kotlin 扩展属性

与扩展方法类似，扩展属性当然也不是真正添加进了原类。

扒开糖衣看看kotlin扩展属性实现原理。

给Date类添加formatDate属性

val sdf = SimpleDateFormat("yyyy-MM-dd HH:mm:ss", Locale.getDefault())
val Date.formatDate: String
    get() = sdf.format(this.time)

调用处

val formatDate = Date().formatDate

反编译成java

@NotNull
public static final SimpleDateFormat getSdf() {
      return sdf;
}

@NotNull
public static final String getFormatDate(@NotNull Date $receiver) {
      Intrinsics.checkParameterIsNotNull($receiver, "receiver$0");
      String var10000 = sdf.format($receiver.getTime());
      Intrinsics.checkExpressionValueIsNotNull(var10000, "sdf.format(this.time)");
      return var10000;
}

静态方法，将调用者当做参数传入。好处就是和扩展方法一样，写起来像是原类调用。

还可以定义var可变类型的扩展属性

var TextView.value: CharSequence
    get() = this.text
    set(value) {
        this.text = value
    }

注意set(value) 本质上没有地方存值，还是在操作this原类的属性。反编译成java，一目了然。

@NotNull
   public static final CharSequence getValue(@NotNull TextView $receiver) {
      Intrinsics.checkParameterIsNotNull($receiver, "receiver$0");
      CharSequence var10000 = $receiver.getText();
      Intrinsics.checkExpressionValueIsNotNull(var10000, "this.text");
      return var10000;
   }

public static final void setValue(@NotNull TextView $receiver, @NotNull CharSequence value) {
      Intrinsics.checkParameterIsNotNull($receiver, "receiver$0");
      Intrinsics.checkParameterIsNotNull(value, "value");
      $receiver.setText(value);
   }

搞点骚操作，int扩展属性

val sdf: SimpleDateFormat
    get() = SimpleDateFormat("yyyy-MM-dd", Locale.getDefault())

val Int.daysAgo: String
    get() = Calendar.getInstance().run {
        this.add(Calendar.DAY_OF_YEAR, -this@daysAgo)
        sdf.format(this.timeInMillis)
    }

val Int.daysLater: String
    get() = Calendar.getInstance().run {
        this.add(Calendar.DAY_OF_YEAR, +this@daysLater)
        sdf.format(this.timeInMillis)
    }

val Int.weekAgo: String
    get() = Calendar.getInstance().run {
        this.add(Calendar.WEEK_OF_YEAR, -this@weekAgo)
        sdf.format(this.timeInMillis)
    }

val Int.weekLater: String
    get() = Calendar.getInstance().run {
        this.add(Calendar.WEEK_OF_YEAR, +this@weekLater)
        sdf.format(this.timeInMillis)
    }

val Int.monthAgo: String
    get() = Calendar.getInstance().run {
        this.add(Calendar.MONTH, -this@monthAgo)
        sdf.format(this.timeInMillis)
    }

val Int.monthLater: String
    get() = Calendar.getInstance().run {
        this.add(Calendar.MONTH, +this@monthLater)
        sdf.format(this.timeInMillis)
    }

val Int.yearAgo: String
    get() = Calendar.getInstance().run {
        this.add(Calendar.YEAR, -this@yearAgo)
        sdf.format(this.timeInMillis)
    }

val Int.yearLater: String
    get() = Calendar.getInstance().run {
        this.add(Calendar.YEAR, +this@yearLater)
        sdf.format(this.timeInMillis)
    }

//打印log看效果
inline fun <reified T> T.ktxLog(msg: String) = Log.d(T::class.java.name, msg)

调用处

ktxLog(3.daysAgo)
ktxLog(3.daysLater)
ktxLog(3.weekAgo)
ktxLog(3.weekLater)
ktxLog(3.monthAgo)
ktxLog(3.monthLater)
ktxLog(3.yearAgo)
ktxLog(3.yearLater)

log

2020-01-09 11:06:01.589 11698-11698/HomeActivity: 2020-01-06
2020-01-09 11:06:01.592 11698-11698/HomeActivity: 2020-01-12
2020-01-09 11:06:01.595 11698-11698/HomeActivity: 2019-12-19
2020-01-09 11:06:01.599 11698-11698/HomeActivity: 2020-01-30
2020-01-09 11:06:01.605 11698-11698/HomeActivity: 2019-10-09
2020-01-09 11:06:01.608 11698-11698/HomeActivity: 2020-04-09
2020-01-09 11:06:01.611 11698-11698/HomeActivity: 2017-01-09
2020-01-09 11:06:01.615 11698-11698/HomeActivity: 2023-01-09

扩展属性也可以搞一堆操作玩出花样。