Picking an Item from the Photo Library

UIImagePickerController的功能：
1.image picker功能主要有两个：一个是捕获picture和medias；一个是弹出一个浏览页，选择一个item

In addition to using a UIImagePickerController instance to capture new pictures and movies, you can use it to present a media browser that lets a user choose an item from their saved photo albums. The steps you take are similar to those for capturing media, as described in Taking Pictures and Movies. The differences are:

• sourceType
• take a picture or movie and save its. present a browser and display its.

Creating and Configuring a Media Browser

• 条件测试（condition test）

  • UIRequiredDeviceCapabilities info.plist

  • isSourceTypeAvailable:

• 实例化（instantiate）

  UIImagePickerController *cameraUI [[UIImagePickerController alloc] init];

• 配置（configure）

  • sourceType

    • UIImagePickerControllerSourceTypePhotoLibrary 可以访问设备的所有相册集，包括有摄像机的设备的相机胶卷

    • UIImagePickerControllerSourceTypeSavedPhotosAlbum 限制范围：带有摄像头的设备的相机胶卷，或没有带有摄像头的设备的相册。

  • mediaType
    check which media types are available by calling the availableMediaTypesForSourceType:，给mediaType设置一个空数组会throw an exception.

  • allowsEditing
    you can custom UI, for example, specify custom cropping.

  • delegate

• 弹出（present）

  • iPhone, iPod touch full-screen present

  • iPad

    • iOS ...-9.0 UIPopoverController

    • iOS 9.0-later UIPopoverPresentationController

• 处理代理方法（conform delegate method）

  • imagePickerController:didFinishPickingMediaWithInfo: