Shulou(Shulou.com)06/02 Report--

How to understand the MotionEvent object, I believe that many inexperienced people do not know what to do, so this article summarizes the causes of the problem and solutions, through this article I hope you can solve this problem.

1. MotionEvent object

When the user touches the screen, a MontionEvent object is created. MotionEvent contains information about where and when the touch occurred, as well as other details of the touch event.

The methods to get the MontionEvent object are:

1. OnTouchEvent (MotionEvent event) method in overloaded Activity

2.View object calls View.setOnTouchListener interface to implement onTouch (View v, MotionEvent event) method

After obtaining the MontionEvent object, you can further obtain the specific information of the touch event through the following common methods:

Event.getAction () / / get the number of touch points such as ACTION_DOWN event.getPointerCount (); / / get the number of touch points, such as 2, where two fingers press the screen event.getPointerId (nID) at the same time; / / for the details of each touch point, we can get the index event.getX (nID) by executing the getPointerId method in a loop; / / get the x position event.getY (nID) of the nid touch point / / get the y position event.getPressure (nID) of the nid point touch; / / LCD can sense the finger pressure of the user, of course, the specific level is determined by the driver and physical hardware event.getDownTime () / / press the start time event.getEventTime () / / event end time event.getEventTime ()-event.getDownTime (); / / it takes time to press the total time

Major related constants in touch objects:

/ * for multi-touch operation * / public static final int ACTION_MASK = 0xff; / * * when the finger is pressed * / public static final int ACTION_DOWN = 0; / * * when the finger is released * / public static final int ACTION_UP = 1 / * / public static final int ACTION_MOVE = 2; / * users may trigger irregular actions. This operation is used to indicate that a touch sequence ends without any actual action. * / public static final int ACTION_CANCEL = 3; / * the touch operation occurs outside the window, but the special case settings for the operation can still be found. * / public static final int ACTION_OUTSIDE = 4; / * / public static final int ACTION_POINTER_DOWN = 5; / * / public static final int ACTION_POINTER_UP = 6; / * / public static final int ACTION_HOVER_MOVE = 7 / * Android3.1 began to introduce constants from input devices (such as mice), not touchscreens. * / public static final int ACTION_SCROLL = 8

II. MotionEvent object processing process

The sample view is as follows:

The activity_main.xml code is as follows:

The custom Button class code is as follows:

Package com.example.d_motionevent;import android.content.Context;import android.util.AttributeSet;import android.util.Log;import android.view.MotionEvent;import android.widget.Button;/** @ author zeng * / public class BooleanButton extends Button {public BooleanButton (Context context, AttributeSet attrs) {super (context, attrs);} protected boolean myValue () {return false } @ Override public boolean onTouchEvent (MotionEvent event) {String myTag = this.getTag () .toString (); Log.e (myTag, "=" + "onTouchEvent method of Button" + "="); Log.e (myTag, MainActivity.describeEvent (this, event)); Log.e (myTag, "parent onTouchEvent () =" + super.onTouchEvent (event) Log.e (myTag, "the button touch =" + myValue ()); return myValue ();}}

TrueButton class code:

Package com.example.d_motionevent;import android.content.Context;import android.util.AttributeSet;/** * onTouchEvent returns the Button of true. * @ author zeng * / public class TrueButton extends BooleanButton {public TrueButton (Context context, AttributeSet attrs) {super (context, attrs);} @ Override protected boolean myValue () {return true;}}

FalseButton class code:

Package com.example.d_motionevent;import android.content.Context;import android.util.AttributeSet;/** * onTouchEvent returns the Button of false. * @ author zeng * / public class FalseButton extends BooleanButton {public FalseButton (Context context, AttributeSet attrs) {super (context, attrs);}}

The MainActivity.class code is as follows:

Package com.example.d_motionevent;import android.os.Bundle;import android.app.Activity;import android.util.Log;import android.view.MotionEvent;import android.view.View;import android.view.View.OnTouchListener;import android.widget.Button;/** * address: http://glblong.blog.51cto.com/3058613/1557683 * @ author zeng * / public class MainActivity extends Activity implements OnTouchListener {@ Override protected void onCreate (Bundle savedInstanceState) {super.onCreate (savedInstanceState) SetContentView (R.layout.activity_main); View layout1 = findViewById (R.id.layout1); layout1.setOnTouchListener (this); Button trueButton1 = (Button) findViewById (R.id.trueButton1); trueButton1.setOnTouchListener (this); Button falseButton1 = (Button) findViewById (R.id.falseButton1); falseButton1.setOnTouchListener (this); View layout2 = findViewById (R.id.layout2) Layout2.setOnTouchListener (this); Button trueButton2 = (Button) findViewById (R.id.trueButton2); trueButton2.setOnTouchListener (this); Button falseButton2 = (Button) findViewById (R.id.falseButton2); falseButton2.setOnTouchListener (this);} @ Override public boolean onTouch (View v, MotionEvent event) {String myTag = v.getTag (). ToString () Log.e (myTag, "=" + "onTouch method of Activity" + "="); Log.e (myTag, "View =" + myTag); Log.e (myTag, describeEvent (v, event)); if ("true" .equals (myTag.substring (0,4)) {Log.e (myTag, "= true"); return true } else {Log.e (myTag, "= = false"); return false;}} @ Override public boolean onTouchEvent (MotionEvent event) {return super.onTouchEvent (event);} protected static String describeEvent (View view, MotionEvent event) {StringBuilder sb = new StringBuilder Sb.append ("Action:") .append (event.getAction ()) .append ("\ n"); / / get touch actions such as ACTION_DOWN sb.append ("relative coordinates:") .append (event.getX ()) .append ("*") .append (event.getY ()) .append ("") Sb.append ("absolute coordinates:") .append (event.getRawX ()) .append ("*") .append (event.getRawY ()) .append ("\ n"); if (event.getX ())

View.getWidth () | | event.getY ()

View.getHeight () {sb.append ("not clicked in the View range");} sb.append ("Edge flags:") .append (event.getEdgeFlags ()) .append (""); / / Edge tag, but depending on the device, it is likely to always return 0 sb.append ("Pressure:") .append (event.getPressure ()) .append (""). / / pressure value, between 0 and 1, depending on the situation, it is likely to always return 1 sb.append ("Size:") .append (event.getSize ()) .append ("\ n"); / / finger pressure range sb.append ("Down time:") .append (event.getDownTime ()) .append ("ms"); sb.append ("Event time:") .append (event.getEventTime ()) .append ("ms") Sb.append ("Elapsed:") .append (event.getEventTime ()-event.getDownTime ()) .append ("ms\ n"); return sb.toString ();}}

Click process Analysis:

1. Click "on true_Btn_" above, and the running log is as follows:

At this point, the onTouch method on Activity is executed and True is returned.

The onTouch () method returns true because the purpose of encoding TrueButton is to return true. Returning true tells Android,MotionEvent that the object is already in use and cannot be supplied to other methods.

It also tells Android to continue sending touch events for this touch sequence to this method. This is why we see other events such as ACTION_UP or ACTION_MOVE after the ACTION_DOWN event.

When touching [on true_Btn_], the button does not change the highlight color. This is because onTouch () is called before any button method is called, and the onTouch () method returns true, so Android no longer calls the onTouchEvent () method of the button [on true_Btn_].

If you add a line of "v.onTouchEvent (event);" in the onTouch () method before returning the row of true, you will see that the button changes color when touched.

two。 Click "on false_Btn_" above, and the running log is as follows:

After clicking, the false_btn button will be on. The effect is as shown in the figure:

Android receives the ACTION_DOWN event in the MotionEvent object and passes it to the onTouch () method in the MainActivity class. OnTouch () returns false after execution.

This procedure tells the Android,onTouch () method that the event is not being used, so Android looks for the next method to call, the onTouchEvent () method overridden in the FalseButton class in the example. See the onTouchEvent () method in BooleanButton.java, which executes the onTouchEvent () method of the parent class and then returns false. The Logcat log printed out at this point is exactly the same as before, because we are still in the same View object FalseButton.

As you can see from the log, the parent class wants to return true from onTouchEvent (), but this method of FalseButton returns false. By returning false, tell Android again that we are not using this event, so Android will not send the ACTION_UP event to our button so that the button does not know if the finger has left the touchscreen. This is why FalseButton stays in the pressed color state when it is pressed.

In short, every time false is returned from the UI object that received the MotionEvent object, Android stops sending the MotionEvent object to that UI object and keeps looking for another UI object to use the MotionEvent object.

Looking back at the log, Android makes two failed attempts to find the consumer of the ACTION_DOWN event, and now it moves on to the next View in the application that might receive the event, the layout true_Layout at the bottom of the button.

At this point, Android calls the onTouch () method in the MainActivity class again, but now the object that receives the event becomes true_Layout. All the information received for the MotionEvent object is the same as before, except for the Y coordinates, because the Y coordinate of the click position is larger relative to the upper-left corner of the layout than to the upper-left corner of the button. Because the onTouch () method of true_Layout returns true, Android sends the rest of the touch event to the layout.

3. Click the upper [true_Btn_] button and touch other areas at the same time.

Touch the [on true_Btn_] button and move the other fingers around the screen before leaving the button. At this point, Logcat will show that the objects that receive the touch event are buttons on the true_Btn_. Even when the finger leaves the button on the true_Btn_ and the other finger is still moving on the screen, the button that receives the touch event is still the button on the true_Btn_.

When true is returned from onTouch (), the touch event sequence is associated with the button [on true_Btn_]. This tells Android that it can stop looking for UI objects that receive MotionEvent objects, and simply send us all future MotionEvent objects for this touch sequence. Even if we encounter another view while dragging our fingers, we still bind to the original view of this sequence.

4. Click the "under false_Btn_" button below to run the log as follows:

The principle is the same as the previous points. If the other fingers continue to touch the screen before the finger pressing the button leaves, the log will no longer be output because Android cannot find the UI object that receives the MotionEvent object and returns the true result until the next new touch sequence is started.

The sample source code is attached. Address: http://glblong.blog.51cto.com/3058613/1557683

III. MotionEvent recovery

There is a recycle () method in the MotionEvent class, but do not recycle the MotionEvent object through this method. If the callback method does not use the MotionEvent object and returns that the false,MotionEvent object may be passed to some other method, view, or our activity.

There is also an obtain () method in the MotionEvent class, which allows you to create a copy of MotionEvent or a brand new MotionEvent that should be recycled after completion.

IV. Summary

Differences between the 1.onTouch () method and the onTouchEvent () method in the View class

OnTouch () is the method called after View.setOnTouchListener. If a View object setOnTouchListener and overrides the onTouchEvent () method of the View class itself, which method will be called first when the screen is touched?

Whenever an event occurs, it is first distributed through the dispatchTouchEvent method. Take a look at the dispatchTouchEvent () source code in the View class, as follows:

/ * * Pass the touch screen motion event down to the target view, or this view * if it is the target. * * @ param event * The motion event to be dispatched. * @ return True if the event was handled by the view, false otherwise. * / public boolean dispatchTouchEvent (MotionEvent event) {/ / is used for test purposes, directly ignoring the second annotation of if (mInputEventConsistencyVerifier! = null) {mInputEventConsistencyVerifier.onTouchEvent (event, 0);} / / the above code. If (onFilterTouchEventForSecurity (event)) {/ / noinspection SimplifiableIfStatement if (mOnTouchListener! = null & & (mViewFlags & ENABLED_MASK) = = ENABLED & & mOnTouchListener.onTouch (this, event)) {/ / if there is a listener, the onTouchEvent method return true of the current view is not executed. } / / execute the onTouchEvent method if (onTouchEvent (event)) {return true;}} / / of the current view for the test item, and directly ignore if (mInputEventConsistencyVerifier! = null) {mInputEventConsistencyVerifier.onUnhandledEvent (event, 0) } return false;}

The dispatchTouchEvent () method determines whether there is a listener before determining onTouchEvent (event). Therefore, the onTouch () method is executed first, and if the listener's onTouch () returns false, the View's onTouchEvent () continues.

two。 The transmission process of touch events

1)。 The touch event is first received by the topmost UI object (such as Btn in the example), and if the View object receives and returns false, it continues to be passed on to the UI object above it (such as layout in the example), and so on, to the lowest Activity.

2)。 When the same View object receives a touch event, if a listener is set, first execute onTouch () in View.setOnTouchListener, and then execute the onTouchEvent (event) method in the View class.

3)。 When a UI object receives a touch event and returns true, other event sequences of simultaneous multi-finger touch will be received by the UI object until the next new touch event sequence is generated.

4)。 When the UI objects of each layer receive touch events and return false, multiple fingers touch at the same time, the resulting sequence of other events will not find the received UI object. Because the current original receiving UI object returns false, the system keeps looking for the next UI object that might receive an event and return true, but cannot find it.

After reading the above, have you mastered how to understand the MotionEvent object? If you want to learn more skills or want to know more about it, you are welcome to follow the industry information channel, thank you for reading!

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