Annotations 註記
常用的註記介紹.
一堆常見的工具庫都有提供註記：google guava, jsr, apache common, google common, android support
NonNull/Nullable
@NonNull 與 @Nullable 在一些工具庫也有提供，不過 Android 主流還是使用 Android Support：
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
...
/**
* Add support for inflating the <fragment> tag.
*/
@Nullable
@Override
public View onCreateView(String name, @NonNull Context context, @NonNull AttributeSet attrs) {
...
這樣的好處是防呆以及省防禦性檢查：
public class MyFragment extends Fragment {
@Nullable
@Override
public View onCreateView(String name, @NonNull Context context, @NonNull AttributeSet attrs) {
if (TextUtils.isEmpty(name)) return null; // 這行編譯就會報錯
if (TextUtils.isEmpty(name)) nam = context.getResources().getString(R.id.app_name); // 不用一一檢查 context != null
// 因為 @NonNull 註記已經確保不讓塞 null
}
}
以及其他：
import android.support.annotation.StringRes;
...
public abstract void setTitle(@StringRes int resId); // 只允許塞 R.string.xxx
import android.support.annotation.IntDef;
...
public abstract class ActionBar {
...
@IntDef({NAVIGATION_MODE_STANDARD, NAVIGATION_MODE_LIST, NAVIGATION_MODE_TABS})
@Retention(RetentionPolicy.SOURCE)
public @interface NavigationMode {}
public static final int NAVIGATION_MODE_STANDARD = 0;
public static final int NAVIGATION_MODE_LIST = 1;
public static final int NAVIGATION_MODE_TABS = 2;
@NavigationMode
public abstract int getNavigationMode(); // 不用新增型別 NavigationMode class，直接標記枚舉防呆
public abstract void setNavigationMode(@NavigationMode int mode);
@IntDef(flag=true, value={ // flag = true 是給 1, 2, 4, 8 等這種旗標性數值使用，所以就算不是枚舉的數字 10 也是合法的，因為 2 + 8
DISPLAY_USE_LOGO,
DISPLAY_SHOW_HOME,
DISPLAY_HOME_AS_UP,
DISPLAY_SHOW_TITLE,
DISPLAY_SHOW_CUSTOM
})
@Retention(RetentionPolicy.SOURCE)
public @interface DisplayOptions {}
@VisibleForTesting
@VisibleForTesting
public void setLogger(ILogger logger) {...}
@Keep
We've also added @Keep to the support annotations. Note however that > that annotation hasn't been hooked up to the Gradle plugin yet (though > it's in progress.) When finished this will let you annotate methods and > > classes that should be retained when minimizing the app.
看起來還在進行中，可以先用筆者的專案：https://github.com/yongjhih/proguard-annotations