目前，我正在研究为我们提供的名为 PaintPot 的 Android 程序的代码，该程序允许用户在他们的 Android 设备上进行手指绘画。

此代码处理当点击屏幕、单击按钮等时将发生的事件。

// Here is the event dispatcher for our app.  We need to Override the method for the Form
// superclass
@Override
public boolean dispatchEvent(Component component, String id, String eventName,
                             Object[] args) {

    //if the canvas is touched by a tapping finger
    if (component.equals(myCanvas) && eventName.equals("Touched")) {
        canvasTouced(((Float) args[0]).intValue(), ((Float) args[1]).intValue());
        return true;

        //if the canvas is touched by a dragging finger, paint the line this way
    } else if (component.equals(myCanvas) && eventName.equals("Dragged")) {
        drawLine(((Float) args[2]).intValue(),
                ((Float) args[3]).intValue(),
                ((Float) args[4]).intValue(),
                ((Float) args[5]).intValue());
        return true;

        //if the canvas is touched while the blue button is selected
    } else if (component.equals(btnBlue) && eventName.equals("Click")) {
        myCanvas.PaintColor(COLOR_BLUE);
        return true;

        //if the canvas is touched while the green button is selected
    } else if (component.equals(btnGreen) && eventName.equals("Click")) {
        myCanvas.PaintColor(COLOR_GREEN);
        return true;

        //if the canvas is touched while the red button is selected
    } else if (component.equals(btnRed) && eventName.equals("Click")) {
        myCanvas.PaintColor(COLOR_RED);
        return true;

        //if the wipe button is selected
    } else if (component.equals(btnWipe) && eventName.equals("Click")) {
        myCanvas.Clear();
        return true;
    }


    return false;
}

代码运行良好，并且“按原样”执行我希望的操作。但我真正不明白的是，如果用户在 Canvas 上点击或拖动他或她的手指的 if 语句处于“同一级别”，或者如果选择了颜色按钮，则在同一 if 语句中。不应该是当用户在屏幕上点击或拖动他的手指时，确定是否制作一条线或一个点的相同代码也应该询问线或点应该是什么颜色，而不仅仅是大小点，取决于选择什么颜色的按钮？

为了更完整的引用，这里是完整的代码，如果有人好奇的话:

import android.graphics.Color;
import com.google.devtools.simple.runtime.components.android.Button;
import com.google.devtools.simple.runtime.components.Component;
import com.google.devtools.simple.runtime.components.HandlesEventDispatching;
import com.google.devtools.simple.runtime.components.android.Canvas;
import com.google.devtools.simple.runtime.components.android.Form;
import com.google.devtools.simple.runtime.components.android.HorizontalArrangement;
import com.google.devtools.simple.runtime.components.android.Label;
import com.google.devtools.simple.runtime.events.EventDispatcher;


import java.util.Random;




public class PaintPotActivity extends Form implements HandlesEventDispatching {


private Canvas myCanvas; //creates a canvas object
private Label lblStatus; //creates a label that discusses the status of the program
private Button btnRed; //creates a button for red paint
private Button btnBlue; // "" for blue paint
private Button btnGreen; // "" for green paint


private Button btnWipe; //creates a button that wipes the screen clean
private Button btnDotSize; // creates a button that changes the dot size




// Variable (field) used to for displaying number of touches
int numTouches; //declares an integer that lists out the number of touches a user made


// The equivalent to a "main" method for App Inventor apps is the $define method.
void $define() { 


    //We are going to place the color buttons in a HorizontalArrangement
    HorizontalArrangement hr = new HorizontalArrangement(this);
    btnRed = new Button(hr); 
    btnBlue = new Button(hr);
    btnGreen = new Button(hr);


    //set their color
    btnRed.BackgroundColor(Color.RED);
    btnBlue.BackgroundColor(Color.BLUE);
    btnGreen.BackgroundColor(Color.GREEN);

    //set the button text
    btnRed.Text("Red");
    btnBlue.Text("Blue");
    btnGreen.Text("Green");


    //canvas into its own HorizontalArrangement
    hr = new HorizontalArrangement(this);
    myCanvas = new Canvas(hr);
    myCanvas.Width(400);
    myCanvas.Height(400);
    myCanvas.LineWidth(10);


    //Wipe and a label into its own HorizontalArrangement
    hr = new HorizontalArrangement(this);
    btnWipe = new Button(hr);
    btnWipe.Text("Wipe");


    lblStatus = new Label(hr);
    lblStatus.Text("  touchX/touchY:");


    // Register for events.  By the second argument can be any string.    The third argument must
    // exactly match the name of the event that you want to handle for that component.  When the event
    // happens, dispatchEvent will be called with these arguments.
    EventDispatcher.registerEventForDelegation(this, "JavaBridge", "Touched");
    EventDispatcher.registerEventForDelegation(this, "JavaBridge", "Click");
    EventDispatcher.registerEventForDelegation(this, "JavaBridge", "Dragged");
}


// Here is the event dispatcher for our app.  We need to Override the method for the Form
// superclass
@Override
public boolean dispatchEvent(Component component, String id, String eventName,
                             Object[] args) {

    //if the canvas is touched by a tapping finger
    if (component.equals(myCanvas) && eventName.equals("Touched")) {
        canvasTouced(((Float) args[0]).intValue(), ((Float) args[1]).intValue());
        return true;

        //if the canvas is touched by a dragging finger, paint the line this way
    } else if (component.equals(myCanvas) && eventName.equals("Dragged")) {
        drawLine(((Float) args[2]).intValue(),
                ((Float) args[3]).intValue(),
                ((Float) args[4]).intValue(),
                ((Float) args[5]).intValue());
        return true;

        //if the canvas is touched while the blue button is selected
    } else if (component.equals(btnBlue) && eventName.equals("Click")) {
        myCanvas.PaintColor(COLOR_BLUE);
        return true;

        //if the canvas is touched while the green button is selected
    } else if (component.equals(btnGreen) && eventName.equals("Click")) {
        myCanvas.PaintColor(COLOR_GREEN);
        return true;

        //if the canvas is touched while the red button is selected
    } else if (component.equals(btnRed) && eventName.equals("Click")) {
        myCanvas.PaintColor(COLOR_RED);
        return true;

        //if the wipe button is selected
    } else if (component.equals(btnWipe) && eventName.equals("Click")) {
        myCanvas.Clear();
        return true;
    }


    return false;
}




/**
 * This method will get the touched touchX, touchY coordinates and will then create a circle
 * of random radius (between 1 to 33) with the color that was selected (RED, BLUE or GREEN).
 * It will also display the touched touchX,touchY coordinates.
 * @param x current x
 * @param y current y
 */
private void canvasTouced(int x, int y) {


    myCanvas.DrawCircle(x, y, new Random().nextInt(33));
    lblStatus.Text("  touchX/touchY:" + x + "/" + y + " touches: " + ++numTouches);


}


/**
 * Method to draw line
 * @param prevX last touch x
 * @param prevY last touch y
 * @param touchX current x
 * @param touchY current y
 */
private void drawLine(int prevX, int prevY, int touchX, int touchY) {
    myCanvas.DrawLine(prevX, prevY, touchX, touchY);
 }


}

最佳答案

dispatchEvent 在应用程序触发事件时被调用。每次按钮点击和与应用程序的交互都会引发单独的事件。每个 if 语句都决定了如何处理不同类型的事件。

Shouldn't it be when the user taps or drags his finger across the screen, the same code that determines if a line or a dot that is made should also ask for what color the line or dot should be, rather than just size of the dot, depending on what color button is selected

在这种情况下，事件之间的状态保存在 myCanvas 中 - 单击 btnBlue 将 Canvas 上的绘画颜色设置为蓝色 (myCanvas.PaintColor( COLOR_BLUE);) 然后等待另一个事件发生。如果用户随后在 Canvas 上拖动，则会绘制一条线(最终 myCanvas.DrawLine(prevX, prevY, touchX, touchY);)。由于 myCanvas 记住了状态，所以它以蓝色绘制线。

关于java - 同一级别的 If 语句如何同时在 Android/Java 程序中执行？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/15008695/