TouchDelegate源码分析

在View的onTouchEvent()中会涉及到这个类，就顺便看了一看，虽然之前没有接触过。

用途

如果有两个View，分别是ViewA，ViewB，可以给ViewA setTouchDelegate(bounds, ViewB)，这样当ViewA中落在bounds的事件就会传给ViewB。

使用场景见官方文档。

源码分析

这个类主要就是一个diapatchTouchEvent()，用来将事件分发给委托View。

/**
 * Helper class to handle situations where you want a view to have a larger touch area than its
 * actual view bounds. The view whose touch area is changed is called the delegate view. This
 * class should be used by an ancestor of the delegate. To use a TouchDelegate, first create an
 * instance that specifies the bounds that should be mapped to the delegate and the delegate
 * view itself.
 * 

 * The ancestor should then forward all of its touch events received in its
 * {@link android.view.View#onTouchEvent(MotionEvent)} to {@link #onTouchEvent(MotionEvent)}.
 * 
 */
// 如果你需要一个View拥有比自己真实区域更大的区域接受触摸事件，你就可以使用这个工具类。
// 触摸区域改变的View叫做委托View。
// 这个类应该被包含委托View的父View使用，父View就应该将它所有在区域内的事件都传递给它。
public class TouchDelegate {

    /**
     * View that should receive forwarded touch events
     */
    // 接受额外区域事件的View，委托View。
    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.
     */
    // 构造器传入的需要传递给委托View的坐标区域。
    private Rect mBounds;

    /**
     * mBounds inflated to include some slop. This rect is to track whether the motion events
     * should be considered to be be within the delegate view.
     */
    // mBounds扩充一些最小滑动距离后的区域，用来判断事件是否应该被考虑在委托View的区域中
    private Rect mSlopBounds;

    /**
     * True if the delegate had been targeted on a down event (intersected mBounds).
     */
    // 如果被指向一个ACTION_DOWN事件，就为true。
    private boolean mDelegateTargeted;

    /**
     * The touchable region of the View extends above its actual extent.
     */
    // 委托View的扩充区域在真实区域之上
    public static final int ABOVE = 1;

    /**
     * The touchable region of the View extends below its actual extent.
     */
    // 委托View的扩充区域在真实区域之下
    public static final int BELOW = 2;

    /**
     * The touchable region of the View extends to the left of its
     * actual extent.
     */
    // 委托View的扩充区域在真实区域左边
    public static final int TO_LEFT = 4;

    /**
     * The touchable region of the View extends to the right of its
     * actual extent.
     */
    // 委托View的扩充区域在真实区域右边
    public static final int TO_RIGHT = 8;

    // 被看作移动事件的最小滑动距离
    private int mSlop;

    /**
     * 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
     */
    /**
     * 构造器，初始化。
     * @param bounds 需要映射到委托View的区域
     * @param delegateView 委托View
     */
    public TouchDelegate(Rect bounds, View delegateView) {
        mBounds = bounds;

        // 获得最小滑动参数
        mSlop = ViewConfiguration.get(delegateView.getContext()).getScaledTouchSlop();
        mSlopBounds = new Rect(bounds);
        // 将区域的每条边向外扩大移动mSlop的距离
        mSlopBounds.inset(-mSlop, -mSlop);
        mDelegateView = delegateView;
    }

    /**
     * Will 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 forwarded to the delegate, false otherwise.
     */
    /**
     * 如果事件在构造器中指定的区域内，就将该事件委托传递给构造器指定的View。
     * @param event 事件
     * @return 是否成功的传递给委托View并消耗事件。
     */
    public boolean onTouchEvent(MotionEvent event) {
        // 获取事件相对于事件分发到的View的坐标
        int x = (int)event.getX();
        int y = (int)event.getY();
        boolean sendToDelegate = false;
        boolean hit = true;
        boolean handled = false;

        switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN:
            Rect bounds = mBounds;
            // 判断事件是否在委派控制的区域内
            if (bounds.contains(x, y)) {
                // 在区域内就置为true
                mDelegateTargeted = true;
                sendToDelegate = true;
            }
            break;
        case MotionEvent.ACTION_UP:
        case MotionEvent.ACTION_MOVE:
            // ACTION_UP和ACTION_MOVE可能位于一个ACTION_MOVE事件之后，所以使用mSlopBounds判断。
            sendToDelegate = mDelegateTargeted;
            // 如果接受过ACTION_DOWN事件，并且在区域内。
            if (sendToDelegate) {
                Rect slopBounds = mSlopBounds;
                // 判断如果不在扩展最小滑动距离的区域中，就将hit置为false。
                if (!slopBounds.contains(x, y)) {
                    hit = false;
                }
            }
            break;
        case MotionEvent.ACTION_CANCEL:
            // 如果是取消事件，sendToDelegate赋值为ACTION_DOWN时赋的值
            sendToDelegate = mDelegateTargeted;
            // 再将mDelegateTargeted重新赋值false，因为此时事件序列应该是要结束了。
            mDelegateTargeted = false;
            break;
        }
        // 如果上面赋值了true，就去将事件传递给委托View。
        if (sendToDelegate) {
            final View delegateView = mDelegateView;

            // 如果该事件为ACTION_MOVE事件后，并且还在区域内。
            if (hit) {
                // Offset event coordinates to be inside the target view
                // 将事件的坐标重新设置为委托View的中心。
                event.setLocation(delegateView.getWidth() / 2, delegateView.getHeight() / 2);
            } else {
                // Offset event coordinates to be outside the target view (in case it does
                // something like tracking pressed state)
                int slop = mSlop;
                // 如果在区域外了就设置在委托区域之外两倍的最小滑动距离上。
                event.setLocation(-(slop * 2), -(slop * 2));
            }
            // 调用委托View去分发处理该事件。
            handled = delegateView.dispatchTouchEvent(event);
        }
        // 返回值true表示将事件分发给了委托View并且委托View处理了该事件。
        // 返回值为false表示委托View没有处理事件，或者因为事件坐标的原因没有交给委托View处理。
        return handled;
    }
}