JavaScript的事件：onLoad

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Navigator 2.0
Navigator 3.0: event handler of Image

In a FRAMESET and FRAME relationship, an onLoad event within a frame (placed in the BODY tag) occurs before an onLoad event within the FRAMESET (placed in the FRAMESET tag).

For images, the onLoad event handler indicates the script to execute when an image is displayed. Do not confuse displaying an image with loading an image. You can load several images, then display them one by one in the same Image object by setting the object's src property. If you change the image displayed in this way, onLoad executes every time an image is displayed, not just when the image is loaded into memory.

If you specify an onLoad事件适用对象an Image object that displays a looping GIF animation (multi-image GIF), each loop of the animation triggers the onLoad event, and the event handler executes once for each loop.

You can use the onLoad event handler to create a JavaScript animation by repeatedly setting the src property of an Image object. See Image for information.

BODY onLoad="window.alert("Welcome to the Brave New World home page!") 示例 2: Display alert when image loads. The following example creates two Image objects, one with the Image constructor and one with the IMG tag. Each Image object has an onLoad event handler that calls the displayAlert function, which displays an alert. For the image created with the IMG tag, the alert displays the image name. For the image created with the Image constructor, the alert displays a message without the image name. This is because the onLoad handler for an object created with the Image constructor must be the name of a function, and it cannot specify parameters for the displayAlert function.

SCRIPT
imageA = new Image(50,50)
imageA.onload=displayAlert
imageA.src="cyanball.gif" function displayAlert(theImage) {
   if (theImage==null) {
      alert('An image loaded')
   }
   else alert(theImage.name + ' has been loaded.')
}
/SCRIPT IMG NAME="imageB" SRC="greenball.gif" ALIGN="top"
   onLoad=displayAlert(this)BR 示例 3: Looping GIF animation. The following example displays an image, birdie.gif, that is a looping GIF animation. The onLoad事件适用对象the image increments the variable cycles, which keeps track of the number of times the animation has looped. To see the value of cycles, the user clicks the button labeled Count Loops.

SCRIPT
var cycles=0
/SCRIPT
IMG ALIGN="top" SRC="birdie.gif" BORDER=0
   onLoad="++cycles"
INPUT TYPE="button" VALUE="Count Loops"
   onClick="alert('The animation has looped ' + cycles + ' times.')" 示例 4: Change GIF animation displayed. The following example uses an onLoad event handler to rotate the display of six GIF animations. Each animation is displayed in sequence in one Image object. When the document loads, !anim0.html is displayed. When that animation completes, the onLoad event handler causes the next file, !anim1.html, to load in place of the first file. After the last animation, !anim5.html, completes, the first file is again displayed. Notice that the changeAnimation function does not call itself after changing the src property of the Image object. This is because when the src property changes, the image's onLoad event handler is triggered and the changeAnimation function is called.

SCRIPT
var whichImage=0
var maxImages=5 function changeAnimation(theImage) {
   ++whichImage
   if (whichImage = maxImages) {
      var imageName="!anim" + whichImage + ".gif"
      theImage.src=imageName
   } else {
      whichImage=-1
      return
   }
}
/SCRIPT IMG NAME="changingAnimation" SRC="!anim0.gif" BORDER=0 ALIGN="top"
   onLoad="changeAnimation(this)" See also示例 for Image.