Kotlin for Android - 0x01 有趣的扩展

在 Kotlin 中， 提供了一个很方便的功能，可以在不继承父类，也不使用类似装饰器这样的设计模式的情况下对指定类进行扩展，扩展新的方法、属性。

扩展方法

用法：类名.方法名

例子

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fun Context.toast(message: CharSequence, duration: Int = Toast.LENGTH_SHORT) {
    Toast.makeText(this, message, duration).show()
}

然后 Context 及其子类 都可以使用 toast 方法了,duration 是默认参数，不传值的话默认为 Toast.LENGTH_SHORT

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toast("Hello world!")
toast("Hello world!", Toast.LENGTH_LONG)

给一个类扩展方法并 不会修改原来的类 ，Kotlin 只不过让类的实例可以通过 . 来调用这个方法
这个方法是 静态添加上的，并不会在运行时改变。什么意思呢，看下面的例子：

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open class C
class D: C()

fun C.foo() = "c"
fun D.foo() = "d"

fun printFoo(c: C) {
  println(c.foo()) //这里调用的是C类的foo()
}

printFoo(D()) // 打印出 c

我们声明了参数类型为 C ，那么 printFoo 方法内直接调用的就是 C 类的 **foo()**。而 D() 是具体运行时传入的，扩展函数内调用的还是 C 的方法。

当定义的扩展方法这个类已经存在了同名的扩展方法，这时候是怎么运行的呢？ 看下面的例子：

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class C {
  fun foo() { println("member") }
}

fun C.foo() { println("extension") }

当我们调用 c.foo() , 将会输出 “member”, 而不是 “extension”， 很明显，类内部定义的方法优先 。

扩展属性

用法：类名.属性名

例如

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val View.ctx: Context
    get() = context

这样 View及其子类 都拥有了 ctx属性。

Kotlin for Android 常用的扩展

下面总结一些 Android 中比较常用的扩展。注意是对哪些类进行的扩展。

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// 调用 toast 方法
fun Context.toast(message: String, duration: Int = Toast.LENGTH_SHORT) {
    Toast.makeText(this, message, duration).show()
}

// 获取颜色
fun Context.color(res: Int): Int = ContextCompat.getColor(this, res) // 注意 getResource().getColor 已经不建议使用了

// 获取 LayoutInflater
fun Context.layoutInflater() = LayoutInflater.from(this)

fun Context.isConnected(): Boolean {
    val manager = getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
    return manager.activeNetworkInfo?.isConnectedOrConnecting ?: true
}

// 网络错误
fun Activity.noNetworkSnackBar(parentView: View, retryListener: () -> Unit) {
    Snackbar.make(parentView, R.string.network_issue, Snackbar.LENGTH_INDEFINITE)
            .setAction(R.string.try_again) {
                retryListener.invoke()
            }.show()
}



fun Int.dpToPx(context: Context) = this * context.resources.displayMetrics.density


fun RecyclerView.Adapter<ViewHolder>.createHolder(view: ViewGroup) = object : RecyclerView.ViewHolder(view) {}
fun <T : View> RecyclerView.Adapter<ViewHolder>.getView(holder: ViewHolder) = holder.itemView as T

// View 可见性
fun View.setVisible() {
    this.visibility = View.VISIBLE
}

fun View.setInvisible() {
    this.visibility = View.INVISIBLE
}

fun View.setGone() {
    this.visibility = View.GONE
}

fun View.toggle(isVisible: Boolean) {
    if (isVisible) setVisible() else setGone()
}

fun View.setSize(width: Int, height: Int) {
    if (width <= 0 && height <= 0) return

    val layoutParams = when (this.layoutParams) {
        is LinearLayout.LayoutParams -> this.layoutParams as LinearLayout.LayoutParams
        is FrameLayout.LayoutParams -> this.layoutParams as FrameLayout.LayoutParams
        is RelativeLayout.LayoutParams -> this.layoutParams as RelativeLayout.LayoutParams
        else ->  this.layoutParams
    }
    val ratio = height / width
    layoutParams.height = layoutParams.width * ratio
    this.layoutParams = layoutParams
}

// 弹出snackbar
fun View.snackbar(message: String, duration: Int = Snackbar.LENGTH_SHORT) {
    Snackbar.make(this,message,duration).show()
}

fun View.snackbar(messageRes: Int, duration: Int = Snackbar.LENGTH_SHORT) {
    Snackbar.make(this,messageRes,duration).show()
}

// 任意类打log
fun Any.log(message: String) {
    Log.e(this.javaClass.simpleName, message)
}



fun Context.getActivityManager() = getSystemService(Context.ACTIVITY_SERVICE) as ActivityManager
fun Context.getAlarmManager() = getSystemService(Context.ALARM_SERVICE) as AlarmManager
fun Context.getAudioManager() = getSystemService(Context.AUDIO_SERVICE) as AudioManager
fun Context.getClipboardManager() = getSystemService(Context.CLIPBOARD_SERVICE) as ClipboardManager
fun Context.getConnectivityManager() = getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager
fun Context.getKeyguardManager() = getSystemService(Context.KEYGUARD_SERVICE) as KeyguardManager
fun Context.getLayoutInflater() = getSystemService(Context.LAYOUT_INFLATER_SERVICE) as LayoutInflater
fun Context.getLocationManager() = getSystemService(Context.LOCATION_SERVICE) as LocationManager
fun Context.getNotificationManager() = getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
fun Context.getPowerManager() = getSystemService(Context.POWER_SERVICE) as PowerManager
fun Context.getSearchManager() = getSystemService(Context.SEARCH_SERVICE) as SearchManager
fun Context.getSensorManager() = getSystemService(Context.SENSOR_SERVICE) as SensorManager
fun Context.getTelephonyManager() = getSystemService(Context.TELEPHONY_SERVICE) as TelephonyManager
fun Context.getVibrator() = getSystemService(Context.VIBRATOR_SERVICE) as Vibrator
fun Context.getWallpaperService() = getSystemService(Context.WALLPAPER_SERVICE) as WallpaperService
fun Context.getWifiManager() = getSystemService(Context.WIFI_SERVICE) as WifiManager
fun Context.getWindowManager() = getSystemService(Context.WINDOW_SERVICE) as WindowManager
fun Context.getInputMethodManager() = getSystemService(Context.INPUT_METHOD_SERVICE) as InputMethodManager
fun Context.getAccessibilityManager() = getSystemService(Context.ACCESSIBILITY_SERVICE) as AccessibilityManager
fun Context.getAccountManager() = getSystemService(Context.ACCOUNT_SERVICE) as AccountManager
fun Context.getDevicePolicyManager() = getSystemService(Context.DEVICE_POLICY_SERVICE) as DevicePolicyManager
fun Context.getDropBoxManager() = getSystemService(Context.DROPBOX_SERVICE) as DropBoxManager
fun Context.getUiModeManager() = getSystemService(Context.UI_MODE_SERVICE) as UiModeManager
fun Context.getDownloadManager() = getSystemService(Context.DOWNLOAD_SERVICE) as DownloadManager
fun Context.getStorageManager() = getSystemService(Context.STORAGE_SERVICE) as StorageManager
fun Context.getNfcManager() = getSystemService(Context.NFC_SERVICE) as NfcManager
fun Context.getUsbManager() = getSystemService(Context.USB_SERVICE) as UsbManager
fun Context.getTextServicesManager() = getSystemService(Context.TEXT_SERVICES_MANAGER_SERVICE) as TextServicesManager
fun Context.getWifiP2pManager() = getSystemService(Context.WIFI_P2P_SERVICE) as WifiP2pManager
fun Context.getInputManager() = getSystemService(Context.INPUT_SERVICE) as InputManager
fun Context.getMediaRouter() = getSystemService(Context.MEDIA_ROUTER_SERVICE) as MediaRouter
fun Context.getNsdManager() = getSystemService(Context.NSD_SERVICE) as NsdManager
fun Context.getDisplayManager() = getSystemService(Context.DISPLAY_SERVICE) as DisplayManager
fun Context.getUserManager() = getSystemService(Context.USER_SERVICE) as UserManager
fun Context.getBluetoothManager() = getSystemService(Context.BLUETOOTH_SERVICE) as BluetoothManager
fun Context.getAppOpsManager() = getSystemService(Context.APP_OPS_SERVICE) as AppOpsManager
fun Context.getCaptioningManager() = getSystemService(Context.CAPTIONING_SERVICE) as CaptioningManager
fun Context.getConsumerIrManager() = getSystemService(Context.CONSUMER_IR_SERVICE) as ConsumerIrManager
fun Context.getPrintManager() = getSystemService(Context.PRINT_SERVICE) as PrintManager
fun Context.getAppWidgetManager() = getSystemService(Context.APPWIDGET_SERVICE) as AppWidgetManager
fun Context.getBatteryManager() = getSystemService(Context.BATTERY_SERVICE) as BatteryManager
fun Context.getCameraManager() = getSystemService(Context.CAMERA_SERVICE) as CameraManager
fun Context.getJobScheduler() = getSystemService(Context.JOB_SCHEDULER_SERVICE) as JobScheduler
fun Context.getLauncherApps() = getSystemService(Context.LAUNCHER_APPS_SERVICE) as LauncherApps
fun Context.getMediaProjectionManager() = getSystemService(Context.MEDIA_PROJECTION_SERVICE) as MediaProjectionManager
fun Context.getMediaSessionManager() = getSystemService(Context.MEDIA_SESSION_SERVICE) as MediaSessionManager
fun Context.getRestrictionsManager() = getSystemService(Context.RESTRICTIONS_SERVICE) as RestrictionsManager
fun Context.getTelecomManager() = getSystemService(Context.TELECOM_SERVICE) as TelecomManager
fun Context.getTvInputManager() = getSystemService(Context.TV_INPUT_SERVICE) as TvInputManager
fun Context.getSubscriptionManager() = getSystemService(Context.TELEPHONY_SUBSCRIPTION_SERVICE) as SubscriptionManager
fun Context.getUsageStatsManager() = getSystemService(Context.USAGE_STATS_SERVICE) as UsageStatsManager
fun Context.getCarrierConfigManager() = getSystemService(Context.CARRIER_CONFIG_SERVICE) as CarrierConfigManager
fun Context.getFingerprintManager() = getSystemService(Context.FINGERPRINT_SERVICE) as FingerprintManager
fun Context.getMidiManager() = getSystemService(Context.MIDI_SERVICE) as MidiManager
fun Context.getNetworkStatsManager() = getSystemService(Context.NETWORK_STATS_SERVICE) as NetworkStatsManager



// 扩展属性


val View.ctx: Context
    get() = context

var TextView.textColor: Int
    get() = currentTextColor
    set(v) = setTextColor(v)

inline fun Notification.build(context: Context, func: NotificationCompat.Builder.() -> Unit): Notification {
      val builder = NotificationCompat.Builder(context)
      builder.func()
      return builder.build()
}

其他资源

更多详细用法，查看官网文档
Kotlin 资源汇总

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