Google JetPack 之 DataBinding

心已赠人 2022-01-22 02:17 135阅读 0赞

# DataBinding #

DataBinding 是以声明的方式，将布局中组件与应用程序源数据绑定在一起的框架库。

作用：  
1.将布局组件与源数据绑定，使源数据变化的同时布局组件及时同步更新。  
2.减少Activity中View的定义(private View view)与初始化(findViewById)，让Activity代码更专注于界面的逻辑更新。  
3.可自定义适配器，实现扩展组件的属性功能。  
4.可自定义事件，实现各种组件的事件触发功能。  
特点：  
1.使用简单，主要以声明的方式实现。  
2.功能强大，可自定义适配器 & 事件 ，兼容各种界面逻辑需求。

## 简单使用 ##

只为研究下原理，然后写了一个简单的 demo

android {
    		//......
        dataBinding {
            enabled = true
        }

layout 代码 activity\_qinxue.xml

<?xml version="1.0" encoding="utf-8"?>
    <layout xmlns:android="http://schemas.android.com/apk/res/android">
        <data>
            <variable
                name="userInput"
                type="String" />
        </data>
    
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:orientation="vertical"
            android:padding="20dp">
    
            <TextView
                android:id="@+id/text_view"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="@{userInput}"/>
    
            <EditText
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:hint="输入"
                android:text="@={userInput}" />
        </LinearLayout>
    </layout>

Activity 代码

public class QinXueActivity extends Activity {
    
        @Override
        protected void onCreate(@Nullable Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            ActivityQinxueBinding binding = ActivityQinxueBinding.inflate(getLayoutInflater());
            setContentView(binding.getRoot());
        }
    }

实现效果  
![效果][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3Fpbnh1ZTI0_size_16_color_FFFFFF_t_70]

## 源码分析 ##

### 生成的中间产物 ###

1、activity\_qinxue.xml

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
    <Layout absoluteFilePath="/Users/qinxue/Downloads/GoogleArchitectureDemo/app_specific/src/main/res/layout/activity_qinxue.xml" directory="layout"
        isMerge="false"
        layout="activity_qinxue" modulePackage="google.architecture.specific">
        <Variables name="userInput" declared="true" type="String">
            <location endLine="5" endOffset="27" startLine="3" startOffset="8" />
        </Variables>
        <Targets>
            <Target tag="layout/activity_qinxue_0" view="LinearLayout">
                <Expressions />
                <location endLine="26" endOffset="18" startLine="8" startOffset="4" />
            </Target>
    	//......
        </Targets>
    </Layout>

/build/intermediates/data-binding-info/debug/activity\_qinxue-layout.xml  
编译出的中间产物，文件会给每一个View设置上tag，这个很重要。但是这个这么重要，View的tag岂不是不能自定义了，感觉不太好。

2、ActivityQinxueBinding.java  
build/generated/source/apt/debug/google/architecture/specific/databinding/ActivityQinxueBinding.java  
编译出的中间产物，做数据绑定的。

### 流程分析 ###

1、inflate 填充View，看到这里调用了bind

public static ActivityQinxueBinding inflate(@NonNull android.view.LayoutInflater inflater, @Nullable android.databinding.DataBindingComponent bindingComponent) {
            return bind(inflater.inflate(google.architecture.specific.R.layout.activity_qinxue, null, false), bindingComponent);
        }

2、bind检查了tag，上面编译的中间产物中已经标明了tag。返回了ActivityQinxueBinding 对象。

public static ActivityQinxueBinding bind(@NonNull android.view.View view, @Nullable android.databinding.DataBindingComponent bindingComponent) {
            if (!"layout/activity_qinxue_0".equals(view.getTag())) {
                throw new RuntimeException("view tag isn't correct on view:" + view.getTag());
            }
            return new ActivityQinxueBinding(bindingComponent, view);
        }

3、ActivityQinxueBinding 初始化

public ActivityQinxueBinding(@NonNull android.databinding.DataBindingComponent bindingComponent, @NonNull View root) {
            super(bindingComponent, root, 0);
            final Object[] bindings = mapBindings(bindingComponent, root, 3, sIncludes, sViewsWithIds);	//遍历view，并保存
            this.mboundView0 = (android.widget.LinearLayout) bindings[0];		//本地变量指向 view
            this.mboundView0.setTag(null);
            this.mboundView2 = (android.widget.EditText) bindings[2];
            this.mboundView2.setTag(null);
            this.textView = (android.widget.TextView) bindings[1];
            this.textView.setTag(null);
            setRootTag(root);		// rootView的tag设置为this，建立两者关系。
            // listeners
            invalidateAll();			//刷新绑定,所有的view都绑定
        }

4、数据绑定

@Override
        public void invalidateAll() {
            synchronized(this) {
                    mDirtyFlags = 0x2L;
            }
            requestRebind();	//关键方法，无论哪个View绑定数据都会调用这个
        }

5、ViewDataBinding.java 里的方法

/**
         * @hide
         */
        protected void requestRebind() {
            if (mContainingBinding != null) {	//没有设置的话就是null
                mContainingBinding.requestRebind();
            } else {
                synchronized (this) {
                    if (mPendingRebind) {		//在mRebindRunnable执行之前都是true,防止多次重新刷新
                        return;
                    }
                    mPendingRebind = true;
                }
                //执行刷新，线面是个版本适配，做的事情是一致的，看下面的mUIThreadHandler就行了,切到主线程
                if (USE_CHOREOGRAPHER) {
                    mChoreographer.postFrameCallback(mFrameCallback);
                } else {
                    mUIThreadHandler.post(mRebindRunnable);	
                }
            }
        }

4、Runnable

private final Runnable mRebindRunnable = new Runnable() {
            @Override
            public void run() {
                synchronized (this) {
                    mPendingRebind = false;	//置标记位允许后面的rebind执行
                }
                processReferenceQueue();
    		
                if (VERSION.SDK_INT >= VERSION_CODES.KITKAT) {
                    // Nested so that we don't get a lint warning in IntelliJ
                    	//if 代码块里是个判断View是否Attach到Window，监听到window，最终还是执行executePendingBindings();
                    if (!mRoot.isAttachedToWindow()) {
                        // Don't execute the pending bindings until the View
                        // is attached again.
                        mRoot.removeOnAttachStateChangeListener(ROOT_REATTACHED_LISTENER);
                        mRoot.addOnAttachStateChangeListener(ROOT_REATTACHED_LISTENER);
                        return;
                    }
                }
                executePendingBindings();
            }
        };

5、ViewDataBinding 的executePendingBindings

/**
         * Evaluates the pending bindings, updating any Views that have expressions bound to
         * modified variables. This <b>must</b> be run on the UI thread.
         */
        public void executePendingBindings() {
            if (mContainingBinding == null) {
                executeBindingsInternal();	
            } else {
                mContainingBinding.executePendingBindings();
            }
        }
        
     private void executeBindingsInternal() {
            if (mIsExecutingPendingBindings) {
                requestRebind();
                return;
            }
            if (!hasPendingBindings()) {
                return;
            }
            mIsExecutingPendingBindings = true;//标记位防止重复进入
            mRebindHalted = false;
            if (mRebindCallbacks != null) {	//这个回调没设置就是null，分析流程可不不管他
                mRebindCallbacks.notifyCallbacks(this, REBIND, null);	//绑定前回调，这里可能重置mRebindHalted，这是设置拦截的逻辑
    
                // The onRebindListeners will change mPendingHalted
                if (mRebindHalted) {
                    mRebindCallbacks.notifyCallbacks(this, HALTED, null);//通知被拦截
                }
            }
            if (!mRebindHalted) {
                executeBindings();// 关键代码，在ViewDataBinding这个类中是个空方法，需查看子类
                if (mRebindCallbacks != null) {
                    mRebindCallbacks.notifyCallbacks(this, REBOUND, null);
                }
            }
            mIsExecutingPendingBindings = false;
        }

6、ActivityQinxueBinding类中，实际绑定代码

@Override
        protected void executeBindings() {
            long dirtyFlags = 0;
            synchronized(this) {
                dirtyFlags = mDirtyFlags;
                mDirtyFlags = 0;
            }
            java.lang.String userInput = mUserInput;
    
            if ((dirtyFlags & 0x3L) != 0) {
            }
            // batch finished
            if ((dirtyFlags & 0x3L) != 0) {
                // api target 1
    
                android.databinding.adapters.TextViewBindingAdapter.setText(this.mboundView2, userInput);
                android.databinding.adapters.TextViewBindingAdapter.setText(this.textView, userInput);//设置数据
            }
            if ((dirtyFlags & 0x2L) != 0) {
                // api target 1
    
                android.databinding.adapters.TextViewBindingAdapter.setTextWatcher(this.mboundView2, (android.databinding.adapters.TextViewBindingAdapter.BeforeTextChanged)null, (android.databinding.adapters.TextViewBindingAdapter.OnTextChanged)null, (android.databinding.adapters.TextViewBindingAdapter.AfterTextChanged)null, mboundView2androidTextAttrChanged);//设置监听
            }
        }

dirtyFlags的解释

a、参考 mDirtyFlags = 0x2L; 所有(dirtyFlags & flag) != 0 都会执行绑定  
b、先看设置数据代码

public void setUserInput(@Nullable java.lang.String UserInput) {
            this.mUserInput = UserInput;
            synchronized(this) {
                mDirtyFlags |= 0x1L; //这里是1
            }
            notifyPropertyChanged(BR.userInput);
            super.requestRebind();
        }

(dirtyFlags & 0x2L) == 0 的，不会执行设置监听的操作，只会执行数据绑定。

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3Fpbnh1ZTI0_size_16_color_FFFFFF_t_70]: /images/20220122/38f82bc96a3d4e29a8c45f6cba820676.png