Kotlin Book Club 2019第一週talk

1. Kotlin在Android中的應⽤用
Cast, Better Callback, Extension functions
Lutas Lin
2019.05.03
Taiwan Kotlin User Group - Kotlin Book Club 2019

2. Sample Code連結
https://github.com/lutas2000/Android-Kotlin-Sample

3. Cast
• 什什麼是Cast?

4. Main Activity
MainFragment KotlinFragmentJavaFragment
PM : 老闆希望紀錄User離開App時的⾴頁⾯面

5. Main Activity
MainFragment KotlinFragmentJavaFragment
Reportable
report()

6. private Fragment currentFragment;
@Override
protected void onDestroy() {
super.onDestroy();
((Reportable)currentFragment).report();
}
public interface Reportable {
public void report();
}
Reportable.java
MainActivity.java

7. Main Activity
MainFragment KotlinFragmentJavaFragment
Reportable
report()
PM : 對了了，MainFragment別回報！

8. Caused by: java.lang.ClassCastException:
lutas.sample.kotlinandroid.MainFragment cannot be cast to
lutas.sample.kotlinandroid.Reportable
糟糕！
MainActivity.java
@Override
protected void onDestroy() {
super.onDestroy();
if (currentFragment instanceof Reportable) {
((Reportable) currentFragment).report();
}
}

9. Kotlin有更更好的寫法嗎？
Safe Cast
(currentFragment as? Reportable)?.report()
Type Check and Smart Cast
if (currentFragment is Reportable) {
currentFragment.report()
}

10. Callback
• 如何寫⼀一個Callback ?
class A {
interface Callback {
public void onDone();
}
private void action(Callback callback) {
// Do something
callback.onDone();
}
}
A a = new A();
a.action(new Callback() {
@Override
public void onDone() {
// Do something
}
});

11. Kotlin有更更好的寫法嗎？
class A {
fun action(onDone: () -> Unit) {
// Do something
onDone()
}
}
val a = A()
a.action {
// Do something
}
Lambda as Parameters

12. Extension Functions
• EditText的TextWatcher Interface好煩！
• 要override三個function: beforeTextChanged,
onTextChanged, afterTextChanged
• afterTextChanged的parameter還不是String！

13. EditTextUtil.java
interface OnChangedListener {
public void onChanged(String text);
}
public static void setOnTextChangedListener(
EditText editText,
final OnChangedListener listener
) {
editText.addTextChangedListener(new TextWatcher() {
@Override
public void beforeTextChanged(…) {
}
@Override
public void onTextChanged(…) {
}
@Override
public void afterTextChanged(Editable s) {
listener.onChanged(s.toString());
}
});
}

14. EditTextUtil.java
EditTextUtil.setOnTextChangedListener(editText,
new EditTextUtil.OnChangedListener() {
@Override
public void onChanged(String text) {
Log.d(TAG, "text changed: "+ text);
}
}
);

15. EditTextExt.kt
fun EditText.setOnTextChangedListener(onChanged: (String) -> Unit) {
addTextChangedListener(object: TextWatcher {
override fun afterTextChanged(s: Editable) {
onChanged(s.toString())
}
override fun beforeTextChanged(…) {
}
override fun onTextChanged(…) {
}
})
}
editText.setOnTextChangedListener {
Log.d(TAG, "text changed: $it")
}
使⽤用情境