Fix the issue when using WebImage with some transition like scaleEffect, each time the new state update will cause unused image fetching

[dreampiggy]

See our demo:

WebImage(url: url, options:[.delayPlaceholder])
    .placeholder(.wifiExclamationmark)
    .scaleEffect(self.scale)
    .gesture(MagnificationGesture(minimumScaleDelta: 0.1).onChanged { value in
        let delta = value / self.lastScale
        self.lastScale = value
        let newScale = self.scale * delta
        self.scale = min(max(newScale, 0.5), 2)
    }.onEnded { value in
        self.lastScale = 1.0
    })

Before this fix, when network load failed, each time you zoom your finder, the new SDWebImage loadImage function call will trigger, this cause huge performance cost, worstly, the UI part will show a placeholder with flashback.

To workaround with this, we roll back to load the image in WebImage's first Appear. However, we have to solve the issue #20 #19

So, this time, we use a trick (smart ?) solution: We query the memory cache firstlly during WebImage init method. This is fast because it just a NSCache objectForKey call.

If memory cache hit, the image does not show placeholder or empty, so fix the editMode issue.

[codecov]

Codecov Report

Merging #92 into master will decrease coverage by 0.82%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master      #92      +/-   ##
==========================================
- Coverage   74.28%   73.45%   -0.83%     
==========================================
  Files           9        9              
  Lines         696      727      +31     
==========================================
+ Hits          517      534      +17     
- Misses        179      193      +14     

Flag	Coverage Δ
#ios	73.45% <75.00%> (-0.83%)	⬇️

Impacted Files	Coverage Δ
SDWebImageSwiftUI/Classes/ImageManager.swift	70.73% <67.85%> (-3.35%)	⬇️
SDWebImageSwiftUI/Classes/WebImage.swift	80.00% <100.00%> (-2.17%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0b11cce...ffeea1a. Read the comment docs.

[dreampiggy]

Why SwiftUI Suck here ? The layout engine create a new View struct, touch the body value, but not trigger the onAppear, what does this used for ?