Android 扩大View可点击区域范围

有时候会遇到这种需求：本身控件显示在很小的范围内，但是要求扩大可点击的区域。根据官方文档https://developer.android.com/develop/ui/views/touch-and-input/gestures/viewgroup?hl=zh-cn#delegate可以得知通过 TouchDelegate 类，让父视图能够将子视图的可触摸区域扩展到子视图的边界之外。当子节点必须很小但需要更大的触摸区域时，这非常有用。
给个例子：
布局文件activity_main.xml如下：设置的TextView对应的宽和高都只有10dp

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  xmlns:tools="http://schemas.android.com/tools"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  tools:context=".MainActivity">

  <TextView
    android:id="@+id/tv_test"
    android:layout_width="10dp"
    android:layout_height="10dp"
    android:background="#e8e8e8"
    android:gravity="center"
    android:layout_centerInParent="true"
    android:text="Hello World" />

RelativeLayout>

对面的activity文件如下：
扩大textView对应的上下左右点击区域为500
● int paddingLeft = 500;
● int paddingRight = 500;
● int paddingTop = 500;
● int paddingBottom = 500;

package com.example.addview;

import androidx.appcompat.app.AppCompatActivity;

import android.graphics.Rect;
import android.os.Bundle;
import android.view.TouchDelegate;
import android.view.View;
import android.view.ViewTreeObserver;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        final TextView testTextView = (TextView) findViewById(R.id.tv_test);
        testTextView.setOnClickListener(new View.OnClickListener() {
            int cnt = 0;
            @Override
            public void onClick(View v) {
                cnt++;
                Toast.makeText(MainActivity.this, "you clicked me" + " " + cnt + " times", Toast.LENGTH_SHORT).show();
            }
        });

        final View parent = (View) testTextView.getParent();
        int paddingLeft = 500;
        int paddingRight = 500;
        int paddingTop = 500;
        int paddingBottom = 500;
        testTextView.post(new Runnable() {
            @Override
            public void run() {
                Rect bounds = new Rect();
                testTextView.getHitRect(bounds);
                bounds.left -= paddingLeft;
                bounds.top -= paddingTop;
                bounds.right += paddingRight;
                bounds.bottom += paddingBottom;
                TouchDelegate mTouchDelegate = new TouchDelegate(bounds,testTextView);
                //设置parent的TouchDelegate,parent执行TouchDelegate的onTouchEvent方法会去调用代理的TextView的dispatchTouchEvent方法
                parent.setTouchDelegate(mTouchDelegate);
            }
        });
    }
}

然后截图就可以看到点击不是该控件显示的空白区域，也可以响应到该view的click事件。

总结：
通过PaddingLeft来扩大Android控件可点击区域的步骤如下：
● 首先，在 xml 文件中添加你的控件，并设置它的 paddingLeft 属性值。这个值应该设置为你希望扩大的像素值。
● 使用 TouchDelegate 类来创建一个代理对象，然后将它与控件绑定。代理对象告诉 Android 系统点击事件的触发范围。
● 然后使用 View.post 方法将扩大控件的宽度和高度的代码放在消息队列中。

那么为什么通过TouchDelegate类来创建的代理对象，与控件绑定以后可以扩大控件的宽度和高度呢？
从源码中可以看到如果设置了TouchDelegate，touchEvent会优先交给TouchDelegate来处理。

public class TouchDelegate {

    /**
     * View that should receive forwarded touch events
     */
    private View mDelegateView;

    /**
     * Bounds in local coordinates of the containing view that should be mapped to the delegate
     * view. This rect is used for initial hit testing.
     */
    // 传入一个Rect对象
    private Rect mBounds;

    /**
     * mBounds inflated to include some slop. This rect is to track whether the motion events
     * should be considered to be within the delegate view.
     */
    private Rect mSlopBounds;

    /**
     * True if the delegate had been targeted on a down event (intersected mBounds).
     */
    @UnsupportedAppUsage
    private boolean mDelegateTargeted;

    /**
     * The touchable region of the View extends above its actual extent.
     */
    public static final int ABOVE = 1;

    /**
     * The touchable region of the View extends below its actual extent.
     */
    public static final int BELOW = 2;

    /**
     * The touchable region of the View extends to the left of its actual extent.
     */
    public static final int TO_LEFT = 4;

    /**
     * The touchable region of the View extends to the right of its actual extent.
     */
    public static final int TO_RIGHT = 8;

    private int mSlop;

    /**
     * Touch delegate information for accessibility
     */
    private TouchDelegateInfo mTouchDelegateInfo;

    /**
     * Constructor
     *
     * @param bounds Bounds in local coordinates of the containing view that should be mapped to
     *        the delegate view
     * @param delegateView The view that should receive motion events
     */
    public TouchDelegate(Rect bounds, View delegateView) {
        // 接收对应的view控件和扩大的区域
        mBounds = bounds;

        mSlop = ViewConfiguration.get(delegateView.getContext()).getScaledTouchSlop();
        mSlopBounds = new Rect(bounds);
        mSlopBounds.inset(-mSlop, -mSlop);
        mDelegateView = delegateView;
    }

    /**
     * Forward touch events to the delegate view if the event is within the bounds
     * specified in the constructor.
     *
     * @param event The touch event to forward
     * @return True if the event was consumed by the delegate, false otherwise.
     */
    public boolean onTouchEvent(@NonNull MotionEvent event) {
        // 获取事件的坐标x,y
        int x = (int)event.getX();
        int y = (int)event.getY();
        boolean sendToDelegate = false;
        boolean hit = true;
        boolean handled = false;

        switch (event.getActionMasked()) {
            // 如果是ACTION_DOWN事件，判断事件的位置x,y是否落在扩大的区域mBounds内
            case MotionEvent.ACTION_DOWN:
                mDelegateTargeted = mBounds.contains(x, y);
                sendToDelegate = mDelegateTargeted;
                break;
            case MotionEvent.ACTION_POINTER_DOWN:
            case MotionEvent.ACTION_POINTER_UP:
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_MOVE:
                sendToDelegate = mDelegateTargeted;
                if (sendToDelegate) {
                    Rect slopBounds = mSlopBounds;
                    if (!slopBounds.contains(x, y)) {
                        hit = false;
                    }
                }
                break;
            case MotionEvent.ACTION_CANCEL:
                sendToDelegate = mDelegateTargeted;
                mDelegateTargeted = false;
                break;
        }
        // 如果落在扩大的区域内
        if (sendToDelegate) {
            if (hit) {
                // 设置该事件对应的触发位置
                // Offset event coordinates to be inside the target view
                event.setLocation(mDelegateView.getWidth() / 2, mDelegateView.getHeight() / 2);
            } else {
                // Offset event coordinates to be outside the target view (in case it does
                // something like tracking pressed state)
                int slop = mSlop;
                event.setLocation(-(slop * 2), -(slop * 2));
            }
            // 拦截点击事件
            handled = mDelegateView.dispatchTouchEvent(event);
        }
        return handled;
    }
    ...
}

从源码中 可以看到，创建TouchDelegate 需要传入一个Rect(left,top,right,bottom) 和delegateView, onTouchEvent触发时，会通过这个Rect来判断点击事件是否落在区域内，如果是 则转发给代理view来处理该事件。