Question

This section provides details for how to implement native plugin code on the iOS platform. Before reading this, see [Plugin Development Guide][plugin-dev] for an overview of the plugin's structure and its common JavaScript interface. This section continues to demonstrate the sample echo plugin that communicates from the Cordova webview to the native platform and back.

An iOS plugin is implemented as an Objective-C class that extends the CDVPlugin class. For JavaScript's exec method's service parameter to map to an Objective-C class, each plugin class must be registered as a <feature> tag in the named application directory's config.xml file.

Plugin Class Mapping

The JavaScript portion of a plugin 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, effectively calling the action method on the service class, with the arguments passed in the args array.

Specify the plugin as a <feature> tag in your Cordova-iOS application's project's config.xml file, using the plugin.xml file to inject this markup automatically, as described in [Plugin Development Guide][plugin-dev]:

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

The feature's name attribute should match what you specify as the JavaScript exec call's service parameter. The value attribute should match the name of the plugin's Objective-C class. The <param> element's name should always be ios-package. If you do not follow these guidelines, the plugin may compile, but Cordova may still not be able to access it.

Plugin Initialization and Lifetime

One instance of a plugin object is created for the life of each UIWebView. Plugins are not instantiated until they are first referenced by a call from JavaScript, unless <param> with an onload name attribute is set to "true" in config.xml. For example,

<feature name="Echo">
    <param name="ios-package" value="Echo" />
    <param name="onload" value="true" />
</feature>

Plugins should use the pluginInitialize method for their startup logic.

Plugins with long-running requests or background activities such as media playback, listeners, or that maintain internal state should implement the onReset method to cancel those long-running requests or to clean up after those activities. The method runs when the UIWebView navigates to a new page or refreshes, which reloads the JavaScript.

Writing an iOS Cordova Plugin

A JavaScript call fires off a plugin request to the native side, and the corresponding iOS Objective-C plugin is mapped properly in the config.xml file, but what does the final iOS Objective-C plugin class look like? Whatever is dispatched to the plugin with JavaScript's exec function is 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];
}

For more details, see CDVInvokedUrlCommand.h, CDVPluginResult.h, and CDVCommandDelegate.h.

iOS CDVPluginResult Message Types

You can use CDVPluginResult to return a variety of result types back to the JavaScript callbacks, using class methods that follow this pattern:

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

You can create String, Int, Double, Bool, Array, Dictionary, ArrayBuffer, and Multipart types. You can also leave out any arguments to send a status, or return an error, or even choose not to send any plugin result, in which case neither callback fires.

Note the following for complex return values:

• messageAsArrayBuffer expects NSData* and converts to an ArrayBuffer in the JavaScript callback. Likewise, any ArrayBuffer the JavaScript sends to a plugin are converted to NSData*.

• messageAsMultipart expects an NSArray* containing any of the other supported types, and sends the entire array as the arguments to your JavaScript callback. This way, all of the arguments are serialized or deserialized as necessary, so it is safe to return NSData* as multipart, but not as Array/Dictionary.

Echo iOS Plugin Example

To match the JavaScript interface's echo feature described in Application Plugins, use the plugin.xml to inject a feature specification to the local platform's config.xml file:

<platform name="ios">
    <config-file target="config.xml" parent="/*">
        <feature name="Echo">
            <param name="ios-package" value="Echo" />
        </feature>
    </config-file>
</platform>

Then we would add the following Echo.h and Echo.m files to the Plugins folder within the Cordova-iOS application directory:

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

#import <Cordova/CDVPlugin.h>

@interface Echo : CDVPlugin

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

@end

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

#import "Echo.h"
#import <Cordova/CDVPlugin.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

The necessary imports at the top of the file extends the class from CDVPlugin. In this case, the plugin only supports a single echo action. It obtains the echo string by calling the objectAtIndex method get the first parameter of the arguments array, which corresponds to the arguments passed in by the JavaScript exec() function.

It checks the parameter to make sure it is not nil or an empty string, returning a PluginResult with an ERROR status if so. If the parameter passes the check, it returns a PluginResult with an OK status, passing in the original echo string. Finally, it sends 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.

iOS Integration

The CDVPlugin class features other methods that your plugin can override. For example, you can capture the pause, resume, app terminate and handleOpenURL events. See the CDVPlugin.h and CDVPlugin.m classes for guidance.

Threading

Plugin methods ordinarily execute in the same thread as the main interface. 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];
    }];
}

Debugging iOS Plugins

To debug on the Objective-C side, you need Xcode's built-in debugger. For JavaScript, you can attach Safari to the app running within the iOS Simulator/Device.

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, as described in Domain Whitelist Guide. If you forget, an error is logged in the Xcode console.