<br />

## Prerequisites

Complete the [custom events setup](https://developers.google.com/ad-manager/mobile-ads-sdk/ios/custom-events/setup.md.txt).

## Request a banner ad

When the custom event line item is reached in the waterfall mediation chain,
the `loadBanner:adConfiguration:completionHandler:` method is called on the
class name you provided when [creating a custom
event](https://developers.google.com/ad-manager/mobile-ads-sdk/ios/custom-events/setup#create). In this case,
that method is in `SampleCustomEvent`, which then calls
the `loadBanner:adConfiguration:completionHandler:` method in
`SampleCustomEventBanner`.

To request a banner ad, create or modify a class that implements
`GADMediationAdapter` and `loadBanner:adConfiguration:completionHandler:`. If a
class that extends `GADMediationAdapter` already exists, implement
`loadBanner:adConfiguration:completionHandler:` there. Additionally, create a
new class to implement `GADMediationBannerAd`.

In our custom event example,
[`SampleCustomEvent`](https://github.com/googleads/googleads-mobile-ios-mediation/blob/main/example/CustomEvent/SampleCustomEvent.m) implements
the `GADMediationAdapter` interface and then delegates to
`SampleCustomEventBanner`.

### Swift

```swift
import GoogleMobileAds

class SampleCustomEvent: NSObject, MediationAdapter {

  fileprivate var bannerAd: SampleCustomEventBanner?
  ...

  func loadBanner(
    for adConfiguration: MediationBannerAdConfiguration,
    completionHandler: @escaping GADMediationBannerLoadCompletionHandler
  ) {
    self.bannerAd = SampleCustomEventBanner()
    self.bannerAd?.loadBanner(
      for: adConfiguration, completionHandler: completionHandler)
  }
}
```

### Objective-C

```objective-c
#import "SampleCustomEvent.h"

@implementation SampleCustomEvent
...

SampleCustomEventBanner *sampleBanner;

- (void)loadBannerForAdConfiguration:
            (GADMediationBannerAdConfiguration *)adConfiguration
                   completionHandler:(GADMediationBannerLoadCompletionHandler)
                                         completionHandler {
  sampleBanner = [[SampleCustomEventBanner alloc] init];
  [sampleBanner loadBannerForAdConfiguration:adConfiguration
                           completionHandler:completionHandler];
}
```

`SampleCustomEventBanner` is responsible for the following tasks:

- Loading the banner ad and invoking a
  [`GADMediationBannerLoadCompletionHandler`](https://developers.google.com/ad-manager/mobile-ads-sdk/ios/api/reference/Protocols/GADMediationAdapter.md.txt) method once
  loading completes.

- Implementing the `GADMediationBannerAd` protocol.

- Receiving and reporting ad event callbacks to Google Mobile Ads SDK.

The optional parameter defined in the Ad Manager UI is
included in the ad configuration.
The parameter can be accessed through
`adConfiguration.credentials.settings[@"parameter"]`. This parameter is
typically an ad unit identifier that an ad network SDK requires when
instantiating an ad object.

### Swift

```swift
class SampleCustomEventBanner: NSObject, MediationBannerAd {
  /// The Sample Ad Network banner ad.
  var bannerAd: SampleBanner?

  /// The ad event delegate to forward ad rendering events to Google Mobile Ads SDK.
  var delegate: MediationBannerAdEventDelegate?

  /// Completion handler called after ad load
  var completionHandler: GADMediationBannerLoadCompletionHandler?

  func loadBanner(
    for adConfiguration: MediationBannerAdConfiguration,
    completionHandler: @escaping GADMediationBannerLoadCompletionHandler
  ) {
    // Create the bannerView with the appropriate size.
    let adSize = adConfiguration.adSize
    bannerAd = SampleBanner(
      frame: CGRect(x: 0, y: 0, width: adSize.size.width, height: adSize.size.height))
    bannerAd?.delegate = self
    bannerAd?.adUnit = adConfiguration.credentials.settings["parameter"] as? String
    let adRequest = SampleAdRequest()
    adRequest.testMode = adConfiguration.isTestRequest
    self.completionHandler = completionHandler
    bannerAd?.fetchAd(adRequest)
  }
}
```

### Objective-C

```objective-c
#import "SampleCustomEventBanner.h"

@interface SampleCustomEventBanner () <SampleBannerAdDelegate,
                                       GADMediationBannerAd> {
  /// The sample banner ad.
  SampleBanner *_bannerAd;

  /// The completion handler to call when the ad loading succeeds or fails.
  GADMediationBannerLoadCompletionHandler _loadCompletionHandler;

  /// The ad event delegate to forward ad rendering events to the Google Mobile
  /// Ads SDK.
  id <GADMediationBannerAdEventDelegate> _adEventDelegate;
}
@end

@implementation SampleCustomEventBanner

- (void)loadBannerForAdConfiguration:
            (GADMediationBannerAdConfiguration *)adConfiguration
                   completionHandler:(GADMediationBannerLoadCompletionHandler)
                                         completionHandler {
  __block atomic_flag completionHandlerCalled = ATOMIC_FLAG_INIT;
  __block GADMediationBannerLoadCompletionHandler originalCompletionHandler =
      [completionHandler copy];

  _loadCompletionHandler = ^id<GADMediationBannerAdEventDelegate>(
      _Nullable id<GADMediationBannerAd> ad, NSError *_Nullable error) {
    // Only allow completion handler to be called once.
    if (atomic_flag_test_and_set(&completionHandlerCalled)) {
      return nil;
    }

    id<GADMediationBannerAdEventDelegate> delegate = nil;
    if (originalCompletionHandler) {
      // Call original handler and hold on to its return value.
      delegate = originalCompletionHandler(ad, error);
    }

    // Release reference to handler. Objects retained by the handler will also
    // be released.
    originalCompletionHandler = nil;

    return delegate;
  };

  NSString *adUnit = adConfiguration.credentials.settings[@"parameter"];
  _bannerAd = [[SampleBanner alloc]
      initWithFrame:CGRectMake(0, 0, adConfiguration.adSize.size.width,
                               adConfiguration.adSize.size.height)];
  _bannerAd.adUnit = adUnit;
  _bannerAd.delegate = self;
  SampleAdRequest *adRequest = [[SampleAdRequest alloc] init];
  adRequest.testMode = adConfiguration.isTestRequest;
  [_bannerAd fetchAd:adRequest];
}
```

Whether the ad is successfully fetched or encounters an error, you
would call `GADMediationBannerLoadCompletionHandler`. In the event of success,
pass through the class that implements `GADMediationBannerAd` with a `nil` value
for the error parameter; in the event of failure, pass through the error you
encountered.

Typically, these methods are implemented inside callbacks from the
third-party SDK your adapter implements. For this example, the Sample SDK
has a `SampleBannerAdDelegate` with relevant callbacks:

### Swift

```swift
func bannerDidLoad(_ banner: SampleBanner) {
  if let handler = completionHandler {
    delegate = handler(self, nil)
  }
}

func banner(
  _ banner: SampleBanner, didFailToLoadAdWith errorCode: SampleErrorCode
) {
  let error =
    SampleCustomEventUtilsSwift.SampleCustomEventErrorWithCodeAndDescription(
      code: SampleCustomEventErrorCodeSwift
        .SampleCustomEventErrorAdLoadFailureCallback,
      description:
        "Sample SDK returned an ad load failure callback with error code: \(errorCode)"
    )
  if let handler = completionHandler {
    delegate = handler(nil, error)
  }
}
```

### Objective-C

```objective-c
- (void)bannerDidLoad:(SampleBanner *)banner {
  _adEventDelegate = _loadCompletionHandler(self, nil);
}

- (void)banner:(SampleBanner *)banner
    didFailToLoadAdWithErrorCode:(SampleErrorCode)errorCode {
  NSError *error = SampleCustomEventErrorWithCodeAndDescription(
      SampleCustomEventErrorAdLoadFailureCallback,
      [NSString stringWithFormat:@"Sample SDK returned an ad load failure "
                                 @"callback with error code: %@",
                                 errorCode]);
  _adEventDelegate = _loadCompletionHandler(nil, error);
}
```

`GADMediationBannerAd` requires implementing a `UIView` property:

### Swift

```swift
var view: UIView {
  return bannerAd ?? UIView()
}
```

### Objective-C

```objective-c
- (nonnull UIView *)view {
  return _bannerAd;
}
```

## Forward mediation events to Google Mobile Ads SDK

Once you've called `GADMediationBannerLoadCompletionHandler` with a loaded ad,
the returned `GADMediationBannerAdEventDelegate` delegate object can then be
used by the adapter to forward presentation events from the third-party SDK to
Google Mobile Ads SDK. The `SampleCustomEventBanner` class implements the
`SampleBannerAdDelegate` protocol to forward callbacks from the sample ad
network to Google Mobile Ads SDK.

It's important that your custom event forwards as many of these callbacks as
possible, so that your app receives these equivalent events from Google Mobile Ads SDK.
Here's an example of using callbacks:

### Swift

```swift
func bannerWillLeaveApplication(_ banner: SampleBanner) {
  delegate?.reportClick()
}
```

### Objective-C

```objective-c
- (void)bannerWillLeaveApplication:(SampleBanner *)banner {
  [_adEventDelegate reportClick];
}
```

This completes the custom events implementation for banner ads. The full example
is available on
[GitHub](https://github.com/googleads/googleads-mobile-ios-mediation/tree/main/example/CustomEvent).
You can use it with an ad network that is already supported or modify it to
display custom event banner ads.