前言
成都創(chuàng)新互聯(lián)主打移動(dòng)網(wǎng)站、網(wǎng)站設(shè)計(jì)、做網(wǎng)站、網(wǎng)站改版、網(wǎng)絡(luò)推廣、網(wǎng)站維護(hù)、空間域名、等互聯(lián)網(wǎng)信息服務(wù)，為各行業(yè)提供服務(wù)。在技術(shù)實(shí)力的保障下，我們?yōu)榭蛻舫兄Z穩(wěn)定，放心的服務(wù)，根據(jù)網(wǎng)站的內(nèi)容與功能再?zèng)Q定采用什么樣的設(shè)計(jì)。最后，要實(shí)現(xiàn)符合網(wǎng)站需求的內(nèi)容、功能與設(shè)計(jì)，我們還會(huì)規(guī)劃穩(wěn)定安全的技術(shù)方案做保障。
最近斷斷續(xù)續(xù)地把項(xiàng)目的界面部分的代碼由JAva改成了Kotlin編寫，并且如果應(yīng)用了kotlin-android-extensions插件,一個(gè)顯而易見的好處是再也不用寫 findViewById()來實(shí)例化你的控件對(duì)象了，直接操作你在布局文件里的id即可，這一點(diǎn)我感覺比butterknife做的還簡潔友好。
Activity
重慶分公司
重慶分公司
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import kotlinx.android.synthetic.main.activity_main.*
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
textview.text="hello world"
}
}其中kotlinx.android.synthetic.main.activity_main.*是kotlin-android-extensions插件自動(dòng)生成的。下面我們來解析下原理。因?yàn)閗otlin也是一門JVM語言，最近也會(huì)和java一樣編譯成class字節(jié)碼，所以我們直接來反編譯看看生成的java文件。
選擇Decompile,解析出來的代碼如下
public final class MainActivity extends AppCompatActivity {
private HashMap _$_findViewCache;
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
this.setContentView(2131296284);
TextView var10000 = (TextView)this._$_findCachedViewById(id.textview);
Intrinsics.checkExpressionValueIsNotNull(var10000, "textview");
var10000.setText((CharSequence)"hello world");
}
public View _$_findCachedViewById(int var1) {
if (this._$_findViewCache == null) {
this._$_findViewCache = new HashMap();
}
View var2 = (View)this._$_findViewCache.get(var1);
if (var2 == null) {
var2 = this.findViewById(var1);
this._$_findViewCache.put(var1, var2);
}
return var2;
}
public void _$_clearFindViewByIdCache() {
if (this._$_findViewCache != null) {
this._$_findViewCache.clear();
}
}
}可以很清楚看到最終還是調(diào)用了findViewById() ，不過獲取View對(duì)象直接調(diào)用的是findCachedViewById,并且創(chuàng)建一個(gè) HashMap 進(jìn)行View對(duì)象的緩存，避免每次調(diào)用 View 時(shí)都會(huì)重新調(diào)用findViewById()進(jìn)行查找。
Fragment
再來看下Fragment中的使用：
import android.os.Bundle
import android.support.v4.app.Fragment
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import kotlinx.android.synthetic.main.fragment_blank.*
class BlankFragment : Fragment() {
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
return inflater.inflate(R.layout.fragment_blank, container, false)
}
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
textview_fra.text="hello world"
}
}反編譯后代碼如下
public final class BlankFragment extends Fragment {
private HashMap _$_findViewCache;
@Nullable
public View onCreateView(@NotNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
Intrinsics.checkParameterIsNotNull(inflater, "inflater");
return inflater.inflate(2131296285, container, false);
}
public void onViewCreated(@NotNull View view, @Nullable Bundle savedInstanceState) {
Intrinsics.checkParameterIsNotNull(view, "view");
super.onViewCreated(view, savedInstanceState);
TextView var10000 = (TextView)this._$_findCachedViewById(id.textview_fra);
Intrinsics.checkExpressionValueIsNotNull(var10000, "textview_fra");
var10000.setText((CharSequence)"hello world");
}
public View _$_findCachedViewById(int var1) {
if (this._$_findViewCache == null) {
this._$_findViewCache = new HashMap();
}
View var2 = (View)this._$_findViewCache.get(var1);
if (var2 == null) {
View var10000 = this.getView();
if (var10000 == null) {
return null;
}
var2 = var10000.findViewById(var1);
this._$_findViewCache.put(var1, var2);
}
return var2;
}
public void _$_clearFindViewByIdCache() {
if (this._$_findViewCache != null) {
this._$_findViewCache.clear();
}
}
// $FF: synthetic method
public void onDestroyView() {
super.onDestroyView();
this._$_clearFindViewByIdCache();
}
}可以看到最終是通過調(diào)用getView().findViewById()來進(jìn)行控件的實(shí)例化。
看下getView()源碼
@Nullable
public View getView() {
return this.mView;
}再看下mView成員變量的賦值時(shí)機(jī)：
void performCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
if (this.mChildFragmentManager != null) {
this.mChildFragmentManager.noteStateNotSaved();
}
this.mPerformedCreateView = true;
this.mViewLifecycleOwner = new LifecycleOwner() {
public Lifecycle getLifecycle() {
if (Fragment.this.mViewLifecycleRegistry == null) {
Fragment.this.mViewLifecycleRegistry = new LifecycleRegistry(Fragment.this.mViewLifecycleOwner);
}
return Fragment.this.mViewLifecycleRegistry;
}
};
this.mViewLifecycleRegistry = null;
this.mView = this.onCreateView(inflater, container, savedInstanceState);
if (this.mView != null) {
this.mViewLifecycleOwner.getLifecycle();
this.mViewLifecycleOwnerLiveData.setValue(this.mViewLifecycleOwner);
} else {
if (this.mViewLifecycleRegistry != null) {
throw new IllegalStateException("Called getViewLifecycleOwner() but onCreateView() returned null");
}
this.mViewLifecycleOwner = null;
}
}可以看到mView其實(shí)就是onCreateView()的返回值，所以我們不能在onCreateView()方法里操作控件ID的方式操作View對(duì)象，會(huì)產(chǎn)生空指針異常。建議在onViewCreated()方法里使用。
其他（動(dòng)態(tài)布局）
除了Activity和Fragment，我們用的最多的UI布局當(dāng)屬Adapter了，kotlin-android-extensions也提供了對(duì)這一類動(dòng)態(tài)布局的支持。因?yàn)檫@一功能是實(shí)現(xiàn)性質(zhì)的，默認(rèn)關(guān)閉，我們需要手動(dòng)打開，在build.gradle中開啟：
androidExtensions {
experimental = true
}然后再recycler.adapter中使用如下：
import kotlinx.android.extensions.LayoutContainer import kotlinx.android.synthetic.main.item_recyclerview.* class MyAdapter(val context: Context, val data: List) : RecyclerView.Adapter () { override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder { val view = LayoutInflater.from(context).inflate(R.layout.item_recyclerview, parent, false) return ViewHolder(view) } override fun onBindViewHolder(holder: ViewHolder, position: Int) { holder.name_tv.text = data[position] holder.itemView.setOnClickListener { Toast.makeText(context,"點(diǎn)擊了第$position 項(xiàng)",Toast.LENGTH_SHORT).show() } } override fun getItemCount(): Int { return data.size } inner class ViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView), LayoutContainer { override val containerView: View = itemView } }
可以看到相比Activity和Fragment，我們的ViewHolder需要多實(shí)現(xiàn)一個(gè)接口LayoutContainer。看下它的源碼：
public final class ViewHolder extends android.support.v7.widget.RecyclerView.ViewHolder implements LayoutContainer {
@NotNull
private final View containerView;
private HashMap _$_findViewCache;
@NotNull
public View getContainerView() {
return this.containerView;
}
public ViewHolder(@NotNull View itemView) {
Intrinsics.checkParameterIsNotNull(itemView, "itemView");
super(itemView);
this.containerView = itemView;
}
public View _$_findCachedViewById(int var1) {
if (this._$_findViewCache == null) {
this._$_findViewCache = new HashMap();
}
View var2 = (View)this._$_findViewCache.get(var1);
if (var2 == null) {
View var10000 = this.getContainerView();
if (var10000 == null) {
return null;
}
var2 = var10000.findViewById(var1);
this._$_findViewCache.put(var1, var2);
}
return var2;
}
public void _$_clearFindViewByIdCache() {
if (this._$_findViewCache != null) {
this._$_findViewCache.clear();
}
}
}總結(jié)
以上就是這篇文章的全部內(nèi)容了，希望本文的內(nèi)容對(duì)大家的學(xué)習(xí)或者工作具有一定的參考學(xué)習(xí)價(jià)值，如果有疑問大家可以留言交流，謝謝大家對(duì)創(chuàng)新互聯(lián)的支持。
