_ERROR];
        }

        [self.commandDelegate sendPluginResult:pluginResult callbackId:command.callbackId];
    }

    @end

Let's take a look at the code. At the top we have all of the necessary Cordova imports. Our class extends from `CDVPlugin` - very important.

This plugin only supports one action, the `echo` action. First, we grab the echo string using the `objectAtIndex` method on our `args`, telling it we want to get the 0th parameter in the arguments array. We do a bit of parameter checking: make sure it is not `nil`, and make sure it is not a zero-length string.

If it is, we return a `PluginResult` with an `ERROR` status. If all of those checks pass, then we return a `PluginResult` with an `OK` status, and pass in the `echo` string we received in the first place as a parameter.

Finally, we send the result to `self.commandDelegate`, which executes the `exec` method's success or failure callbacks on the JavaScript side. If the success callback is called, it passes in the `echo` parameter.

## Threading

Plugin methods are executed in the same thread as the UI. If your plugin requires a great deal of processing or requires a blocking call, you should use a background thread. For example:

    - (void)myPluginMethod:(CDVInvokedUrlCommand*)command
    {
        // Check command.arguments here.
        [self.commandDelegate runInBackground:^{
            NSString* payload = nil;
            // Some blocking logic...
            CDVPluginResult* pluginResult = [CDVPluginResult resultWithStatus:CDVCommandStatus_OK messageAsString:payload];
            // The sendPluginResult method is thread-safe.
            [self.commandDelegate sendPluginResult:pluginResult callbackId:command.callbackId];
        }];
    }

## Advanced Plugin Functionality

See other methods that you can override in:

1. CDVPlugin.h
2. CDVPlugin.m

For example, you can hook into the `pause`, `resume`, app terminate and `handleOpenURL` events.

## Debugging Plugins

To debug the Objective-C side, you would use Xcode's built-in debugger. For JavaScript, on iOS 5.0 you can use Weinre, an Apache Cordova Project or iWebInspector, a third-party utility

For iOS 6, you would use Safari 6.0 to simply attach to your app running in the iOS 6 Simulator.

## Common Pitfalls

- Don't forget to add your plugin's mapping to config.xml. If you forget, an error is logged in the Xcode console.
- Don't forget to add any hosts you connect to in the whitelist. If you forget, an error is logged in the Xcode console.

## Deprecated Plugin Signature Note

The old (deprecated) signature is:

        - (void) myMethod:(NSMutableArray*)arguments withDict:(NSMutableDictionary*)options;

The Objective-C plugin method's `options` parameter is deprecated, and should not be used. For legacy reasons, the last JavaScript object passed in the `args` array is passed in as the `options` dictionary of the Objective-C method. You must make sure that any JavaScript object that is passed in as an element in the `args` appears as the last item in the array. Otherwise it throws off the array index for all subsequent parameters of the Objective-C array. Only one JavaScript object is supported for the options dictionary, and only the last one encountered will be passed to the native method. Errors stemming from this problem are the reason `options` has been deprecated.