Developing a Plugin on iOS

Developing a Plugin on iOS

Writing a plugin requires an understanding of the architecture of Cordova-iOS. Cordova-iOS consists of a UIWebView where intercept commands passed in as url changes. These plugins are represented as class mappings in the Cordova.plist file, under the Plugins key.

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

Plugin Class Mapping

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

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

This will marshal 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 options parameter for the Objective-C plugin method is being deprecated, and it should not be used. For legacy reasons - the last JavaScript object passed in the args Array will be passed in as the options dictionary of the method in Objective-C. You must make sure that any JavaScript object that is passed in as an element in the args array occurs as the last item in the Array, if not it will throw off the array index of all subsequent parameters of the Array in Objective-C. Only one JavaScript object is supported for the options dictionary, and only the last one encountered will be passed to the native method. It is because of these error-prone reasons that they are being deprecated.

The plugin must be added to Plugins key (a Dictionary) of the Cordova.plist file in your Cordova-iOS application's project folder.

<key>service_name</key>
<string>PluginClassName</string>

The key service_name should match what you use in the JavaScript exec call, and the value will be the name of the Objective-C class of the plugin. Without this added, the plugin may compile but will not be reachable by Cordova.

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 Cordova.plist 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. Most method implementations look like this:

- (void) myMethod:(NSMutableArray*)arguments withDict:(NSMutableDictionary*)options
{
    NSString* callbackId = [arguments objectAtIndex:0];

    CDVPluginResult* pluginResult = nil;
    NSString* javaScript = nil;

    @try {
        NSString* myarg = [arguments objectAtIndex:1];

        if (myarg != nil) {
            pluginResult = [CDVPluginResult resultWithStatus:CDVCommandStatus_OK];
            javaScript = [pluginResult toSuccessCallbackString:callbackId];
        } 
    } @catch (id exception) {
        pluginResult = [CDVPluginResult resultWithStatus:CDVCommandStatus_JSON_EXCEPTION messageAsString:[exception reason]];
        javaScript = [pluginResult toErrorCallbackString:callbackId];
    }

    [self writeJavascript:javaScript];
}

Echo Plugin iOS Plugin

We would add the following to the Plugins key (a Dictionary) of the project's Cordova.plist file:

<key>Echo</key>
<string>Echo</string>

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/CDVPlugin.h>

@interface Echo : CDVPlugin

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

@end

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

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

@implementation Echo

- (void) echo:(NSMutableArray*)arguments withDict:(NSMutableDictionary*)options
{
    NSString* callbackId = [arguments objectAtIndex:0];

    CDVPluginResult* pluginResult = nil;
    NSString* javaScript = nil;

    @try {
        NSString* echo = [arguments objectAtIndex:1];

        if (echo != nil && [echo length] > 0) {
            pluginResult = [CDVPluginResult resultWithStatus:CDVCommandStatus_OK messageAsString:echo];
            javaScript = [pluginResult toSuccessCallbackString:callbackId];
        } else {
            pluginResult = [CDVPluginResult resultWithStatus:CDVCommandStatus_ERROR];
            javaScript = [pluginResult toErrorCallbackString:callbackId];
        }
    } @catch (NSException* exception) {
        pluginResult = [CDVPluginResult resultWithStatus:CDVCommandStatus_JSON_EXCEPTION messageAsString:[exception reason]];
        javaScript = [pluginResult toErrorCallbackString:callbackId];
    }

    [self writeJavascript:javaScript];
}

@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 callbackId parameter, which is always the 0th item in the arguments array. Next, we grab the echo string using the objectAtIndex method on our args, telling it we want to get the 1st 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. Then, we convert the PluginResult to JavaScript by calling either the toSuccessCallbackString (if it was OK) or toErrorCallbackString (if it was an error) methods.

Finally we write the JavaScript back to the UIWebView, which will execute the JavaScript that will callback to success or failure callbacks of the exec method on the JavaScript side. If the success callback was called, it will pass the echo parameter as a parameter.

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, you can use Weinre, an Apache Cordova Project or iWebInspector, a third-party utility

Common Pitfalls